Bridge Engineering MCQ with answer and solution

Concept

A flexible cable changes its shape easily when a moving load changes position. A stiffening girder distributes the moving concentrated load over the cable to prevent excessive shape changes.

Step-by-step solution

1. A suspended cable is a flexible structural member that takes the shape of a parabola or catenary under uniform loading. 2. When a moving concentrated load acts on a pure cable, the profile of the cable distorts significantly as the load moves. 3. To prevent this severe distortion and keep the road surface level, a stiffening girder (or truss) is attached to the suspenders. 4. The stiffening girder spreads the point load over the length of the cable so that the cable retains its structural stability. Answer: (D) Stiffening girder

Explanation

Flexible cables change shape easily under moving point loads. A stiffening girder is used in suspension bridges to distribute live loads evenly along the cable. This prevents large deflections and maintains bridge geometry.

Why the other options are wrong

A: A thicker cable increases weight but still distorts locally under moving loads. B: Adding more cables does not prevent localized shape changes caused by moving point loads. C: Reducing sag increases horizontal tension but does not stop cable profile distortion.

Shortcut

Remember: Suspension cables need stiffening girders to handle moving traffic loads smoothly.

Common mistake

Thinking that using a thicker cable or reduced sag prevents local shape distortion, whereas cables always deform under point loads unless stiffened.