Question No. 01
How do engineer determine the number of cells for concrete box girder bridges?
Answer: 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.
Question No. 02
What is the quantity of cement in Mix M20 concrete?
Answer: For nominal Mix Cement consumption for M25 will be 570 kg per cum. and for Design Mix it can be optimized to 510 kg per cum.
Question No. 03
What glue is least sticky?
Answer: Elmer’s glue is the least sticky
Question No. 04
When a drainage system (i.e. u-channels with catch-pits) is connected to a main drainage channel ,a segment of short pipe is used. What is the reason of such arrangement?
Answer: 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.
Question No. 05
For column reinforcements, why is helical reinforcement sometimes designed instead of normal links?
Answer: 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.
Question No. 06
What are the three major types of reinforcement used in pre-stressing?
(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 poissons effects or by the shape of the stress trajectories
(ii) Equilibrium reinforcement Equilibrium reinforcement is required where there are several anchorages in which pre-stressing loads are applied sequentially.
(iii) Bursting Reinforcement Tensile stresses are induced during pre-stressing 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.
Question No. 07
How many Lbs are there in a yard?
Answer: A pound (Lb) is a unit of measurement for weight while a yard is a unit of measurement for distance. If you are referring to common measurements used on a construction site, a “yard” is a common abbreviation for a “cubic yard”, which is a unit of measurement of volume. (A cubic yard equals 27 cubic feet.) Since dirt weighs about 110 pounds per cubic feet, a cubic yard of dirt weighs about 2970 Lbs. Since concrete is heavier (150 PCF), a cubic yard of concrete weighs about 4050 Lbs.
Question No. 08
Sometimes the side of concrete bridges is observed to turn black in colour. What is the reason for this phenomenon?
Answer: 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.
Question No. 09
What are the components of a waterproofing system in the roof of a typical pumping station?
Answer: In the design of a waterproofing system at the roof of a pumping station, normally the following components are:
(i) Above the structural finish level of the concrete roof, a screed of uniform thickness is applied to provide a smooth surface for the application of waterproofing membrane. (Screed of varying thickness can also be designed on the roof to create a slope for drainage.) The screed used for providing a surface for membrane should be thin and possess good adhesion to the substrate. Moreover, the screed aids in the thermal insulation of the roof.
(ii) Above the screed, waterproofing membrane is provided to ensure water-tightness of the roof.
(iii) An insulation board may be placed on top of waterproof membrane for thermal insulation. In cold weather condition where the loss of heat at the roof is significant, the insulation board helps to reduce these losses. On the contrary, in summer the roof is heated up by direct sunlight and the insulation layer reduces the temperature rise inside the pumping station.
Question No. 10
If the contractor is liable for defective works for 12 years with contract under seal (6 years with contract not under seal), then what is the significance of Maintenance Period?
Answer: Defective works constitute a breach of contract in accordance with Limitation Ordinance An action founded on simple contract (not under seal) shall not be brought after expiration of 6 years while an action founded with contract under seal shall not be brought after expiration of 12 years. For construction works, the date of counting these actions should be the date of substantial completion.
To answer the above question, one should note that under the contractual requirement, the contractor during Maintenance Period has the right to rectify the defects and the employer has also the right to request the contractor to make good defective work. However, after the expiry of Maintenance Period, in case of any arising of defects, the employer has to employ others to rectify these works and bring the action to court to claim the contractor for the costs associated.