Question

Explain why should the beams used in the construction of the bridges have large depth and small breadth.

Answer 1

To resist a given bending moment and also to keep deflections 
small, it has been stated that the deepest beam is the most 
suitable. If, however, the depth is made too great in 
proportion to the breadth, the beam may buckle sideways due to 
a column effect as the result of compressive stresses in the 
top fibres.

Answer 2

The beams used in bridge construction should have large depth and small breadth

• Large depth and small breadth of the beams contributes in increasing the second moment of area of the beams

• The increase in second moment of area increases the stiffness of beams to the loads in vertical direction that are primary loads on most of the bridges

• This also helps in resisting the moment caused by transverse loading