why are girders for supporting roofs made in the shape of I

This is to reduce buckling. Repeated loading can make a depression on the supporting roofs . THIS IS BUCKLING. IT IS given by the equation SIGMA= wl³/ 4 Ybd³where w= weight, l= length, y=youngs modulus, b= breadth , d= depth. so increasing the depth can reduce buckling effectively. so an I shaped cross section is preffered which will give enough depth and broader area of coss section.