Beams used in the construction of bridges have large depth and small breadth. Explain why?

Explanation

Consider a rectangular bar of length l, breadth b and thickness d supported at both ends. When bar is loaded by W at centre, the depression produced is
`sigma=(W l^3)/(4Y bd^3)`
Since `sigma` is proportional to `d^(-3)` and `b^(-1)`, so `sigma` can be decreased by increasing .d. rather than .b., So beams used in bridges have large depth.