Bearing Pressure Design - Semi-Fexible Rafts.

Rafts are intended to take local stress concentrations, i.e. line and point loads, and spread them over a larger area by the time they reach the formation level, so as not to exceed the allowable bearing pressure. This is done through the combined influence of concrete thickness and profile, rein- forcement, and thickness of hardcore/granular fill. Where local bending of the raft is utilized to spread the loads over a wider area, an ultimate limit-state analysis is carried out to size the necessary reinforcement.

(1) Slabs
While the slab thickness does contribute to the spread  of concentrated loads, the main factor is the presence or absence of bottom reinforcement. In slabs with top rein- forcement only, the load is assumed to spread through  the slab at 45°, i.e. as if it were mass concrete. In slabs with bottom reinforcement, the reinforcement can act with the slab to form a local spread footing, distributing the load over a wider distance. These cases are shown in Fig. 13.1.

Fig. 13.1 Bearing pressure design for internal walls on
slabs without thickenings.

(2) Internal beam thickenings
The width over which the load is assumed to spread is  primarily governed by the arrangement of reinforcement.

The presence or absence of transverse reinforcement in  the thickenings, and bottom reinforcement in the slabs, determines whether the load can be spread merely over the bottom soffit of the thickening, or additionally over any sloping sides to the thickening or the adjacent slab. These cases are shown in Fig. 13.2. It is recommended that the slab is made thick enough to resist the applied shear, without the use of shear reinforcement.

Fig. 13.2 Bearing pressure design for internal beam thickenings.

(3) External beam thickenings
With external beam thickenings, the effect of eccentric loads is often the most dominant factor. With reference to Fig. 13.3 (a), the bearing pressure is initially checked assuming a uniform pressure distribution of width 2x located concentrically below the load. If the bearing pressure must be reduced, this is done either by increasing x (the projection of the thickening beyond the line of action of the load) or by spreading the load further into the raft, and using the slab reinforcement to transfer a moment to a suitable reaction to balance the vertical loading eccentricity (see Fig. 13.3 (b) and (c)). This latter approach tends to be the more economic provided:

(a) The opposite edge thickening has a similar load intensity, to balance the moment within the slab, and
(b) The slab reinforcement is sufficient for this moment to develop.

Fig. 13.3 Bearing pressure design for external beam thickenings.

(4) Effect of compacted hardcore/granular fill
The presence of compacted hardcore or granular fill below a raft enables further spreading of concentrated loads, reducing bearing pressures and slab bending moments.

Although almost all rafts have some thickness of hard- core/compacted fill, the beneficial effect of this thickness  is often ignored in practice except where this thickness is substantial. Where the thickness is taken into account a 60° spread is usually assumed (see Fig. 13.4).

Fig. 13.4 Effect of compacted hardcore fill on bearing
pressure design.