Design - Nominal Crust Raft – Semi-Flexible

1 Design decisions
As discussed previouly the nominal crust raft is used where loadings are relatively light and ground conditions reasonable. The raft is lightly reinforced and consists of a basic ground slab with nominal thickenings (see  Fig. 13.15).

Fig. 13.15 Nominal crust raft design example.

2 Sizing the design
Such rafts can be designed either from experience simply by adopting a known raft which has performed successfully on similar ground conditions and subjected to similar loadings or by calculation as discussed previously.

Many local authorities have ‘standard’ designs which have been approved for use and are often agreed as ‘deemed to satisfy’ building insurers’ requirements. It is wise to agree with building control the design methodology prior to preparation of detailed calculations.

The calculated design assumes that the slab and thickening should be capable of spanning and cantilevering over any local depressions which may occur as a result of the loading and/or sub-strata conditions.

Such rafts are used generally for relatively lightly loaded conditions on reasonable ground. In such conditions these lightly reinforced rafts can prove more economic than strip footings particularly where the ground is reasonably level, where the basic ground slab is used as the main body of  the raft and where small straight thickenings replace  complicated layouts of wall strips.

The layout of the downstands is determined from the  overall raft stiffness requirements and while heavy load lines and point loads will have a bearing on the location  of such downstands they should not be allowed to dictate the design. For example, the strip wall loadings, shown in Fig. 13.10, zig-zags across the building and if the downstand thickenings were made to follow these lines an overall weakness in the thickenings would result at each change in direction and hence the overall behaviour of the raft would be adversely affected.

Fig. 13.10 Zig-zag wall layout.

It is therefore important that a common straight line across the building is used for the downstand which caters for  the local heavy loads and overall stiffness (see Figs 13.11 and 13.12).

Fig. 13.11 Straight thickenings below raft.

Fig. 13.12 Irregular wall layout but straight
thickenings.

With regard to overall thickening layouts it may be necessary when considering total raft behaviour to introduce thickenings purely for stiffness and in locations where no vertical load lines exist (see Fig. 13.13).

Fig. 13.13 Thickening layout for raft stiffness.

For raft foundations adequate protection from weathering and frost effects on most granular soils, sandy clays and insensitive clays can be achieved with 450 mm cover, similar to road construction. Over-emphasis on clay shrinkage must not be allowed to change the engineering judgement in such soils particularly where past performance has been proven.

The raft is considered as a single element in determining overall behaviour taking account of the stiffness of the  raft, and then breaking the foundation down into a num- ber of small elements to simplify the design. These local conditions tend to dictate the cross-section dimensions of the foundations with the overall behaviour being devel- oped and incorporated into the design on the drawing board. For example, if we take the raft shown in Fig. 13.13 and adopt the internal thickening layout discussed it can be seen that the reinforcement details for overall slab behaviour should ensure that beam thickenings can act continuously. In particular the design should avoid local weakenings in the concrete profile or reinforcement in vulnerable locations such as the internal angles of the raft (see Fig. 13.14). The detail therefore must ensure strong inter-sections at these locations where the overall shape of the raft tends to weaken structural behaviour.

Fig. 13.14 Weak and improved thickening layouts.