Transfer of forces

The ultimate destination of all forces developed by a structure is to the ground. Just as we take all of our

Figure 2.27 Graduated tell-tale bridging a fracture.

weight through our feet, so does a structure through its footings. The analogy can be taken further in that the load onto our toes is, for most of the time, greater than on our heels. Thus it is also for most walls, or even an individual column should it be receiving some other load. If there is a change in the distribution of foundation forces at ground level, this toe pressure will be the first affected. This regularly occurs due to softening of the ground at the base of a wall, or trenching in front, or loss of face mortar (Figure 2.28). The structural forces will attempt to redistribute load to compensate for the loss of support by increasing the toe compression, which often involves a forward rotation of the wall.

In Figure 2.28 it can be seen that the front wall separates as the wall leans forward, and tension cracks open in the return walls. This phenomenon increases the stress on the toe, as a consequence of which a significant vertical shear acts on the face of the structure, as demonstrated by the spectacular shear cracks on the buttress shown in Figure 2.29.

Figure 2.28 Forward lean of wall with tension cracks showing in return wall.
Figure 2.29 Shear cracks in buttress of leaning wall base.

Regrettably, this realisation fails to get home to large numbers of professionals, and it is not unusual to find excavations carried out at the toe of walls, with no regard to the tons of force from a cupola or vault at its top. It is important to realise that, in nearly all cases, the greatest vertical stress will be here at the toe and that there will be a corresponding horizontal component also. Even more common is the gross disregard given to discharge of roof water, which invariably happens just where it can do the greatest damage: at the toe of the wall. Some basic facts are worth recapping in this regard, if we look at some primary structural components.

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