Equivalent strut behaviour of infill wall

When a masonry infill wall sits inside an RC frame and the frame sways under lateral load, the frame and the infill separate at the tension corners, and the infill touches the frame only at the two opposite (compression) corners. The wall then acts like a diagonal compression strut between these corners. This is the equivalent strut (equivalent diagonal) model.

  +------------+
  |\           |
  |  \  strut  |
  |    \       |      width w = 0.175 (alpha_h)^-0.4 d
  |      \     |      d = diagonal length of infill
  +------------+

The strut has a thickness $t$ equal to that of the wall and a width $w \approx 0.175\,\alpha_h^{-0.4}\,d$, where $\alpha_h = h\left[\dfrac{E_m t \sin 2\theta}{4E_f I_c h_m}\right]^{1/4}$ ($h$ = column height, $E_m$ = masonry modulus, $E_f$ = frame modulus, $I_c$ = column moment of inertia, $h_m$ = infill height, $\theta$ = angle of the diagonal with the horizontal). It is modelled as a pin-ended diagonal brace in the frame analysis.

Is it advantageous? Yes, in general: the strut greatly increases the lateral stiffness and strength of the frame, reduces drift and dissipates energy, so some damage to the infill can be tolerated. But the benefit depends on the infill being well built and uniformly distributed. Irregular infill can cause soft-storey, short-column and torsion problems, and the wall may fail out of plane. These effects must be considered in design.