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|