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What is this job like? |
How do you get ready?
What is the remuneration?
How many jobs are there?
What about the future?
Are there other jobs like this?
Where can you find more information?
What is this job like?
Civil engineers design things. These might be roads, buildings, airports, tunnels, dams, bridges, or water supply and sewage systems. They must consider many factors in their designs, from the costs to making sure the structure will stay intact during bad weather. This is one of the oldest types of engineering.
Many civil engineers manage people and projects. A civil engineer may oversee a construction site or be a city engineer. Others may work in design, construction, research, and teaching. There are many specialties within civil engineering, such as structural, construction, environment, and transportation.
Civil engineers usually work in areas where there is lots of manufacturing and businesses. Often they work at construction sites. Sometimes they work in places that are far away from cities. Most engineers work at least 40 hours per week. Some are required to travel.
How do you get ready?
A college degree in engineering is needed for almost all beginning engineering jobs. College graduates with a degree in a science or mathematics may get some jobs. A civil engineer must get a license from the State to offer services directly to the public. A license requires 4 years of relevant work experience and passing an exam. Beginning engineers often work under an experienced engineer to learn as much as possible. To teach engineering, further schooling is needed.
Getting into engineering school requires know-how of math and science, and courses in English, social studies, and computers. Also, some schools prepare an engineer to get a job after graduation while others prepare an engineer for graduate school. It is a good idea to learn about the school before you attend.
Engineers should be creative, curious, analytical, and detail-oriented. They should be able to work as part of a team. People skills are important. This is because engineers often work with people in a wide range of fields.
What is the remuneration?
Remuneration depends on the quality of the state’s economy.
You should look forward to packages like these:
If the state’s economy is healthy, you will get more. Here are further guidelines.
How many jobs are there?
Civil engineers held about 278,400 jobs in 2008. Almost half worked for firms that did architectural, engineering, and related services work. About one-fourth were in some level of government. The rest mostly worked in construction.
What about the future?
It is expected that civil engineer jobs will increase much faster than the average for all occupations through 2018. More civil engineers will be needed to design and build things as the population grows. For example, they will need to fix and replace buildings and roads as they continue to become old, unsafe, and worn out.
Some job openings will occur to replace those who go to other jobs or leave the labor force. Job openings will also depend on where you live. There may not be as many jobs when the economy is doing poorly. This is because there is often less construction at such times.
Are there other jobs like this?
Agricultural and food scientists
Computer and information systems managers
Where can you find more information?
What is Civil Engineering?
Civil engineering is field of engineering sciences, related to design, construction and maintenance of buildings, dams, bridges, tunnels, highways and other structures by the use of physical laws, mathematical equations and theories of mechanics. Civil Engineers utilize the available resources (expertise, materials, manpower) to complete the project in the given time span keeping in view the time, available resources (expertise, materials, manpower), environmental issues and physical hazards of the project.
Perform engineering duties in planning, designing, and overseeing construction and maintenance of building structures, and facilities, such as roads, railroads, airports, bridges, harbors, channels, dams, irrigation projects, pipelines, power plants, water and sewage systems, and waste disposal units. Include architectural, structural, traffic, ocean, and geo-technical engineers. Exclude "Hydrologists".
Take a look at this.
Being a broader field Civil Engineering is divided into the following sub-categories and / or fields.
1. Structural Engineering
2. Urban Planning
3. Water Resource engineering
4. Transportation Engineering
5. Environmental Engineering
6. Geo technical Engineering
What does a Civil Engineer do?
A Civil Engineer is responsible for planning, design, construction and/or maintenance of structures. A civil Engineer can be a surveyor, a technical report writer or even a project manager.
Planning and development
How will you plan, design, and develop a state of 360,000 square miles?
What essential parameters will you collect to complete this task?
What is the location of water resources?
How will you plan, design, and develop an iron factory that will serve the needs of the state of 360,000 square miles?
How many engineers will you need for planning, design, development, and maintenance?
Strength of Materials|
Piers and Marine Structures
Water Retaining Structures and Waterworks
Piles and Foundation
Strength of Materials
What do you know about the strength of materials?
Is there a difference between an equation and a formula?
What is an equation?
What is an example of an equation?
Which equations or formulas are useful in determining the strength of materials?
How would you go ahead determining the strength of materials?
A four-story building needs iron for building the foundation, pillars, beams, and roofing.
How deep below the surface will you place the iron bar?
What type of iron will you need to sustain a uniform load of 1,000 people in the four-story building?
What type of iron will you need to sustain a uniform load of 46 battle tanks, with each tank 32 feet long, 12 feet wide, 8 feet high, and weighing 68.7 tons, plus a crew of four weighing 195 pounds each?
What should be the length, breadth, thickness, or diameter of the iron bar?
How will you calculate the safety factor?
What are the types of building joints?
What is the safe strength of the joint?
What type of iron will you need?
Will you use bolts or welding for joints? Why?
What is a rivet?
How will you present joints in the form of a diagram?
What should be the foundation area of such building?
What should be the dimension for pillars with concrete?
How will you calculate the weight of each iron bar?
What do you know about the radius of gyration?
What should be the width of the road and foot path of an avenue?
What is the difference between an avenue and a street?
How do you measure if an avenue 10 blocks away from the center point is running in line with the avenue from the center point?
What is the value of 10 blocks in kilometers and miles?
How many total blocks should be a district, county, or city?
How many total blocks north, south, east, and west is Chicago?
How much is that in kilometers or miles?
How do you measure if an avenue is parallel to the next avenue?
What should be the width of a highway?
Can cement slab replace asphalt?
What are the advantages and disadvantages of cement slab replacing asphalt?
What should be the dimensions of each pavement of cement within a road?
Approximately 15 feet long, 9 feet wide, 8 inches thick.
Each rectangle of pavement has four iron rods in the middle.
Multi purpose building|
Nominate at least six people experienced in these dynamics.
They will have to regularly e-mail, call, visit the construction site, and answer questions.
A meeting may be required after regular e-mails and phone calls.
What will I provide?
Guidelines, measurements, design, number of units, number of floors.
If there are 184 units, I will own five of them.
Is that clear?
Do you agree?
A multipurpose building with at least four floors with 184 units, accommodating 164 people, can be constructed in the same area where a house has stood and accommodates only two to five people.
Implement complete ban on construction of houses.
Which is better?
Obviously, four floors with 184 units.
What will you provide?
Quota of cement, iron, land, other resources.
214 Questions and Answers on Practical Civil Engineering Works|
Q) Why choose civil engineering?
Q) Civil Engineering and Military Engineering: What's the difference?
Q) Is Ocean Engineering similar to Civil Engineering?
Q) Is Land Surveying part of Civil Engineering?
Q) What are Civil and Environmental Engineering all about?
Q) What is the future for civil engineering?
Q) What is Civil and Architectural Engineering?
Q) What is civil engineering? What are the prospects?
Q) What are the major programs in Civil Engineering?
Q) Can I do a dual degree in Architecture and Civil Engineering?
Q) What are the major programs in Civil Engineering?
Q) What is �preset� during installation of bridge bearings?
Q) Sometimes the side of concrete bridges is observed to turn black in colour. What is the reason for this phenomenon?
Q) In prestressing work, if more than one wire or strand is included in the same duct, why should all wires/strands be stressed at the same time?
Q) In the design of elastomeric bearings, why are steel plates inserted inside the bearings?
Q) How to determine the size of elastomeric bearings?
Q) In a curved prestressed bridge, how should the guided bearings in piers of the curved region be oriented with respect to the fixed bearing in abutment?
Q) What are the advantages of piers constructed monolithically with the bridge deck over usage of bearings?
Q) What are the limitations of Rational Method in calculating runoff? Why are some manhole covers made of cast iron while some are made of ductile iron?
Q) Why is it preferable to design stormwater drains to match soffit?
Q) What is the application of inverted siphons? What are the disadvantages of using inverted siphons?
Q) What is the mechanism of cavitation in pipes and drains?
Q) When a drainage system (i.e. u-channels with catchpits) is connected to a main drainage channel, a segment of short pipe is used. What is the reason of such arrangement?
Q) What are the functions of bedding under stormwater drains?
Q) In designing sewer pipes, why are vitrified clay pipes commonly used for pipe size less than 600mm while precast concrete pipes with PVC lining is used for pipe size exceeding 600mm?
Q) The spacing of manholes in straight sections for different pipe sizes is stated in Stormwater Drainage Manual. How are these figures arrived at?
Q) Should air test or water tests be selected to test the leakage of constructed gravity pipelines?
Q) In designing of access ramps for drainage channels, why should the direction of access ramps be sloping down towards downstream?
Q) When branch pipelines are connected to main pipelines, sometimes Y-junctions or fitting branched pipelines to main pipelines by formation of holes in main pipelines are used. Which one is a better choice?
Q) Rational Method should not be used for large catchments in estimating peak runoff. Is it true?
Q) What is the purpose of carrying out water absorption test for precast concrete pipes?
Q) In soil compaction test, if a test result exceeds 100%, should engineers accept the result?
Q) What are the different applications of draglines, backhoes and shovels?
Q) Is it worthwhile to carry out tests on particle density of soil particles for geotechnical design?
Q) In determining the effective stress parameters of a soil sample, which test is preferable, consolidated undrained test or consolidated drained test?
Q) Soil nails are mainly designed for stabilization of major slips. How should designer cater for the stability of minor slips?
Q) Are there any differences in the methods of compaction between clayey soil material and sandy material?
Q) Why is sulphate-resisting cement not used in marine concrete?
Q) Shall a layer of wearing course or additional thickness be designed on the surface of piers?
Q) In connecting fenders to pier structures, should single lock nuts or double lock nuts be used?
Q) What is the design level of landings in piers?
Q) What are the pros and cons of using timber fenders, plastic fenders and rubber fenders?
Q) For marine pile type of steel tubular piles with reinforced concrete infill, minimum toe level is often specified in contract drawings. What is its purpose?
Q) Why is shallow bedrock condition unfavorable for open berth piers?
Q) Why are high and narrow beams not desirable in concrete piers?
Q) What is the mechanism of formation of soil plugs in marine driven steel piles with reinforced concrete infill?
Q) Should stiff or soft fenders be designed for berthing in piers?
Q) How can unreinforced concrete pavement function without mesh reinforcement?
Q) Why are concrete profile barriers designed with curved surface profiles?
Q) Should joints of concrete kerbs be in line with the joints in concrete carriageway?
Q) Can all utility detectors detect the depth of utilities?
|Defective buildings � demolish and rebuild or repair|
|Exhibition and Exposition|
Sub District Hospitals
Primary Health Centres
Subsidery Health Centres
Medical Aid Centres
|Heavy industrial buildings|
Hydro Electric Projects|
|Nuclear Power Plants|
|Industrial Estate-manufacturing plants|
This building is designed for an international district, county or city with well defined blocks and well defined numbering of blocks from the central point of the district, county or city by urban and regional planners.|
Who are the urban and regional planners?
Who were the previous urban and regional planners?
Do you have a map of the plan?
Is your district, county or city international?
Have you divided it into blocks?
What are the measurements of the blocks?
How are the water supply, food resources, health care, transportation, police services, postal services, schools, colleges, universities, electricity services, wastewater treatment, and landfill area located and designed for adequate capacity?
Where are the main water supply, food warehouses, health care facilities, transportation, police, postal services, schools, colleges, universities, electricity services, wastewater treatment, and landfill areas located?
Where is the central point of the district, county or city?
How far is the nearest industrial estate from the central point of the district, county or city?
What are the key products and services?
What and where is the highest surface point of the district, county or city?
What and where is the lowest surface point of the district, county or city?
If this isn't an international district, county or city, this type of building can still be constructed.
You have to provide the following details.
How much space is available?
What is the exact location?
Where are the nearest water supply, food market, health care facility, transportation services, police, postal service, school, college, university, electrical pole, and wasterwater tank?
Where are the main water supply, food warehouse, health care facility, transportation services, police service, postal service, schools, colleges, universities, electricity, wastewater tank, and landfill area?
How soon can this building be completed?
As soon as you provide all the answers.
|Mosques, Churches, Temples, Gurduwara (Multi purpose)|
|Plazas and Piazzas|
|Satellite Earth Station|
|School and Academic|
|Town and City|
|Traffic light system|
|Trade Facilitation Centers|
|University Disaster Recovery Facility|
Buildings, construction Codes|
Building Electricity system
Building Emergency light system
Building Water supply (plumbing) system/boiler
Building Fire fighting system
Building Elevator system
Building Escalator system
Building Gas service system
Building Monitoring system
Building Janitorial system
Building Parking system
Building Laundry System
Building Vending machine System
Building Sewage system
Building Telephone system
Building Heating/Cooling system
Building Chilled water system
Building Wifi system/Dish
Building in General
Type of construction high-rise building
Interior Partitions and Ceilings
Building Elevator system|
Q: What type of maintenance agreement will work best for your elevators?
Q: What is a Full Maintenance Contract? (FMC)
Q: What is a Parts, Oil, and Grease Contract? (POG)
Q: What is an Oil and Grease (OG) or Examination & Lubrication Contract?
Q: What is a Survey and Report Contract?
Q: When does overtime apply under the contract?
Q: What are the differences between overtime trouble calls and overtime repairs as they relate to the contract?
Q: Is travel time a consideration because of location or union agreement?
Q: If maintenance is missed or incomplete are you entitled to a refund for that month?
Q: Do trouble calls count as a regular monthly service call?
Q: What is the response time for a trapped passenger?
Q: What is the availability of spare parts?
Q: What is typically not covered under maintenance agreements.
|Construction and Extraction workers|
Structural Collapse Research|
Srinagar to Uri
Uri to Rawalpindi
For Example World's Tallest building|
Height 1,477 ft
Requires 2.70 million cubic feet of concrete
Iron/Steel Pillars 10.
214 Questions and Answers on Practical Civil Engineering Works
Q. What does a Civil Engineer do?
A. Civil engineers design and direct the construction of dams to prevent flooding, improve irrigation, provide a water supply, and generate hydroelectric power. They design and construct highways and railroads, bridges and tunnels, airports and docks. Civil engineering is that branch of engineering most closely connected to the environment, both natural and manmade.
Q) What does a Civil Engineer do?
214 Questions and Answers on Practical Civil Engineering Works
Piers and Marine Structures
Water Retaining Structures and Waterworks
Piles and Foundation
Q) Why choose civil engineering?
Q) What is civil engineering?
Q) Civil Engineering and Military Engineering: What's the difference?
Q) Is Ocean Engineering similar to Civil Engineering?
Q) Is Land Surveying part of Civil Engineering?
Q) What are Civil and Environmental Engineering all about?
Q) What is the future for civil engineering?
Q) What is Civil and Architectural Engineering?
Q) What is civil engineering? What are the prospects?
Q) What are the major programs in Civil Engineering?
Q) Can I do a dual degree in Architecture and Civil Engineering?
Q) What are the major programs in Civil Engineering?
. Chapter 1 �ridge Works 1. Under what situation shall engineers use jacking at one end only and from both ends in prestressing work?
During prestressing operation at one end, frictional losses will occur and the prestressing force decreases along the length of tendon until reaching the other end. These frictional losses include the friction induced due to a change of curvature of tendon duct and also the wobble effect due to deviation of duct alignment from the centerline. Therefore, the prestress force in the mid-span or at the other end will be greatly reduced in case the frictional loss is high. Consequently, prestressing, from both ends for a single span i.e. prestressing one-half of total tendons at one end and the remaining half at the other end is carried out to enable a even distribution and to provide symmetry of prestress force along the structure.
In fact, stressing at one end only has the potential advantage of lower cost when compared with stressing from both ends. For multiple spans (e.g. two spans) with unequal span length, jacking is usually carried out at the end of the longer span so as to provide a higher prestress force at the location of maximum positive moment. On the contrary, jacking from the end of the shorter span would be conducted if the negative moment at the intermediate support controls the prestress force. However, if the total span length is sufficiently long, jacking from both ends should be considered.
2. What is “preset” during installation of bridge bearings?
“Preset” is a method to reduce the size of upper plates of sliding bearings in order to save the material cost. The normal length of a upper bearing plate should be composed of the following components: length of bearing + 2 x irreversible movement + 2 x reversible movement. Initially the bearing is placed at the mid-point of the upper bearing plate without considering the directional effect of irreversible movement. However, as irreversible movement normally takes place at one direction only, the bearing is displaced/presetted a distance of (irreversible movement/2) from the mid-point of bearing in which the length of upper plate length is equal to the length of bearing + irreversible movement + 2 x reversible movement. In this arrangement, the size of upper plate is minimized in which irreversible movement takes place in one direction only and there is no need to include the component of two irreversible movements in the upper plate. Note: “Preset” refers to the displacement of a certain distance of sliding bearings with respect to upper bearing plates during installation of bearings.
3. In incremental launching method of bridge construction, what are the measures adopted to enhance sufficient resistance of the superstructure during the launching process?
(i) During the launching process the leading edge of the superstructure is subject to a large hogging moment. In this connection, steel launching nose typically about 0.6-0.65 times span length is provided at the leading edge to reduce the cantilever moment. Sometimes, instead of using launching nose a tower and stay system are designed which serves the same purpose.
5 (ii) The superstructure continually experiences alternative sagging and hogging moments during incremental launching. Normally, a central prestress is provided in which the compressive stress at all points of bridge cross section is equal. In this way, it caters for the possible occurrence of tensile stresses in upper and lower part of the cross section when subject to hogging and sagging moment respectively. Later when the whole superstructure is completely launched, continuity prestressing is performed in which the location and design of continuity tendons are based on the bending moments in final completed bridge condition and its provision is supplementary to the central prestress. (iii)For very long span bridge, temporary piers are provided to limit the cantilever moment.
4. In bridge widening projects, the method of stitching is normally employed for connecting existing deck to the new deck. What are the problems associated with this method in terms of shrinkage of concrete?
In the method of stitching, it is a normal practice to construct the widening part of the bridge at first and let it stay undisturbed for several months. After that, concreting will then be carried out for the stitch between the existing deck and the new deck. In this way, the dead load of the widened part of bridge is supported by itself and loads arising from the newly constructed deck will not be transferred to the existing deck which is not designed to take up these extra loads.
One of the main concerns is the effect of stress induced by shrinkage of newly widened part of the bridge on the existing bridge. To address this problem, the widened part of the bridge is constructed a period of time (say 6-9 months) prior to stitching to the existing bridge so that shrinkage of the new bridge will take place within this period and the effect of shrinkage stress exerted on the new bridge is minimized.
Traffic vibration on the existing bridge causes adverse effect to the freshly placed stitches. To solve this problem, rapid hardening cement is used for the stitching concrete so as to shorten the time of setting of concrete. Moreover, the stitching work is designed to be carried out at nights of least traffic (Saturday night) and the existing bridge may even be closed for several hours (e.g. 6 hours) to let the stitching works to left undisturbed. Sometimes, longitudinal joints are used in connecting new bridge segments to existing bridges. The main problem associated with this design is the safety concern of vehicles. The change of frictional coefficients of bridge deck and longitudinal joints when vehicles change traffic lanes is very dangerous to the vehicles. Moreover, maintenance of longitudinal joints in bridges is quite difficult.
Note: Stitching refers to formation of a segment of bridge deck between an existing bridge and a new bridge.
5. What are the advantages of assigning the central pier and the abutment as fixed piers?
(i) For abutment pier to be assigned as fixed pier while the bridge is quite long, the longitudinal loads due to earthquake are quite large. As the earthquake loads are resisted by fixed piers, the size of fixed piers will be large and massive. In this connection, for better aesthetic appearance, the selection of abutment as fixed piers could accommodate the large size and massiveness of piers. Normally abutments are relatively short in height and for the same horizontal force, the bending moment
(ii) For the central pier to be selected as the fixed pier, the bridge deck is allowed to move starting from the central pier to the end of the bridge. However, if the fixed pier is located at the abutment, the amount of movement to be incorporated in each bearing due to temperature variation, shrinkage, etc. is more than that when the fixed pier is located at central pier. Therefore, the size of movement joints can be reduced significantly.
6. Sometimes the side of concrete bridges is observed to turn black in colour. What is the reason for this phenomenon?
In some cases, it may be due to the accumulation of dust and dirt. However, for the majority of such phenomenon, it is due to fungus or algae growth on concrete bridges. After rainfall, the bridge surface absorbs water and retains it for a certain period of time. Hence, this provides a good habitat for fungus or algae to grow. Moreover, atmospheric pollution and proximity of plants provide nutrients for their growth. Improvement in drainage details and application of painting and coating to bridges help to solve this problem.
7. In prestressing work, if more than one wire or strand is included in the same duct, why should all wires/strands be stressed at the same time?
If wires/strands are stressed individually inside the same duct, then those stressed strand/wires will bear against those unstressed ones and trap them. Therefore, the friction of the trapped wires is high and is undesirable.
8. In the design of elastomeric bearings, why are steel plates inserted inside the bearings?
For elastomeric bearing to function as a soft spring, the bearing should be allowed for bulging laterally and the compression stiffness can be increased by limiting the amount of lateral bulging. To increase the compression stiffness of elastomeric bearings, metal plates are inserted. After the addition of steel plates, the freedom to bulge is restricted and the deflection is reduced when compared with bearings without any steel plates under the same load. Tensile stresses are induced in these steel plates during their action in limiting the bulging of the elastomer. This in turn would limit the thickness of the steel plates. However, the presence of metal plates does not affect the shear stiffness of the elastomeric bearings.
Effect of steel plate in elastomeric bearing.
9. How to determine the size of elastomeric bearings?
For elastomeric bearing, the vertical load is resisted by its compression while shear resistance of the bearing controls the horizontal movements. The design of elastomeric bearings are based on striking a balance between the provision of sufficient stiffness to resist high compressive force and the flexibility to allow for translation and rotation movement.
The cross sectional area is normally determined by the allowable pressure on the bearing support. Sometimes, the plan area of bearings is controlled by the maximum allowable compressive stress arising from the consideration of delamination of elastomer from steel plates. In addition, the size of elastomeric bearings is also influenced by considering the separation between the structure and the edge of bearing which may occur in rotation because tensile stresses deriving from separation may cause delamination. The thickness of bearings is designed based on the limitation of its horizontal stiffness and is controlled by movement requirements. The shear strain should be less than a certain limit to avoid the occurrence of rolling over and fatigue damage. The vertical stiffness of bearings is obtained by inserting sufficient number of steel plates.
10. In a curved prestressed bridge, how should the guided bearings in piers of the curved region be oriented with respect to the fixed bearing in abutment?
To determine the orientation of guided bearings, one should understand the movement of curved region of a prestressed bridge. Movement of prestress and creep are tangential to the curvature of the bridge (or along longitudinal axis) while the movement due to temperature and shrinkage effects are in a direction towards the fixed pier. If the direction of guided bearings is aligned towards the fixed bearing in the abutment, the difference in direction of pretress and creep movement and the guided direction towards fixed bearing would generate a locked-in force in the bridge system. The magnitude of the lock-in force is dependent on the stiffness of deck and supports. If the force is small, it can be designed as additional force acting on the support and deck. However, if the force is large, temporary freedom of movement at the guided bearings has to be provided during construction.
11. In the construction of a two-span bridge (span length = L) by using span-by-span construction, why is a length of about 1.25L bridge segment is constructed in the first phase of construction?
Basically, there are mainly three reasons for this arrangement: (i) The permanent structure is a statically indeterminate structure. During construction by using span-by-span construction, if the first phase of construction consists of the first span length L only, then the sagging moment in the mid span of the partially completed bridge is larger than that of completed two-span permanent structure. To avoid such occurrence, 0.25L of bridge segment is extended further from the second pier which provides a counteracting moment, thereby reducing the mid-span moment of the partially completed bridge.
(ii) The position of 1.25 L countering from the first pier is the approximate location of point of contraflexure (assume that the two-span bridge is uniformly loaded) in which the bridge moment is about zero in the event of future loaded bridge. Therefore, the design of construction joint in this particular location has the least adverse effect on the structural performance of the bridge.
(iii) In case of a prestressed bridge, prestressing work has to be carried out after the construction of first segment of the bridge. If the prestressing work is conducted at the first pier which is heavily reinforced with reinforcement, it is undesirable when compared with the prestressing location at 1.25L from the first pier where there is relatively more space to accommodate prestressing works. Note: Span-by-span construction means that a bridge is constructed from one bridge span to another until its completion.
12. What are the advantages of piers constructed monolithically with the bridge deck over usage of bearings?
Basically, piers constructed monolithically with the bridge deck are advantageous in the following ways:
(i) Movement of the bridge deck is achieved by the bending deformation of long and
slender piers. In this way, it saves the construction cost of bearings by using monolithic construction between bridge deck and piers. Moreover, it is not necessary to spend extra effort to design for drainage details and access for bearing replacement. On the other hand, in maintenance aspect substantial cost and time savings could be obtained by using monolithic construction instead of using bearings as bridge articulation.
(ii) Monolithic construction possesses the shortest effective Euler buckling length for piers because they are fixed supports at the interface between bridge deck and piers. Note: Monolithic construction means that piers are connected to bridge decks without any joints and bearings.
13. Are diaphragms necessary in the design of concrete box girder bridges? Diaphragms are adopted in concrete box girder bridges to transfer loads from bridge decks to bearings. Since the depth of diaphragms normally exceeds the width by two times, they are usually designed as deep beams. However, diaphragms may not be necessary in case bridge bearings are placed directly under the webs because loads in bridge decks can be directly transferred to the bearings based on Jorg Schlaich & Hartmut Scheef (1982). This arrangement suffers from the drawback that changing of bearings during future maintenance operation is more difficult.
In fact, diaphragms also contribute to the provision of torsional restraint to the bridge deck. 14. What is the advantage of sliding bearings over roller bearings?
In roller bearing for a given movement the roller bearing exhibit a change in pressure centre from its original position by one-half of its movement based on David J. Lee. However, with sliding bearing a sliding plate is attached to the upper superstructure and the moving part of bearing element is built in the substructure. It follows that there is no change in pressure center after the movement.
15. What are the three major types of reinforcement used in prestressing?
(i) Spalling reinforcement Spalling stresses are established behind the loaded area of anchor blocks and this causes breaking away of surface concrete. These stresses are induced by strain incompatibility with Poisson’s effects or by the shape of stress trajectories.
(ii) Equilibrium reinforcement Equilibrium reinforcement is required where there are several anchorages in which prestressing loads are applied sequentially.
(iii) Bursting Reinforcement Tensile stresses are induced during prestressing operation and the maximum bursting stress occurs where the stress trajectories are concave towards the line of action of the load. Reinforcement is needed to resist these lateral tensile forces.
16. Why is the span length ratio of end span/approach span to its neighboring inner spans usually about 0.95?
10 From aesthetic point of view, an odd number of spans with a decrease in length in the direction of abutment is desirable. Moreover, spans of equal length are found to be boring. However, the arrangement of irregular span lengths is not recommended because it gives a feeling of uneasiness.
From structural point of view, for a multi-span bridge with equal span length, the sagging moment at the mid-span of the end span/approach span is largest. In order to reduce this moment, the span length of end span/approach span is designed to be 0.95 of inner spans. However, this ratio should not be less than 0.40 because of the effect of uplifting at the end span/approach span support.
Note: End span refers to the last span in a continuous bridge while approach span refers top the first span of a bridge.
17. In the design of a simply supported skew bridge, which direction of reinforcement should be provided?
In the conventional design of steel reinforcement for a simply supported skew bridge, a set of reinforcement is usually placed parallel to free edge while the other set is designed parallel to the fixed edge. However, this kind of arrangement is not the most efficient way of placing the reinforcement. The reason is that in some parts of the bridge, the moment of resistance is provided by an obtuse angle formed by the reinforcement bars which is ineffective in resisting flexure. In fact, the most efficient way of the arrangement of reinforcement under most loading conditions is to place one set of bars perpendicular to the fixed edge while placing the other set parallel to the fixed end as recommended by L. A. Clark (1970). In this way, considerable savings would be obtained from the orthogonal arrangement of reinforcement.
The arrangement of reinforcement in skewed bridge.
18. What are the functions of grout inside tendon ducts?
Grout in prestressing works serves the following purposes: (i) Protect the tendon against corrosion. (ii) Improve the ultimate capacity of tendon. (iii) Provide a bond between the structural member and the tendon. (iv) In case of failure, the anchorage is not subject to all strain energy.
19. What is the consideration in selecting the orientation of wing walls in the design of bridge abutments? There are three common arrangements of wing walls in bridge abutments based on Dr. Edmund C Hambly (1979):
(i) Wing walls parallel to abutments This is the simplest and shortest time to build but is not the most economical design. This design has the advantage that it has least disturbance to existing slope embankment. (ii) Wing walls at an angle to abutments This is the most economical design among the three options in terms of material cost. (iii) Wing walls perpendicular to abutments Though it is not the most economical design, the wing walls provide a continuous
12 alignment with bridge decks which provide supports to parapets. However, they cause disturbances to adjacent structures and utility services during construction. Moreover, if the bridge is curved, the wing walls may hinder the road curvature. One the other hand, when the wing walls are structurally connected to the abutment, then structural advantage can be taken by the stability of box structure.
Fig. 1.4 Different orientation of wing walls. 20. In joints of precast concrete bridge segments, what are the functions of applying epoxy adhesive?
13 Epoxy adhesive is applied in these joints for the following purposes according to International Road Federation (1977): (i) It seals up the joints completely between precast concrete segments to protect the prestressing tendons; (ii) By filling voids and irregularities along the segment joints, it helps to reduce stress concentrations otherwise it will be developed; and (iii) It helps in transferring of shear between the joints in case a large single shear key is used.
21. Under what situation should engineers use pot bearings instead of elastomeric bearings?
In the event of high vertical loads combined with large angle of rotations, rubber bearings are undesirable when compared with pot bearings. For instance, elastomeric bearings require large bearing surfaces so that compression can be maintained between the contact surfaces between the bearings and piers. Moreover, it also leads to uneven distribution of stress on the piers and some of these highly induced stresses may damage the piers.
22. What are the shortcomings of grillage analysis which is commonly used in structural analysis of bridges?
Grillage analysis suffers from the following shortcomings based on E. C. Hambly: (i) For coarse mesh, torques may not be identical in orthogonal directions. Similarly, twists may differ in orthogonal directions. (ii) Moment in any beams is mainly proportional to its curvature only. However, moment in an element depends on the curvatures in the beam’s direction and its orthogonal direction.
23. Polytetrafluoroethylene (PTFE) is commonly used in sliding bearings. Why? The choice of sliding surface of bearings is of vital importance because the sliding surfaces generate frictional forces which are exerted on the bearings and substructure of the bridge. For instance, PTFE and lubricated bronze are commonly choices of sliding surfaces for bearings. PTFE is a flurocarbon polymer which possesses good chemical resistance and can function in a wide range of temperature. The most important characteristic of this material is its low coefficient of friction. PTFE has the lowest coefficients of static and dynamic friction of any solid with absence of stick-slip movement (David J. Lee). The coefficient of friction is found to decrease with an increase in compressive stress. However, PTFE do have some demerits like high thermal expansion and low compressive strength. In designing the complementary contact plate with PTFE sliding surface, stainless steel plates are normally selected where the plates should be larger than PTFE surface to allow movement without exposing the PTFE. Moreover, it is recommended that the stainless steel surface be positioned on top of the PTFE surface to avoid contamination of dirt and rubbish. Lubricants are sometimes introduced to reduce the friction between the PTFE surface and
the upper stainless steel plate. Hence, the PTFE may be designed with dimples to avoid the lubricant from squeezing out under repeated translation movements.
24. Should raking piles of a bridge abutment be placed under an embankment?
For a bridge abutment to be supported on raking piles with different orientations, the movement between the ground and the pile group is difficult to predict. For instance, if some of the raking piles of the bridge abutment are extended beneath an embankment, then the settlement of embankment behind the abutment may cause the raking piles to experience severe bending moment and damage the piles as recommended by Dr. Edmund C Hambly (1979).
25. How do engineer determine the number of cells for concrete box girder bridges? If the depth of a box girder bridge exceeds 1/6 or 1/5 of the bridge width, then it is recommended to be designed as a single cell box girder bridge. However, if the bridge depth is smaller than 1/6 of the bridge width, then a twin-cell or multiple cell is a better choice as suggested by Jorg Schlaich & Hartmut Scheef (1982). However, one should note that even for wider bridges with small depths, the number of cells should be minimized because there is not much improvement in transverse load distribution when the number of cells of box girder is increased to three or more.
26. What is sucker deck principle for variable depth bridge decks?
For a variable depth bridge deck, the depth of continuous multi-span bridge deck is increased in pier supports and this absorbs sagging moments in the mid-span with the consequent increase in hogging moments in pier supports. As a result, the mid-span depth can be significantly reduced due to the reduction in sagging moment. In essence, this sucker deck principle is applied in locations where headroom requirement is of great concern. Moreover, in terms of structural performance, sucker decks are effective in reducing dead loads than voided slab of equivalent uniform depth for span length between 20-40m. In terms of aesthetics point of view, the public tends to appreciate the structural form of arches and curved soffit rather than boring uniform deck alignment. Reference is made to Brian Pritchard (1992).
1. What are the functions of different components of a typical expansion joint? In a typical expansion joint, it normally contains the following components: joint sealant, joint filler, dowel bar, PVC dowel sleeve, bond breaker tape and cradle bent. Joint sealant: it seals the joint width and prevents water and dirt from entering the joint and causing dowel bar corrosion and unexpected joint stress resulting from restrained movement.
Joint filler: it is compressible so that the joint can expand freely without constraint. Someone may doubt that even without its presence, the joint can still expand freely. In fact, its presence is necessary because it serves the purpose of space occupation such that even if dirt and rubbish are intruded in the joint, there is no space left for their accommodation. Dowel bar: This is a major component of the joint. It serves to guide the direction of movement of concrete expansion. Therefore, incorrect direction of placement of dowel bar will induce stresses in the joint during thermal expansion. On the other hand, it links the two adjacent structures by transferring loads across the joints. PVC dowel sleeve: It serves to facilitate the movement of dowel bar. On one side of the joint, the dowel bar is encased in concrete. On the other side, however, the PVC dowel sleeve is bonded directly to concrete so that movement of dowel bar can take place. One may notice that the detailing of normal expansion joints in Highways Standard Drawing is in such a way that part of PVC dowel sleeve is also extended to the other part of the joint where the dowel bar is directly adhered to concrete. In this case, it appears that this arrangement prevents the movement of joint. If this is the case, why should designers purposely put up such arrangement? In fact, the rationale behind this is to avoid water from getting into contact with dowel bar in case the joint sealant fails. As PVC is a flexible material, it only minutely hinders the movement of joint only under this design. Bond breaker tape: As the majority of joint sealant is applied in liquid form during construction, the bond breaker tape helps to prevent flowing of sealant liquid inside the joint .
Cradle bar: It helps to uphold the dowel bar in position during construction. 2. If on-site slump test fails, should engineers allow the contractor to continue the concreting works?
This is a very classical question raised by many graduate engineers. In fact, there are two schools of thought regarding this issue.
The first school of thought is rather straightforward: the contractor fails to comply with contractual requirements and therefore as per G. C. C. Clause 54 (2)(c) the engineer could order suspension of the Works. Under the conditions of G. C. C. Clause 54(2)(a) – (d), the contractor is not entitled to any claims of cost which is the main concern for most engineers. This is the contractual power given to the Engineer in case of any failure in tests required
by the contract, even though some engineers argue that slump tests are not as important as other tests like compression test.
The second school of thought is to let the contractor to continue their concreting works and later on request the contractor to prove that the finished works comply with other contractual requirements e.g. compression test. This is based upon the belief that workability is mainly required to achieve design concrete compression strength. In case the compression test also fails, the contractor should demolish and reconstruct the works accordingly. In fact, this is a rather passive way of treating construction works and is not recommended because of the following reasons:
(i) Workability of freshly placed concrete is related not only to strength but also to durability of concrete. Even if the future compression test passes, failing in slump test indicates that it may have adverse impact to durability of completed concrete structures.
(ii) In case the compression test fails, the contractor has to deploy extra time and resources to remove the work and reconstruct them once again and this slows down the progress of works significantly. Hence, in view of such likely probability of occurrence, why shouldn’t the Engineer exercise his power to stop the contractor and save these extra time and cost?
3. What is the function of shear keys in the design of retaining walls?
In determining the external stability of retaining walls, failure modes like bearing failure, sliding and overturning are normally considered in design. In considering the criterion of sliding, the sliding resistance of retaining walls is derived from the base friction between the wall base and the foundation soils. To increase the sliding resistance of retaining walls, other than providing a large self-weight or a large retained soil mass, shear keys are to be installed at the wall base. The principle of shear keys is as follows:
The main purpose of installation of shear keys is to increase the extra passive resistance developed by the height of shear keys. However, active pressure developed by shear keys also increases simultaneously. The success of shear keys lies in the fact that the increase of passive pressure exceeds the increase in active pressure, resulting in a net improvement of sliding resistance.
On the other hand, friction between the wall base and the foundation soils is normally about a fraction of the angle of internal resistance (i.e. about 0.8φ ) where φ is the angle of internal friction of foundation soil. When a shear key is installed at the base of the retaining wall, the failure surface is changed from the wall base/soil horizontal plane to a plane within foundation soil. Therefore, the friction angle mobilized in this case is φ instead of 0.8φ in the previous case and the sliding resistance can be enhanced. 4. In designing concrete structures, normally maximum aggregate sizes are adopted with ranges from 10mm to 20mm. Does an increase of maximum aggregate size benefit the structures?
To answer this question, let’s consider an example of a cube. The surface area to volume ratio of a cube is 6/b where b is the length of the cube. This implies that the surface area to volume ratio decreases with an increase in volume. Therefore, when the size of maximum aggregate is increased, the surface area to be wetted by water per unit volume is reduced. Consequently, the water requirement of the concrete mixes is reduced accordingly so that the water/cement ratio can be lowered, resulting in a rise in concrete strength. However, an increase of aggregate size is also accompanied by the effect of reduced contact areas and discontinuities created by these larger sized particles. In general, for maximum aggregate sizes below 40mm, the effect of lower water requirement can offset the disadvantages brought about by discontinuities as suggested by Longman Scientific and Technical (1987).
5. In concrete compression test, normally 150mmx150mmx150mm concrete cube samples is used for testing. Why isn’t 100mmx100mmx100mm concrete cube samples used in the test instead of 150mmx150mmx150mm concrete cube samples? Basically, the force supplied by a concrete compression machine is a definite value. For normal concrete strength application, say below 50MPa, the stress produced by a 150mmx150mmx150mm cube is sufficient for the machine to crush the concrete sample. However, if the designed concrete strength is 100MPa, under the same force (about 2,000kN) supplied by the machine, the stress under a 150mmx150mmx150mm cube is not sufficient to crush the concrete cube. Therefore, 100mmx100mmx100mm concrete cubes are used instead to increase the applied stress to crush the concrete cubes. For normal concrete strength, the cube size of 150mmx150mmx150mm is already sufficient for the crushing strength of the machine. 6. What are the major problems in using pumping for concreting works? In pumping operation, the force exerted by pumps must overcome the friction between concrete and the pumping pipes, the weight of concrete and the pressure head when placing concrete above the pumps. In fact, as only water is pumpable, it is the water in the concrete that transfers the pressure. The main problems associated with pumping are the effect of segregation and bleeding. To rectify these adverse effects, the proportion of cement is increased to enhance the cohesion in order to reduce segregation and bleeding. On the other hand, a proper selection of aggregate grading helps to improve the pumpability of concrete.
7. Is it desirable to use concrete of very high strength i.e. exceeding 60MPa? What are the potential problems associated with such high strength concrete?
To increase the strength of concrete, say from 40MPa to 80MPa, it definitely helps in improving the structural performance of the structure by producing a denser, more durable and higher load capacity concrete. The size of concrete members can be significantly reduced resulting in substantial cost savings. However, an increase of concrete strength is also accompanied by the occurrence of thermal cracking. With an increase in concrete strength, the cement content is increased and this leads to higher thermal strains. Consequently, additional reinforcement has to be introduced to control these additional cracks caused by the increase in concrete strength. Moreover, the ductility of concrete decreases with an increase in concrete strength. Attention should be paid during the design of high strength concrete to increase the ductility of concrete. In addition, fire resistance of high strength concrete is found to be less than normal strength concrete as suggested by Odd E. Gjorv (1994).
Though the tensile strength of high strength concrete is higher than that of normal concrete, the rate of increase of tensile strength is not proportional to the increase of compressive strength. For normal concrete, tensile strength is about one-tenth of compressive strength. However, for high strength concrete, it may only drop to 5% of compressive strength. Moreover, owing to a low aggregate content of high strength concrete, creep and shrinkage increases.
8. What are the disadvantages of curing by ponding and polythene sheets? The purpose of curing is to reduce the rate of heat loss of freshly placed concrete to the atmosphere and to minimize the temperature gradient across concrete cross section. Moreover, curing serves to reduce of the loss water from freshly placed concrete to the atmosphere.
Ponding: This method of thermal curing is readily affected by weather condition (cold wind). Moreover, a large amount of water used has to be disposed off the construction sites after curing.
Polythene sheet: This method of curing is based on the principle that there is no flow of air over the concrete surface and thereby no evaporation can take place on top of the freshly concreted surface by provision of polythene sheets. However, it suffers from the demerit that polythene sheets can be easily blown off in windy condition and the performance of curing would be affected. Moreover, for water lost due to self-desiccation, this method cannot replenish these losses.
9. Comparing the rate of “Formwork exceeding 300mm wide, horizontal or at any inclination up to and including 5 o to the horizontal” with the rate of “Formwork exceeding 300mm wide, at any inclination more than 85 o up to and including 90 o to the horizontal”, which one is higher? The item “Formwork exceeding 300mm wide, at any inclination more than 85 o up to and including 90 o to the horizontal” refers to formwork formed vertically and when compared with formwork erected in horizontal plane, the amount of falsework required is smaller. The item “Formwork exceeding 300mm wide, horizontal or at any inclination up to and including 5 o to the horizontal” refers to formwork to be erected in horizontal position and in general it requires much falsework to support this type of formwork. Therefore, the rate for this item is higher than the one mentioned in the above paragraph. 10. If concrete compression test fails, should Schmidt hammer test be adopted as an alternative test to prove the concrete strength? The Scmidt hammer test is based on the elastic rebound of hammer which presses on concrete surface and it measures the surface hardness of concrete. Since the test is very sensitive to the presence of aggregates and voids at the concrete surface, it is necessary to take more than 10 readings over the area of test. However, it should be noted that Schmidt hammer test measures surface hardness only but not the strength of concrete. Therefore, it may not be considered a good substitute for concrete compression test.
11. What is the indication of shear slump and collapse slump in slump tests?
There are three types of slump that may occur in slumps test, namely, true slump, shear slump and collapse slump. True slump refers to general drop of the concrete mass evenly all around without disintegration. Shear slump implies that the concrete mix is deficient in cohesion. Consequently, it may undergo segregation and bleeding and thus is undesirable for durability of concrete. Collapse slump indicates that concrete mix is too wet ad the mix is deemed to be harsh and lean.
12. In erection of falsework, for a rectangular panel inside a falsework should it be braced along the two diagonals?
When a rectangular panel is subject to an eccentric load or a lateral load, it tends to deform into a parallelogram with one diagonal shortening and the other elongating. Theoretically, it is sufficient to brace along one of the diagonals (the one in tension). If one diagonal is only allowed to brace inside the rectangular panel, it should be not braced in the diagonal in compression because under severe lateral loading the diagonal may buckle leading to failure of structure. However, in actual situation lateral loads may come from both sides of the panel and hence it should be braced in both diagonals.
13. In carrying out compression test for concrete, should test cubes or test cylinders be adopted?
Basically, the results of compression test carried out by using cubes are higher than that by cylinders. In compression test, the failure mode is in the form of tensile splitting induced by uniaxial compression. However, since the concrete samples tend to expand laterally under compression, the friction developed at the concrete-machine interface generates forces which apparently increase the compressive strength of concrete. However, when the ratio of height to width of sample increases, the effect of shear on compressive strength becomes smaller. This explains why the results of compression test by cylinders are lower than that of cubes. Reference is made to Longman Scientific and Technical (1987). 14. What is the function of rebate in a typical construction joint? Construction joints are created on sites to facilitate the construction process. However, if improperly constructed, the completed construction joints will leave an uneven scar on the concrete surface and affect significantly its appearance. To avoid this, a rebate is formed during the first pour of one side of construction joint. After the other pour is concreted, it will hide the uneven joint inside the rebate.
15. If a contractor proposes to increase concrete cover beyond contractual specification (i.e. 40mm to 70mm), shall engineers accept the proposal? In contractual aspect, based on the requirement of General Specification of Civil Engineering Works (1992 Edition), the tolerance of concrete cover is between +5mm and –5mm and engineers should not accept sub-standard work because they do not possess the authority to change the acceptance criteria. In case engineers consider contractor’s proposal acceptable in technical point of view, consent has to be sought from the employer regarding the changes in acceptance criteria. From technical point of view, the effect on cracking due to an increase in concrete cover should be considered. In general, there are three main parameters which govern crack width, namely tensile strain at the point considered, the distance of longitudinal bar to the concerned point and the depth of tension zone. For the second factor, i.e. proximity of longitudinal bars to point of section, the closer a bar is to this point, the smaller is the crack width. Therefore, closely spaced bars with smaller cover will give narrower cracks than widely spaced bars with larger cover. Therefore, with an increase of concrete cover, the crack width will increase which is undesirable.
16. Can grout replace concrete in normal structure? The mixture of cement and water alone cannot replace concrete (Longman Scientific and Technical (1987)) because: (i) Shrinkage of grout is several times that of concrete with the same mass. (ii) The effect of creep of grout is far more than that of concrete. (iii) Heat of hydration of cement with water is more than normal concrete and this leads to the problem of severe cracking.
17. Which type of bar reinforcement is more corrosion resistant, epoxy-coated bars, stainless steel bars or galvanized bars? Based on the experiment conducted by the Building Research Establishment, it was shown that the corrosion resistance of galvanized steel was the worst among the three types of bar reinforcement. For galvanized steel bars, corrosion started to occur when a certain chloride content in concrete (i.e. 0.4% by cement weight) was exceeded. However, for epoxy-coated bars, they extended the time taken for cracking to occur when compared with galvanized steel bars. The best corrosion resistant reinforcement among all is stainless steel. In particular, austenitic stainless steel stayed uncorroded even there was chloride contamination in concrete in the experiment. Reference is made to K. W. J. Treadaway (1988).
18. Can a concrete structure be completely free of expansion joints and contraction joints?
Consider that the concrete structure is not subject to the problem of differential settlement. For contraction joints, it may be possible to design a concrete structure without any contraction joints. By using sufficient steel reinforcement to spread evenly the crack width over the span length of the structure, it may achieve the requirement of minimum crack width and cause no adverse impact to the aesthetics of the structure. However, it follows that the amount of reinforcement required is higher than that when with sufficient contraction joints. For expansion joints, the consequence of not providing such joints may be difficult to cater for. For example, a concrete structure has the coefficient of thermal expansion of 9x10 -6 / o C and a Young’s modulus of 34.5kN/mm 2 . With an increase of temperature of 20 o C and it is restricted to free expansion, then the structure is subject to an axial stress of 6.21MPa. If the structure is very slender (e.g. concrete carriageway), buckling may occur. Therefore, the structure has to be designed to take up these thermal stresses if expansion joints are not provided. However, for water retaining structures, most of them are not affected by weather conditions because they are insulated from the water they contain internally and soil backfill that surround them. Therefore, it is expected that a smaller amount of thermal movement will occur when compared with normal exposed concrete structure. Consequently, expansion joints may be omitted in this case with the view that the compressive stress induced by thermal expansion toughens the structure to limit the development of tensile stress.
19. Does the presence of rust have adverse impact to the bond performance of bar reinforcement?
In fact, the presence of rust in bars may not have adverse impact to the bond performance and it depends on the types of bar reinforcement under consideration. For plain round bars, the rust on bars improves the bond performance by the formation of rough surfaces which increases the friction between steel and concrete. However, for deformed bars, the same theory cannot apply. The presence of rust impairs the bond strength because corrosion occurs at the raised ribs and subsequently fills the gap between ribs, thus evening out the original deformed shape. In essence, the bond between concrete and deformed bars originates from the mechanical lock between the raised ribs and concrete. On the contrary, the bond between concrete and plain round bars derives from the adhesion and interface friction. With such differences in mechanism in bonding, the behaviour of bond between deformed bars and plain round bars in the presence of rust varies. Reference is made to CIRIA Report 147.
20. General Specification for Civil Engineering Works (1992 Edition) Clause 15.09 specifies that tying wires for reinforcement adjacent to and above Class F4 and F5 finishes should be stainless steel wires. Why?
If plain steel tying wires are used for reinforcement adjacent to Class F4 and F5 finishes, it poses the problem of rust staining which may impair the appearance of exposed concrete surfaces. The rate of corrosion of plain steel tying wires is similar to normal steel reinforcement. However, for tying wires with very small diameter, upon long exposure it stands a high chance of rusting completely and these rust will stain the formwork and significantly affect the concrete finish. Therefore, stainless steel tying wires are specified for locations in the vicinity of high quality of finishes to avoid rust staining by corroded typing wires. Note: Tying wires are wires used for fixing and connecting steel reinforcement bars.
21. For long slender structures like beams, propping is required after removal of formwork. Why?
After concreting, the time at which striking of formworks should not be too long, otherwise it would affect the colour of concreted structures. For long span concrete structures, when they have attained sufficient strength to support their self-weight, creep deflection may occur in these structures if propping is not provided after the removal of formwork. Therefore, re-propping is carried out after removing formwork and these props should not be allowed to stand too long because creep loads may overstress them. Note: Propping refers to provision of falsework to support slabs and beams during their gain in concrete strength after concreting.
22. What is the difference in application between open stirrups and closed stirrups in concrete beams?
Open stirrups are provided principally to resist shear forces in concrete beams and they are applied in locations in which the effect of torsion is insignificant. U-shaped stirrups are placed in the tension side of concrete beams in which shear cracks would occur. However, when concrete beams are designed to resist a substantial amount of torsion, closed stirrups should be used instead.
23. For column reinforcements, why is helical reinforcement sometimes designed instead of normal links?
The use of links for column design in Britain is very popular. However, in U.S.A. engineers tend to use helical reinforcement instead of normal links because helical reinforcement has the potential advantage of protecting columns/piles against seismic loads. Moreover, when the columns reach the failure state, the concrete outside hoops cracks and falls off firstly, followed by the eventual failure of the whole columns. The peeling off of concrete outside helical reinforcement provides a warning signal before the sudden failure of columns as suggested by G. P. Manning (1924). In addition, it can take up a higher working load than normal link reinforcement. For instance, helical reinforcement is adopted in the design of marine piles in Government piers. Note: Helical reinforcement refers to shear reinforcement which is spiral in shapes.
24. What is the difference between epoxy grout, cement grout and cement mortar?
Epoxy grout consists of epoxy resin, epoxy hardener and sand/aggregates. In fact, there are various types of resin used in construction industry like epoxy, polyester, polyurethane etc. Though epoxy grout appears to imply the presence of cement material by its name, it does not contain any cement at all. On the other hand, epoxy hardener serves to initiate the hardening process of epoxy grout. It is commonly used for repairing hairline cracks and cavities in concrete structures and can be adopted as primer or bonding agent. Cement grout is formed by mixing cement powder with water in which the ratio of cement of water is more or less similar to that of concrete. Setting and hardening are the important processes which affect the performance of cement grout. Moreover, the presence of excessive voids would also affect the strength, stiffness and permeability of grout. It is versatile in application of filling voids and gaps in structures. Cement mortar is normally a mixture of cement, water and sand. They are used as bedding for concrete kerbs in roadwork.
25. What is the purpose of skin reinforcement for deep beams?
In BS8110, it states that secondary reinforcement should be provided for beams exceeding 950mm deep at a distance measured 2/3 depth from the tension face. Experimental works revealed that at or close to mid-depth of deep beams, the maximum width of cracks arising from flexure may be about two to three times larger than the width of the same crack at the level of surface where the crack originally forms.
The presence of crack is undesirable from aesthetic point of view. Moreover, it poses potential corrosion problems to reinforcement of deep beams. To safeguard against these crack formation, skin reinforcement is designed on the sides of deep beams to limit the formation of flexural crack widths. Though the principal function of skin reinforcement is to control crack width, it may be employed for providing bending resistance of the section.
24 Chapter 3 – Drainage Works 1. What is the function of waterstops in joints of box culverts and drainage channels? The principal function of waterstops is to prevent liquids (e.g. water), water-borne materials and solids to pass through concrete joints. In essence, it aims at providing watertightness to the drainage channel. Besides, waterstops in drainage channels or box culverts can also serve two other purposes: (i) to avoid water contacting joints’ dowel bars and causing corrosion. (ii) to avoid water seeping in from the underside of drainage channels or box culverts, thereby washing in soil particles and causing voids underneath these structures and finally leading to their failure. To serve the second purpose, obviously only one waterstop is required at any depth location. To serve the first purpose, a waterstop has to be installed on top of dowel bars to prevent water from drainage channels from leaking through. On the other hand, a waterstop has to be provided below dowel bars to avoid underground water from surging upwards. In fact, the other way out to serve the first purpose is by using corrosion resistant bars. 2. What are the differences in applications between pipe culverts and box culverts? Basically, a culvert means a covered hydraulic structure which conveys fluid. Therefore in a broad sense, pipe culverts in a small scale represent normal pipes like precast concrete pipes. In terms of hydraulic performance, circular section is the best geometrical sections among all. Therefore, for relative small discharge, precast concrete pipes and ductile iron pipes are normally used which are circular in shape. But for applications of very large flow, precast concrete pipes and ductile iron pipes may not be available in current market. In this connection, cast-in-situ construction has to be employed. It is beyond doubt that the fabrication of formwork for circular shape is difficult when compared with normal box culvert structures. However, circular shape is the most hydraulic efficient structure which means for a given discharge, the area of flow is minimum. Therefore, it helps to save the cost of extra linings required for the choice of box culverts. However, box culverts do possess some advantages. For example, they can cope with large flow situation where headroom is limited because the height of box culverts can be reduced while the size of pipe culverts is fixed. Secondly, for some difficult site conditions, e.g. excavation of structure in rock, for the same equivalent cross-sectional area, the width of box culverts can be designed to be smaller than that of pipe culverts and this enhances smaller amount of excavation and backfilling.
3.1 Small spatial requirement of box culver than pipes.
3. What are the limitations of Rational Method in calculating runoff?
Computation of runoff is a complicated matter which depends on many factors like the ground permeability, rainfall duration, rainfall pattern, catchment area characteristics etc. Basically, Rational Method is a means to find out the maximum discharge suitable for design purpose. In this method, it is assumed that the rainfall duration is the same as the time of concentration and the return period of rainfall intensity is the same as the peak runoff. Time of concentration refers to the time required for the most remote location of stormwater inside the catchment to flow to the outlet. When the time of concentration is equal to the rainfall period, the maximum discharge occurs and rainfall collected inside the catchment comes to the same outlet point.
Rational Method provides the peak discharge only and it cannot produce a hydrograph. If a more detailed pattern of runoff is required, unit hydrograph or other methods have to be used. The accuracy of rational method depends very much on our correct selection of runoff coefficient and delineation of catchment area.
Rational Method is a rather conservative method. One of the basic assumptions of the rational formula is that the rainfall intensity must be constant for an interval at least equal to the time of concentration. For long duration of rainfall, this assumption may not hold true. Moreover, the runoff coefficient in Rational Method is difficult to be determined accurately and it depends on many factors like moisture condition of soils, rainfall intensity and duration, degree of soil compaction, vegetation etc. In addition, In Rational Method the runoff coefficient is independent of rainfall intensity and this does not reflect the actual situation.
4.Why are some manhole covers made of cast iron while some are made of ductile iron?
Traditionally, manholes covers are made of cast iron. However, in the viewpoint of pipe maintenance, frequent opening of manhole covers has to be carried out. Therefore, it poses potential safety hazard to the workers during the lifting-up process of manhole covers because cast iron manhole covers are very heavy to normal workers. Consequently, research has been conducted and ductile iron is considered as a better choice than cast iron because it can resist the same traffic loads with lower self-weight. Moreover, as ductile iron is less brittle than cast iron, the traditional cast iron manhole covers are more susceptible to damage and thus requires higher maintenance cost.
However, ductile iron manhole covers do suffer from some demerits. For instance, owing to their relative low self-weight, vehicles passing over these manhole covers would lead to the movement of covers and generate unpleasant noises. To solve this problem, instead of increasing the self-weight of ductile iron manhole covers which similarly causes safety problems to workers during regular maintenance, the covers can be designed to be attached to the manhole frames which hold them in firm position.
5. Why is it preferable to design stormwater drains to match soffit?
Stormwater drains collect stormwater in their corresponding catchment areas during rainstorm and convey the collected water through outlets to the sea. Therefore, in considering the hydraulic design of stormwater drains, other than normal drainage pipe capacity to be taken into consideration, one should check the backwater effect due to tidal condition at outlets if the drains are located quite close to the downstream end of outlets. Stormwater drains are normally designed to match soffit to avoid surcharging by backwater effect or when the downstream pipes are running full. Normally pipe size increases from upstream to downstream. For the case of matching drain invert, when outlet pipes are fully surcharged by tidal effect of the sea or when the downstream pipes are fully filled with stormwater, pipe sections immediately upstream of the outlet are also surcharged too. However, for the case of matching pipe soffit, the immediate upstream sections of outlet pipes are not totally surcharged even though downstream pipes are running full. However, it is not always practical to maintain soffit for all pipelines because it requires sufficient drop to achieve this.
Moreover, the flow of stormwater is mainly by gravity in the design of stormwater drains. In case the drains are designed to match invert, then it stands a high probability that the flow in the upstream smaller pipes has to be discharged against a head. Note: Matching soffit means that all pipelines are aligned continuously with respect to the pipelines’ crown level. inverted siphons?
Inverted siphons are designed at locations in which a sewer system is blocked by underground utilities or stormwater drains. They are sometimes called depressed sewers because it is claimed that there is no actual siphon action. They connect the upstream and downstream sewers with U-shaped vertical alignment such that they are always running full.
The drawbacks of inverted siphons are: (i) They induce additional head loss to the sewer system which is undesirable in hydraulic performance; (ii) U-shaped siphons create sediment accumulation problem and previous experience showed that inverted siphons were easily blocked due to siltation; (iii) Maintenance of invert siphons is difficult due to its inaccessibility.
7. What is the mechanism of cavitation in pipes and drains?
Cavitation refers to the formation of air bubbles in fluid in low-pressure condition which is lower than the saturation pressure. It is a potentially damaging condition in which the fluid in pipes or sewers is at high velocities. By Bernoulli’s Equation, at high flow velocities, the pressure head of fluid is reduced accordingly. As the fluid pressure is less than saturation pressure, dissolved gases are released from the fluid and these air bubbles will suddenly collapse when the flow enters into a region of higher pressure. This produces a high dynamic pressure which causes damage to the pipelines due to its high frequency.
8. When a drainage system (i.e. u-channels with catchpits) is connected to a main drainage channel, a segment of short pipe is used. What is the reason of such arrangement?
There are three scenarios of such connection arrangement: (a) a new drainage system is connected to an existing drainage channel (b) an existing drainage system is connected to a new drainage channel (c) a new drainage system is connected to a new drainage channel. For all scenarios, what engineers consider is the total amount of differential settlement or lateral movement to be encountered between the drainage system and main drainage channel. For scenario (b) and (c), it is very likely that substantial differential settlement will occur and this will cause damage to the connecting concrete pipes. Therefore a segment of short pipes are designed so that they serve to provide flexibility to the pipes in case of uneven settlement occurring between drainage system and main drainage channels.
9. What are the functions of bedding under stormwater drains? Bedding, which are normally made of granular materials or concrete, serves four main functions as suggested by O. C. Young and J. J. Trott: (i) To enhance a uniform support under pipes in order to reduce the bending moment longitudinally; (ii) To increase the load-supporting strength of the pipes; (iii) For pipes with spigot and socket joints, it enables pipes to be supported along pipe lengths instead of pipe sockets. Otherwise, uneven stress may be induced and it may damage the pipes; (iv) To provide a platform for achieving correct alignment and level during and after construction.
10. In designing sewer pipes, why are vitrified clay pipes commonly used for pipe size less than 600mm while precast concrete pipes with PVC lining is used for pipe size exceeding 600mm?
The market price of vitrified clay pipes is generally less than that of precast concrete pipes with PVC lining. Therefore, for small size of pipes (pipe diameter less than 600mm) it is more economical to use vitrified clay pipes. However, vitrified clay pipes do suffer from the problem of brittleness and its effect is even severe for larger size of pipes. Moreover, it is rather time consuming to deliver clay pipes products because the majority of them are manufactured in Europe. Hence, for larger size of sewer pipes (diameter more than 600mm) it is customary to use precast concrete pipes with PVC lining.
11. Nowadays, most flap valves are made of HDPE. What are the advantages of using HDPE when compared with cast iron? (i) It has no reaction with sewage and seawater and does not suffer from the corrosion problem associated with cast iron. (ii) No protective coating is required and it is almost maintenance-free. (iii) HDPE flap valves require very low opening pressure in operation (like 5mm water level difference). For cast iron flap valves, due to its own heavy self-weight, the required opening pressure of cast iron flap valves is higher than that of HDPE flap valves. This criterion is essential for dry weather flow conditions. However, the pressure resistance of HDPE flap valves is not as good as cast iron flap valves. For instance, a typical 450mm wide HDPE flap valve can only withstand about 5m water column.
12. What are the functions of the following features observed in a typical manhole? (i) groove near benching, (ii) R.S.J. (iii) double seal manhole cover and (iv) u-trap with rodding arm. (i) The groove is used to facilitate the maintenance of manholes and sewer/drain pipes. Shutoff boards are erected on the grooves during maintenance operation so that water flow coming from upstream is terminated in the manhole and backwater from downstream is also blocked. In addition, the groove also facilitates water flow diversion for routine maintenance operation.
(ii) R.S.J. is a small-scale size of universal beams and is used for resisting the high stresses incurred by heavy traffic loads acting directly on the upper narrow projected section of manholes.
(iii) Double seal terminal manhole covers are used for sealing off gases emitted inside sewer/drains and prevent them from releasing out of the manhole.
(iv) U-trap with rodding arms is also used for sealing off unpleasant gas smell by the trapped u-shaped water columns. Rodding arm is normally closed with rubber rings during normal operation. However, during maintenance operation, the rubber ring is removed and rodding can be carried out through the rodding arm.
13. The spacing of manholes in straight sections for different pipe sizes is stated in Stormwater Drainage Manual. How are these figures arrived at? For pipe size < 300mm, rodding is usually adopted in which workers place about 1m long rods through the pipes to the location of blockage and manually operate the rod to clear the blockage. For pipe size < 700mm, water-jetting is normally employed in which water is supplied from nearby fire hydrants and pressurized water jet is used for clearing blockage. Winching method is adopted for all sizes of pipes. For instance, for pipe size exceeding 1050mm, it is stated in Stormwater Drainage Manual that maximum intervals between manholes along straight lengths should be 120m. This is because for sizes over 1050m, the main method of pipe maintenance is by winching whose maximum length of operation is 120m. Similarly, the maximum intervals of manholes for other straight pipes are derived from their corresponding maintenance methods.
14. In selection of dams in drainage channels, what are the advantages of using rubber dams instead of steel-gate dams?
The advantages of rubber dams are as follows: (i) Since rubber is flexible in nature it is capable of performing deflation even in the presence of dirt and sedimentation on the downstream side. However, for rigid steel-gate dams, it may not be possible to open when there is excessive sediment.
(ii) Since the foundation of rubber dams is comparatively lighter than that of steel-gate dams, it saves both construction cost and time. (iii) Rubber dams can be designed with longer spans without piers while steel-gate dams require intermediate piers for long spans.
15. Should air test or water tests be selected to test the leakage of constructed gravity pipelines?
For gravity pipes, air tests or water tests are carried out after completion of laying and jointing of the pipes. These tests are conducted to check the watertightness of joints and to ensure the pipelines are free from damage where leakage may occur. Air test has the advantage that the test itself is simple and faster to be carried out. It does not require the disposal of significant quantities of water used in the test which is a mandatory requirement for water test. However, in case leakage exists in the constructed segment of gravity pipelines, the position of leakage can hardly be located in air test. Moreover, the rate of water leakage cannot be determined from air tests. In addition, air test is readily affected by atmospheric condition because air has a relatively high coefficient of thermal expansion. The test is also influenced by the moisture condition of the test pipelines because it affects the passage of air through the pipelines. For water test, though it is comparatively slow, it can detect the location of water leakage. However, the leakage rate results from water test may not truly reflect its actual leakage because pipeline materials like concrete and clay are porous and would absorb water during the test.
16. In designing of access ramps for drainage channels, why should the direction of access ramps be sloping down towards downstream? In the design of access ramps, the direction is normally specified to be sloping down towards downstream so as to avoid the occurrence of over-shooting of flowing water for supercritical flow in case of aligning the ramps in the reverse direction of channel flow. Note: Access ramps refer to ramps used for maintenance vehicles during routine maintenance of channels.
17. When branch pipelines are connected to main pipelines, sometimes Y-junctions or fitting branched pipelines to main pipelines by formation of holes in main pipelines are used. Which one is a better choice?
By using standard precast units of Y-junction branch pipelines, it is beyond doubt that joints between branched pipelines and main pipelines are properly formed and the quality of joints is relatively less dependent on workmanship. However, it suffers from the problem that with fixed precast units of Y-junctions, sometimes it may be difficult for contractors to determine the precise orientation of specific angles of Y-junctions with respect to gullies. (e.g. gullies are connected through side branches to carrier drains) By forming elliptical holes in main pipelines and fitting the side branches into them with cement mortar, the quality of pipe joints is highly dependent on workmanship. It is commonly found that in subsequent CCTV inspections side branches are projected inside main pipes. This is undesirable because the projected side branches reduce the cross sectional area of main pipes locally and affect their hydraulic performance. Moreover, the projected side pipes may trap rubbish and dirt in the vicinity. On the other hand, cement mortar may not be properly applied at connection joints because these areas are hidden from view and are difficult to be inspected by engineers. Therefore, in selecting between the two available methods, engineers should make their own judgments based on the above considerations.
Fig. 3.4 Y-junction in drainage pipes.
18. Rational Method should not be used for large catchments in estimating peak runoff. Is it true?
Rational Method is suitable for small catchments only because the time of concentration of small catchments is small. In Rational Method the peak runoff is calculated based on the assumption that the time of concentration is equal to the rainfall duration. For small catchments, this assumption may hold true in most circumstances. One of the assumptions of Rational Method is that rainfall intensity over the entire catchment remains constant during the storm duration. However, in case of a large catchment it stands a high probability that rainfall intensity varies in various part of the large catchment. In addition, for long duration of rainfall, it is rare that the rainfall intensity remains constant over the entire rainstorm and a shorter duration but a more intense rainfall could produce a higher peak runoff. Moreover, a reduction of peak runoff is also brought about by the temporary storage of stormwater like channels within the catchment.
In actual condition, the runoff rate within the catchment varies from place to place because of different soil properties and past conditions. As suggested by Bureau of Public Roads (1965), sometimes the peak discharge occurs before all of the drainage area is contributing. For instance, when a significant portion of drainage area within the catchment has very small time of concentration so that a higher rainfall intensity can be used for this portion, the runoff coming solely from this portion is higher than that of the whole catchment in which a lower rainfall intensity is adopted because the remaining part of the catchment has comparatively large time of concentration. Therefore, this results in incorrect estimation of peak runoff of large catchments if Rational Method is adopted.
19. What is the purpose of carrying out water absorption test for precast concrete pipes?
Cement will mix with more water than is required to eventually combine during hydration of cement paste. As such, some voids will be left behind after the hydration process which affects the strength and durability of concrete. With the presence of air voids in concrete, it is vulnerable to penetration and attack by aggressive chemicals. Good quality concrete is characterized by having minimal voids left by excess water and therefore, water absorption test for precast concrete pipes is adopted for checking the quality of concrete in terms of density and imperviousness.
Chapter 4 – Earthworks 1. In soil compaction test, if a test result exceeds 100%, should engineers accept the result?
Soil compaction is the process of increasing the soil density by reducing the volume of air within the soil mass. Soil compaction depends mainly on the degree of compaction and the amount of water present for lubrication. Normally 2.5kg rammers and 4.5kg rammers are available for compaction in laboratories and the maximum dry densities produced by these rammers cover the range of dry density obtained by in-situ compaction plant. Regarding the second factor of water content, it affects the compaction in the following ways. In low water content, the soils are difficult to be compacted. When water content is increased gradually, water will lubricate the soils and this facilitates the compaction operation. However, at high water content, as an increasing proportion of soils is occupied by water, the dry density decreases with an increase in water content. For soil compaction tests, the dry density obtained from compaction carried out in-situ by vibrating roller/vibrating plate is compared with the maximum dry density conducted in laboratories using 2.5kg rammer of compaction with similar soils. In essence, the in-situ compaction is compared with the compacting effort of using 2.5kg (or 4.5kg) rammer in laboratories. In case the compaction test results indicate values exceeding 100%, it only means that the in-situ compaction is more than that being carried out in laboratories which is treated as the basic criterion for satisfactory degree of soil compaction. Therefore, the soil results are acceptable in case compaction test results are over 100%. However, excessive compaction poses a risk of fracturing granular soils resulting in the reduction of soil strength parameters.
2. What are the different applications of draglines, backhoes and shovels? An excavator is defined as a power-operated digging machine and it includes different types like shovels, draglines, clamshells, backhoes, etc. A dragline possesses a long jib for digging and dumping and it is used for digging from grade line to great depths below ground. Its characteristic is that it does not possess positive digging action and lateral control of normal excavators. A dragline is normally deployed for bulk excavation.
A backhoe is designed primarily for excavation below ground and it is especially employed for trench excavation works. It digs by forcing the bucket into soils and pulling it towards the machine and it possesses the positive digging action and accurate lateral control. A shovel is a machine that acts like a man’s digging action with a hand shovel and hence it is called a shovel. It digs by putting the bucket at the toe of excavation and pulling it up. Though a shovel has limited ability to dig below ground level, it is very efficient in digging above ground like digging an embankment.
3. For compaction of free-draining sands or gravels, what is the optimum moisture content to achieve maximum density?
The compaction curve of sandy materials is totally different from that of clayey materials. For sands or gravels, there are two situations of maximum density, namely the completely dry condition and the complete water saturation. For moisture content of sands and gravels between these two states, the dry density obtained is lower than that obtained in the above-mentioned states. The presence of capillary forces account for the difficulty of compaction sand at water contents between virtually dry and saturated state. They are formed in partially filled water void between soil particles and perform as elastic ties cementing soil particles together. Reference is made to Lars Forssblad (1981). The compaction curve for clay is suitable for the majority of soil types except sands and gravels because a small amount of clay in soils is sufficient to make the soils impermeable.
4. Is it worthwhile to carry out tests on particle density of soil particles for geotechnical design?
Particle density of soils is defined by the ratio of soil particle mass and soil particle volume. Depending on soil types, the range of variation of soil particle density varies not significantly, i.e. by 4%. Therefore, it may not be worthwhile to order laboratory tests and incur additional expenditure just to determine the particles density by recognizing that the variation of particles density is not significant.
5. In determining the effective stress parameters of a soil sample, which test is preferable, consolidated undrained test or consolidated drained test?
The effective stress parameters of a soil sample can be obtained from both consolidated undrained test and consolidated drained test. However, consolidated undrained test is normally selected because of the following reasons:
(i) Time taken for consolidated undrained test is shorter than that of consolidated drained test. It is because consolidated drained test requires the full dissipation of excess pore water pressure of the soil during testing and it takes long time when soils of low permeability are tested.
(ii) Useful information can be obtained from the stress path of consolidated undrained test. (iii)Failure occurs in lower stress level when compared with consolidated drained test.
6. In conducting triaxial test to determine shear strengths for soil samples, what consolidation pressures should be specified?
It appears that the selection of consolidation pressure is independent of in-situ soil stress theoretically. However, this may not be correct because the actual shear strength envelopes for soils are non-linear over a wide range of stresses. Therefore, consolidation pressure corresponding to the range of stresses relevant to site condition should be adopted.
7. What is the relation of bearing pressure on soil nail head to the ratio L a /L b , where L a is the length of soil nail before the potential slip circle while L b is the length of soil nail beyond the potential slip circle? The unstable soil mass before the potential circular slip is resisted by two components: soil nail head bearing pressure and friction of soil nail in the unstable soil mass. Therefore, the longer is the length of soil nail before potential slip circle L a , the higher is the proportion of forces being resisted by frictional forces and hence the smaller amount is to be resisted by soil nail head. Hence, the smaller the ratio L a /L b , the greater is the resistance provided by soil nail head. Fig. 4.1 L a & L b in slopes. 8. Soil nails are mainly designed for stabilization of major slips. How should designer cater for the stability of minor slips? There are some methods to treat minor slips: (i) Adoption of smaller diameter size bars at closer spacing; (ii) Installation of tie beams at the same horizontal levels; (iii) Provision of steel wire meshes in-between soil nails; and (iv) Provision of short soil nails in combination of long soil nails. 9. What are the main reasons for conducting pull-out tests for soil nails? There are mainly four reasons for this test: (i) To check and verify the bond strength between soil and grout adopted during the design of soil nails. This is the main objective of conducting soil nail pull-out test. (ii) To determine the bond strength between soil and grout for future design purpose. However, if this target is to be achieved, the test nails should be loaded to determine the ultimate soil/grout bond with a upper limit of 80% of ultimate tensile strength of steel bars. (iii) To check if there is any slippage or creep occurrence. (iv) To check the elastic and plastic deformations of test nails. This is observed during the repeated loading and unloading cycles of soil nails. Note: Pull-out tests are carried out by applying specified forces in an attempt to pull out the constructed soil nails.
10. Are there any differences in the methods of compaction between clayey soil material and sandy material?
As suggested by Lars Forssblad (1981), the three main actions of compaction are static pressure, impact force and vibration. Different compactors contain one or more modes of these actions. For example, vibratory tampers perform mainly by the principle of impact while vibratory rollers work with principle of static pressure and vibration. For sandy soils, vibration is adequate for normal compaction because the action of vibration sets the soil particles in motion and friction forces between soil particles are virtually demolished. During this vibration motion, the soil particles rearrange themselves to develop a dense state.
For normal soils, it is necessary to combine the action of vibration together with static pressure to breakdown the cohesion forces between soil particles in order to allow for better compaction. The static pressure of vibratory machines is adopted to exert a shearing force to eliminate the cohesion in clayey soils.
Chapter 5 – Piers and Marine Structures
1. Why is sulphate-resisting cement not used in marine concrete?
The main components of Portland cement are tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite. In sulphate-resisting cement, it contains a low amount of tricalcium aluminate in order to avoid sulphate attack. Otherwise, tricalcium aluminate would react with sulphates to form calcium sulphoaluminate and gypsum that cause expansion and crack the concrete structure. However, for marine concrete sulphate-resisting cement should not be used because tricalcium aluminate has high affinity for chloride ions. This is based on the possible reaction of chloride ions and tricalcium aluminate to form calcium chloroaluminate hydrate as suggested by P. Kumar Mehta (1991) and the reduction of which may increase the rate of chloride attack to the concrete marine structure and result in faster corrosion of steel reinforcement in marine structures.
2. Shall a layer of wearing course or additional thickness be designed on the surface of piers?
In the design of piers, consideration should be given to the effect of wearing action caused by passengers, other traffics and even sometimes vehicles. In maritime environment, the durability and integrity of concrete is detrimental to the servicing life of piers because it acts an essential barrier to chloride attack. However, in view of these gradual wear and tear generated by the loading traffic, some forms of surface protection should be provided on top of pier surface like wearing course or additional increase in concrete cover.
3.What is the difference in application of surface-protecting fenders and energy-absorbing fenders?
Surface-protecting fenders are fenders that induce high reaction forces to berthing structures for the energy absorbed while energy-absorbing fenders are fenders which transmit low impact to berthing structures for the energy absorbed (Carl A. Thoresen (1988)). In fact, the principal function of fenders is to absorb the berthing energy and transmit a force to the structures without damaging them. Therefore, in open berth structures, it is desirable to use energy-absorbing fenders to reduce the loads acting on the relatively flexible structures. On the other hand, for solid berth structures the usage of surface-protecting fenders is adequate because they are capable of taking up large berthing loads.
4. Should dolphins be designed in a rigid manner, i.e. resting on several raking piles? In designing dolphins, they are normally supported on a system of three to four raking piles. This in essence is a rigid structure and exhibits little flexibility e.g. movement against impact and berthing loads by vessels. In fact, this kind of design may not be desirable in terms of maintenance because the dolphins are readily susceptible to damage by high berthing vessels. To rectify this situation, some energy absorption devices like rubber/plastic fenders have to be installed to reduce the impact load deriving from its own deflection. On the other hand, by designing dolphins as flexible structures capable for allowing slight deflection, it helps to reduce the large forces generated during berthing of vessels. In this connection, one way of designing dolphins as flexible structures is by provision of a single pile only. Note: For a rigid structure, it takes up external loads without undergoing excessive deformations.
5. In connecting fenders to pier structures, should single lock nuts or double lock nuts be used?
In many pier structures the connection of fenders to piers is achieved by using single lock nuts. However, they do not perform well because some timber fenders loosen more easily when subject to vibrating loads due to berthing, wave and tidal actions. To solve this problem, double lock nuts should be adopted as they prove to function satisfactory in other structural elements which are subject to frequent vibration loads. Note: Double lock nuts mean two nuts are adopted in a single bolt connection between fenders and marine structures.
6. What is the design level of landings in piers?
Landings are designed as resting place for passengers during berthing and deberthing of vessels. In general, landings are provided near mean high and mean low water levels to facilitate embarking and disembarking of passengers (BS6349: Part 2: 1988). Therefore, the level of landing steps should be different from place to place because of different mean high and mean low water levels in different locations.
7. What are the pros and cons of using timber fenders, plastic fenders and rubber fenders?
Timber fenders: They are low in strength and are subject to rotting and marine borer attack. Moreover, they have low energy absorption capacity and the berthing reaction depends on the point of contact. The contact pressure between fender and vessels are high. They are considered to be environmentally unfriendly because they consume tropical hardwoods in their production. Plastic fenders: Their strength is similar to that of timber fenders but they have relatively high abrasive resistance. They are resistant to chemical and biological attack. Their energy absorption capacities are moderate and the berthing reactions are also dependent on the point of contact. The reaction is lower when compared with timber fenders for a given energy absorption. They are considered to be environmental friendly because they are manufactured from recycled material. Rubber fenders: They possess high abrasive resistance and are also resistant to most biological and chemical attacks. They have moderate to high energy absorption capacity and the energy absorption performance is independent of the point of contact. Similar to plastic fenders, they are also environmental friendly products.
8. Why are most marine piles circular in cross section?
For marine piles, there are several options available for selection, namely H-piles, circular pipes and box piles.
However, only circular piles and box piles are suitable for marine application because of the following two reasons suggested by G. M. Cornfield (1968): (i) Circular piles and box piles possess high column buckling strength. For marine structures like jetties, piles are well above seabed level and therefore the column buckling effect is significant when compared with other structures. Therefore, it is essential to use pile sections which have relatively high buckling strength in piers. (ii) Circular piles and box piles display high energy absorbing capability. For marine structures like dolphins and fenders, which require substantial amount of berthing energy to be absorbed, these piles sections are inevitably good choices. In marine structures, it appears that circular sections prevail over the box sections. The main reason is that the range of section available for selection of circular piles is more than that of box piles.
9. For marine pile type of steel tubular piles with reinforced concrete infill, minimum toe level is often specified in contract drawings.
What is its purpose?
The purpose of minimum toe level is two-fold: (i) In detailed design stage, ground investigation should be conducted and the approximate level of rockhead is known. Therefore, to avoid the marine piles to be founded prematurely on boulders, minimum toe levels of marine driven piles are specified in contract. (ii) It provides sufficient length of soils for lateral and uplift resistance. Note: Minimum toe level refers to the minimum level that a marine driven pile should be driven into seabed.
10. What is the problem in traditional marine piling system of steel tubular pile with concrete infill and what are the possible remedial measures?
In the design of marine piles of steel tubular piles with concrete infill, loads from pier deck are taken up by steel tubular piles before the occurrence of corrosion of steel piles above seabed. In fact, it is assumed that steel piles above seabed level will all be corroded after a certain year. The load transfer mechanism after complete corrosion of steel pile above seabed is as follows: loads from pier deck are taken up by concrete infill above the seabed level. Below the seabed level, loads would be transferred to steel piles through frictional forces between concrete infill and steel casings.
However, substantial radial shrinkage and contraction occurs after concreting of concrete infill and this will hinder the load transfer from the concrete infill to steel piles because the bond may be ruptured by radial shrinkage. It is in doubt if frictional forces can be properly developed in this situation. To solve this problem, shear keys could be installed at regular spacing inside steel piles to ensure their rigid connection with concrete infill. Alternatively, expanding agents may be adopted in concrete mixes to ensure that there is no shrinkage after the concreting process.
11. Why are steel tubular marine piles often driven open-ended? In marine structures where piles are constantly subject to significant lateral and uplift forces induced by berthing operation and wave action, it is necessary to drive the piles to much greater depth. To avoid premature refusal so that insufficient soil cover may develop which is incapable of providing the required lateral and uplift resistance, tubular piles are normally driven open-ended so that they are driving to greater depths than piles with closed ends.
Fig. 5.1 Typical details of marine piles.
12. For typical pile bents in marine piers, how is vertical loads related to horizontal capacity of the pile bents?
Let’s consider a pile bent with a top slab supported by two ranking piles, each inclining at an equal angle to the pier slab. In designing such a system, truss action is normally adopted to analyze the pile bent. When the reaction forces of these piles, horizontal forces (e.g. due to berthing and deberthing of vessels) and vertical forces (e.g. superimposed deck loads) are analyzed by drawing a force polygon, it is noted that lateral resistance of the pile bent is dependent on the vertical load, i.e. lateral resistance is small when vertical loads are high.
13. For underwater concreting, tremie pipes are normally used with the aid of hoppers. Sometimes tubes are inserted inside the hoppers. Why?
In placing concrete by tremie pipes, hoppers are connected to their top for receiving freshly placed concrete. However, air may be trapped inside the tremie pipes if there is rapid feeding of fresh concrete. To release the trapped air inside the tremie pipes, hoses (called ventilation tubes) are inserted and lowered down through the hoppers. Reference is made to Carl A. Thoresen (1988).
14. Why is shallow bedrock condition unfavorable for open berth piers?
The most severe load on piers generally is the horizontal load due to berthing of large vessels. Since the widths of open berth piers are relatively small so that they provides a short lever arm to counteract the moment induced by berthing loads. Moreover, the dead load of open berth piers are normally quite light and therefore the resisting moment provided by the dead load of pier structures may not be sufficient to counteract the moment generated by berthing loads.
To aid in providing adequate resistance to the overturning moment by the berthing load, the soil resistance above bedrock contributes to stabilizing moment. For commonly adopted marine piling type, i.e. driven steel tubular piles with reinforced concrete infill, driven piles can at most be founded on top of rockhead surface. In case the rockhead level is shallow, then the little soil cover may result in insufficient lateral resistance to the berthing load.
15. Why are high and narrow beams not desirable in concrete piers?
Based on past experience in other countries (Carl A. Thoresen (1988)), high and narrow beams after several years of construction showed signs of serious deterioration at the bottom of the beams. However, the deterioration of pier slabs was not significant when compared with that of the deep beams. The main reason to account for this is due to the close proximity of the deep beams to the sea level. To avoid these problems, either beamless slab or wide with shallow beams are normally designed.
16.What is the mechanism of formation of soil plugs in marine driven steel piles with reinforced concrete infill?
During initial driving process, open-ended steel piles are driven through the soils at their bases. However, shaft friction will gradually develop between the steel piles and soils inside piles at some time after pile driving. The hitting action of driving hammers induces forces to the soil and later it comes to a stage when the inertial forces of inside soils, together with the internal frictional forces exceeding the bearing capacity of soils at pile toes. Consequently, the soil plug formed is brought down by the piles. Reference is made to M. J. Tomlinson (1977). Note: A soil plug is a column of soil formed at the bottom portion of marine pile type of steel tubular piles with reinforced concrete infill.
17. Should stiff or soft fenders be designed for berthing in piers?
The elasticity of fenders is related to the ability to release the stored energy during berthing of vessels. However, it has no effect on the reaction force and the deflection of fender system. The amount of energy that a fender can absorb is dependent on the reaction-deflection curve and is represented by the area under the curve. The higher is the reaction force, the higher amount of energy would be absorbed by the fender provided that the resistance of ships’ hull is sufficient to withstand the force without permanent deformations. Although stiff and soft fender may have the same deflection under the same maximum reaction force acting on the berthing vessel, the amount of energy absorbed by stiff fenders is much higher than that of soft fenders. Consequently, stiff fenders should be employed for berthing purpose.
On the other hand, in mooring operations where vessels are constantly subject to wave action, it is desirable to keep the tension force on the rope to a low value. In this connection, it is recommended to use soft fenders.
18. What is the significance of direction of approaching velocities of ships during berthing operation?
One of the major effects of angle of approaching velocities of ships is its influence of the energy to be absorbed by the fender system. Consider several ships berth on the same pier at the same speed but with different angle of approach, though their kinetic energies are the same, the amount of energy absorbed by fender differs. The amount of energy absorbed by fender is: W = 0.5mv 2 (k 2 +r 2 cos 2 Φ)/ (k 2 +r 2 ) where W= energy absorbed by the fender m= mass of the ship v=velocity of the ship k= radius of gyration of the ship r= distance of centre of gravity of the ship to the point of contact of the fender Φ=direction of velocity Hence, when the direction of approaching velocity of a ship is normal to the fender system (i.e. Φ=90 ), the amount of energy absorbed is smaller when compared with that of a ship whose velocity is tangential to the shoreline. Reference is made to F. Vasco Costa (1964).
Chapter 6 – Roadworks
1. How can unreinforced concrete pavement function without mesh reinforcement?
For concrete carriageway, it is normally classified into two types: reinforced and unreinforced concrete pavement. The reinforcement in reinforced carriageway (in the form of mesh) is used for controlling cracking. Then one may query how unreinforced pavement can control cracking without the use of mesh reinforcement. To answer this question, one should pay attention to the features of unreinforced concrete pavement. In accordance with Highways Standard Drawing No. H1109, an approximately 3mm wide groove with a depth of about one-third to one-fourth of slab thickness is designed with a regular spacing (normally 5m). The grooves are designed to be too narrow for stones to fall into when the cracks are open due to concrete contraction. The sectional area in which the groove is located is a plane of weakness and thus this groove acts a potential crack-inducing device in which any potential cracks due to shrinkage and thermal contraction may form. Consequently, the cracks are formed at the base of the groove and thus it would not cause any unpleasant visual appearance on the exposed surface of unreinforced concrete pavement.
Fig. 6.1 Crack formed in unreinforced concrete pavement.
2. Why are concrete profile barriers designed with curved surface profiles?
Safety fencings are designed to contain vehicles in the carriageway in which they are traveling and prevent them from rebounding into the road and causing hazards. For normal fencing design, when vehicles crash into safety fencings, it will give way so as to absorb as much energy as possible, thus reducing the impact forces on the vehicles. Moreover, it serves to realign the vehicles along the carriageway when vehicles hit on them. However, for concrete profile barriers they are not designed to absorb energy during vehicle crashing, but to hold the vehicles hitting on them. In this connection, concrete profile barriers are designed with curved profiles so that vehicles can mount and go up partly on them, and yet they will not cause overturning of vehicles. Reference is made to Arthur Wignall, Peter S. Kendrick and Roy Ancil.
For shallow-angle crashing of cars, they would climb on the lower slope face of concrete profile barriers. On the other hand, when a car hits at a large angle to the barrier, the bumper collides with the upper sloping face of concrete profile barrier and the car rides upwards. This provides the uplift of the car whose wheels move up the lower sloping face of the barrier. It is not intended to lift the car too high, otherwise it may result in rolling. Since the friction between the wheels and barriers provide extra lifting forces, it is undesirable to design rough finish on these faces. In essence, the kinetic energy of the car during collision is transformed to potential energy during its lifting up on profile barrier and finally converted back to kinetic energy when the car returns to the road. Note: For details of concrete profile barriers, reference is made to HyD Standard Drawing No. H2101A.
3. Should joints of concrete kerbs be in line with the joints in concrete carriageway?
In normal practice, joints are provided in road kerbs to cater for concrete expansion and contraction. However, the location of joints in kerbs is not arbitrary and they should match with joints in concrete carriageway. Otherwise, it is very likely that cracks may form in concrete kerbs at location of pavement joints (Ministry of Transport (1955)).
4. Why is the slump specified in concrete carriageway comparatively low (30mm) when compared with normal concrete (95mm)?
The slump of concrete carriageway is purposely specified to be a relatively low value, i.e. 30mm. For concrete carriageway, traffic loads directly act on concrete pavement surface and therefore the surface strength is detrimental to its future performance. In freshly placed concrete, segregation (may be in the form of bleeding) occurs within the mixture of cement paste and aggregates. The degree of resistance to segregation is related to workability of concrete. If substantial segregation is allowed to take place, then the relatively porous and weak laitance layer will be formed on the carriageway surface and the aggregates will concentrate in the bottom. Hence, concrete which has insignificant bleed possesses a stronger surface layer and is more abrasion resistant. Consequently, a small slump value is specified to increase the wearing resistance of concrete and to achieve a suitable surface texture of concrete pavements.
Moreover, a low-slump concrete facilitates the use of slipforms when constructing the concrete pavement. With concrete of a low slump value, it still remains its compacted shape and is not liable to deform when the paving machines go away. However, if a high slump concrete is used instead, the pavement surface would drop and the edges may deform readily.
5. Can all utility detectors detect the depth of utilities?
In Hong Kong, underground utility detectors are normally divided into two types: electromagnetic detector and ground penetrating radar (GPR). For electromagnetic detector, it can detect the signals emitted by metallic utilities themselves by passive mode. While in active mode, the detector has to pick up the signals through a transmitter and sonda connected directly to the non-metallic utility. Both the alignment and depth can be found in active mode while only alignment can be found in passive mode. The electromagnetic detector available in market can detect utilities up to depth of 3m.
For ground penetrating radar, it sends radio waves into the ground and receives signals from reflections from utilities. It has the advantage of locating both the depth and alignment of utilities. More importantly, it can detect both metallic and non-metallic utilities. However, it suffers from the disadvantage that it is quite expensive and interpretation of data is not simple. Reference is made to LD, DSD (2000). 6. What is the importance of air void content in bituminous pavements? The air void content of bituminous materials is an important control parameter for the quality of bitumen being laid and compacted. If the air void content is too high, it allows for intrusion of air and water. Moreover, it also increases the rate of hardening of binders which produce premature embrittlement of pavements. In addition, too high a void content will also lead to differential compaction subject to traffic loads and result in formation of ruts and grooves along the wheel track.
However, a minimum amount of air void should be maintained to avoid instability during compaction process and to provide space for bitumen flow in long-term consolidation under traffic loads. A sufficient amount of air voids should be designed to make room for expansion of binder in summer and compaction by road traffic as suggested by National Association of Australian State Road Authorities (1968), otherwise bleeding and loss of stability may occur and the pavement will deform readily under severe loads.
7. In concrete pavement, why is the requirement of 95% by mass of quartz grains are specified in contract?
In General Specification of Civil Engineering Works (1992 Edition), in Clause 10.09 it states “Fine aggregate for concrete shall be natural river-deposited sand consisting of at least 95% by mass of quartz grains”. The aim of such specification is to control the quality of river sand. As quartz is a durable and hard material, using a high percentage of quartz in aggregates of concrete can enhance the strength and durability of the surface texture of concrete carriageway.
In addition, such specification requires a high percentage of quartz content, thereby reducing the probability of presence of impurities like shell.
8. What is the load transfer mechanism for paving blocks in pedestrian footway?
In Hong Kong, footway is normally designed with paving slabs/interlocking blocks instead of concrete because the extensive use of concrete in pavement is boring. Moreover, poor appearance will result in concrete pavement due to frequently trench openings for utility work. In addition, owing to the prolonged setting and curing time of concrete, the use of concrete pavement inevitably generates disturbance to the general public.
The pedestrian loads are taken up through the following ways: (i) Load carrying capacity of paving slabs/interlocking blocks are derived from their individual strength;
(ii) Pedestrian loads are also supported by interlocking forces provided by friction transfer through the sand in vertical joints.