Question

Explain why should the beam used in construction of bridge have large death and small breadth

Answer 1

The strength of a beam depends on a geometric property called MI (Moment of Inertia.)MI is an index to the resistance the beam offers to bending when subjected to transverse loads.This property depends on its width W and and its depth, D in the case of rectangular section of width W and depth DThe formula is MI = W x D^3 / 12That is : width W multiplied by the cube of the Depth , divided by 12Now it will be clear. Doubling the width, W, merely doubles the MI but if you double the Depth D, the MI goes up 8 times, while the area remains the same.So for optimum usage of material (defined by Area) it is better to have depths greater than widths.