Without changing the height, the width of each steel side-plate will be multiplied by the modular ratio n.
For calculation purposes, each 0.25-in.-thick steel plate is therefore equivalent to a wood plate 3.75-in. thick.
The transformed cross section composed entirely of wood is structurally equivalent to the original wood-and-steel cross section. So, what have we accomplished by this transformation?
The flexure formula applies to homogeneous beams only. To calculate stresses using the flexure formula, we must develop a cross section comprised of just one material (i.e., one elastic modulus E) that is equivalent to our actual cross section.