Civil engineering interview question set 2

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

induced is smaller.

(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. Reference is made to Sandberg Consulting Engineers Report 18380/X/01.

 

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.

 

 

Civil engineering interview question set 2