Modes of failure due to settlement

Settlement of a structure can take three forms, depending on how the footings settle relative to each other.

1. Uniform (total) settlement

All footings settle by the same amount. The structure moves down without distortion and there is little damage. It affects only the connections to services and the level of the building.

 Before:  |======|      After:  |======|
          ########             ########
                               (all down)

2. Tilting (rotation)

One side settles more than the other, so the building rotates as a rigid body. It occurs with eccentric loads, non-uniform soil, or adjacent heavy loads. Tall structures such as chimneys and towers are most affected, and the Leaning Tower of Pisa is an example.

     /|
    / |  <- tilted rigid body
   /__|
  ######

3. Differential settlement

Different footings settle by different amounts, giving angular distortion $\delta/L$ between points. It distorts the frame and causes cracks and damage to beams, walls and floors. Two shapes are common:

• Sagging (dishing): the centre settles more than the ends. Cracks appear in the lower part of the wall.
• Hogging: the ends settle more than the centre. Cracks appear in the upper part.

 Sagging (centre down)              Hogging (centre up)

 ========            ========               ________
         \          /                      /        \
          \________/             =========/          \=========

The differential settlement is limited to about 1/300 to 1/500 of the span to avoid cracking (about 20 mm for footings in sand and 40 mm in clay).