Question No. 01
In the design of service reservoirs, horizontal reinforcement in walls of reservoirs is placed at the outer layer. Why?
Answer: Since service reservoirs are designed as water-retaining structures with stringent requirement of crack width control, the design of reinforcement of service reservoirs is under the control of serviceability limit state. For the walls of service reservoirs, contraction and expansion of concrete are more significant in the horizontal direction of walls because of their relatively long lengths when compared with heights. In this connection, in order to minimize the usage of reinforcement, horizontal bars are placed at the outmost layer so that the distance of reinforcement bars to concrete surface is reduced. Since the shorter is the distance to the point of concern, the smaller is the crack width and hence with such reinforcement arrangement advantages are taken if the reinforcement bars in the critical direction are placed closest to concrete surface.
Question No. 02
Can grout replace concrete in normal structure?
Answer: 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.
Question No. 03
Under what situation should engineers use pot bearings instead of elastomeric bearings?
Answer: 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. Consequently, pot bearings are better alternatives than elastomeric bearings in such a scenario as suggested by David J. Lee.
Question No. 04
How high should the water tank be to provide a 60 PSI at base of this water tower?
Answer: 138.60 ft 2.31ft of water = 1 PSI
Question No. 05
What is sucker deck principle for variable depth bridge decks?
Answer: 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 20-40 m. 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).
Question No. 06
What is a projection line?
Answer: Projection line is the way, in which the earth is shown on a flat piece of Paper.
Question No. 07
In precast concrete jacking pipes, sometimes grout holes are designed inside these precast pipes. Why?
Answer: Grout holes are present in precast jacking pipes for the following reasons:
(i) They serve as the locations for injection of bentonite or other lubricant. Lubricant is used for both granular soils and cohesive soils to trim down the frictional resistance. For cohesive soils, the soils cannot get onto the pipes by the presence of lubricant and the shearing plane lies within the lubricant as suggested by R. N. Craig (1983). On the other hand, for granular soils, the lubricant mixes with soils with a significantly reduced friction. With the use of lubricant, longer pipe lengths can be jacked without the use of intermediate jacking station.
(ii) They provide the inlet locations for subsequent grouting works after completion of pipe jacking to fill completely the void space between the pipes and surrounding soils.
(iii)They are used as lifting holes when placing the precast jacking pipes into rails inside the jacking pits.
Question No. 08
In designing reservoirs, the indirect tensile strength of the concrete mix is specified to be less than a specific value (e.g. 2.8 N/mm²) for potable water. Why should engineers put an upper limit of indirect tensile strength?
Answer: The crack width formation is dependent on the early tensile strength of concrete. The principle of critical steel ratio also applies in this situation. The amount of reinforcement required to control early thermal and shrinkage movement is determined by the capability of reinforcement to induce cracks on concrete structures. If an upper limit is set on the early tensile strength of immature concrete, then a range of tiny cracks would be formed by failing in concrete tension. However, if the strength of reinforcement is lower than immature concrete, then the subsequent yielding of reinforcement will produce isolated and wide cracks which are undesirable for water-retaining structures. Therefore, in order to control the formation of such wide crack widths, the concrete mix is specified to have an indirect tensile strength at 7 days not exceeding a certain value
(e.g. 2.8N/mm² for potable water). Reference is made to R. D. Anchor, A.W. Hill and B. P. Hughes (1979).
Question No. 09
How many kilo-Newtons equals 1000 kilogram?
Answer: When the 1000 kg are at rest, on Earth with gravitational acceleration 9.81 m/s2, 1000 kg equal 9.81 kN.
Question No. 10
What are the shortcomings of grillage analysis which is commonly used in structural analysis of bridges?
Answer: 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 elements depends on the curvature in the beam’s direction and its orthogonal direction.