Brick masonry infill walls in RC frames

Masonry infill is brick or block masonry built within an RC (or steel) frame to form partitions and enclosure. It is usually considered non-structural and ignored in design, but it strongly changes the behaviour of the frame under lateral loads.

Role and effect on frame behaviour

• Under lateral load the infill acts as a diagonal compression strut between the frame corners, so stiffness and lateral strength rise and storey drift is reduced.
• The initial stiffness is 5 to 10 times that of a bare frame; the fundamental period is reduced, so the seismic force attracted rises.
• Cracking and crushing of the infill dissipate energy.

  lateral    +------------+
  load  -->  |\           |
             |  \         |
             |    \       |    infill acts as a
             |      \     |    diagonal compression strut
             |        \   |    (equivalent width w)
             |          \ |
             +------------+
              frame + infill

Equivalent strut width (Mainstone): $w \approx 0.175\,\alpha_h^{-0.4} D$, where $D$ is the diagonal length of the infill and $\alpha_h$ measures the stiffness of the infill relative to the frame.

Advantages

1. Increases the stiffness, strength and energy dissipation of the frame.
2. Reduces drift and damage to non-structural items in low-intensity shaking.
3. Gives thermal and sound insulation, fire resistance and enclosure, at low cost with local materials.

Disadvantages

1. Irregular or non-uniform placement (open ground storey) creates a soft storey and torsion.
2. Partial-height infill leads to the short column effect and shear failure of columns.
3. It is brittle and prone to out-of-plane failure if not tied to the frame (it needs bands, ties and mesh as per NBC 105 and IS 4326).
4. It adds mass and so increases the seismic force; heavy walls need strong frames.
5. Diagonal strut action may overload the column and joint.