The longer side of cross-section of the girder is used as depth. Why?

Explanation

When a load W is attached at the middle point of a girder of length l. breadth b and depth d, supported horizontally. The depression `delta` produced at middle point in the girder is given
by `delta = (Wl^(3))/(4Ybd^(3))" so "delta prop 1/(bd^(3))`
Therefore, depression of the girder will be small.
When depth d is larger, because `delta prop 1/(d^3)`
Therefore longer side of cross-section of the girder is used as depth, so that depression is small for safety of buildings.