Design Example: Nominal Crust Raft.

A new housing estate, consisting of two-storey semi- detached properties, is to be built on a green field site.
The ground conditions consist of a soft to firm clayey sand.

The net allowable bearing pressure for raft design is estimated at na = 100 kN/m2.

The foundation for each pair of houses is to be designed  as a raft foundation. Taking into account the ground conditions and the relatively light loading, a nominal crust raft  is considered adequate. The wall layout, loadings, and  corresponding raft layout are shown in Fig. 13.15.

Fig. 13.15 Nominal crust raft design example.

Loadings
The foundation load due to slab self-weight and imposed
load is

fQ as a percentage of  f is 100fQ/f = 42%. From Fig. 10.20,  the combined partial safety factor for the foundation load is γF = 1.48.

Wall line load, P = (wall dead load) + (wall imposed load)
     = G + Q
     = 15.5 + 10.5
     = 26.0 kN/m

Q as a percentage of P is 100Q/P = 40%. From Fig. 10.20, the combined partial safety factor for the superstructure loads is γP = 1.48.

Fig. 13.20 Blanket raft construction.

Bearing pressure design
Because of the low level of loading, no explicit check on bearing capacity is considered necessary.

Design span for local depressions
With reference to Table 13.1, the soil conditions are taken  to be medium Class B. From Fig. 13.5, assuming 150 mm of hardcore, the design span L is 1.6 m.

Table 13.1 Design diameter for local depressions

Slab design
It is intended for the slab to have top mesh reinforcement only. Figure 13.5 indicates that, for a design span of 1.6 m,  a minimum average effective depth of 100 mm is required to comply with the deflection requirements of BS 8110:  Part 1(2). A slab thickness of 125 mm will therefore be adopted, with 20 mm top cover. For shrinkage purposes, Table 13.2 indicates, for a 12 m long slab, that A142 mesh is adequate.

Table 13.2 Shrinkage reinforcement for raft slabs

Because of the low level of distributed load, and the absence of any significant wall line loads on the slab, there is no need to carry out a local spanning check on the slab under ultimate loads. (If however, the internal thickenings were omitted, and the slab required to carry their load, a check should be carried out in a similar manner to Design Example 2 in section 13.3.3.)

Beam thickening design
Similarly, the low level of loading, and the absence of concentrated point loads on the thickenings, indicate that these can be sized and reinforced on a nominal basis.

For external thickenings, use pre-bent B503 mesh, as shown in Fig. 13.15, with the main T8 bars at 100 mm centres  running along the length of the beam. This will result in  at least three T8 longitudinal bars in the top and bottom  of the beam.

For internal thickenings, again use pre-bent B503 mesh,  as shown in Fig. 13.15, with the main bars again running longitudinally to help span over local depressions.