Simple timber beam and flitched beam

Simple timber beam

A simple beam is a single rectangular piece of sawn timber, simply supported on two supports, that carries the load by bending.

        load
   v  v  v  v  v  v
  +------------------+
  |  solid timber    |   b x d
  +------------------+
  ^                  ^
 support          support

It is used for short spans of up to 4 to 5 m. Section modulus $Z = bd^2/6$, and the design checks bending, shear, bearing and deflection (IS 883:1994 cl. 7.5).

Flitched beam

A flitched beam is a built-up beam in which one or more steel plates (flitch plates) are sandwiched between timber members and bolted through so that all parts act as one. The steel plates add strength and stiffness without increasing the depth.

   timber  steel  timber  steel  timber
   +-----+-+-----+-+-----+       bolts through all
   |     | |     | |     |       parts at intervals
   |     | |     | |     |
   +-----+-+-----+-+-----+

• Timber and steel have the same strain at the same level, so with the modular ratio $m = E_s/E_t$ each steel plate is replaced by timber of width $m\,t_s$ (equivalent section). The load is shared in proportion to the stiffness $EI$ of each part.
• Bending stresses: $f_t = \dfrac{M y}{I_{eq}}$ for timber and $f_s = m\,\dfrac{M y}{I_{eq}}$ for steel, with $I_{eq} = I_t + m\,I_s$; each must not exceed its permissible stress.
• The bolts (or screws), placed at a regular pitch, hold the parts together and take the horizontal shear.
• It is used where a deeper timber beam is not available, or for long spans and heavy loads.