Flitched Beams:

A wooden beam might be strengthened by the addition of mild steel plates. This composite beam is called as flitched beam. The mild steel plates might be added to the sides of the wooden beam, as illustrated in Figure.

           Figure  

The wooden beam & the mild steel plates are associated together so that there is no slipping among them. They act together as a single beam.

Consider b be the width and d be the depth of wooden beam & let t be the thickness & d be the depth of each steel plate.

Consider σ_w is the bending stress in wood & σ_s be the bending stress in steel plate, at a distance y from the neutral axis.

In wood and steel, the strain at a distance y from the neutral axis shall be equal, as they behave as a single beam.

Consider E_w be the modulus of elasticity of wood & E_s be the modulus of elasticity of steel plate.

After that,       Strain in wood = Strain in steel

                              σw/ Ew  =  σs/ Es

                              σ _s  =  (Es/E_w)   σ _w

                        ∴  σ_s  = m σ_w

where m is the modular ratio among steel & wood and equals Es/E_w