Grillage bases are rarely economic these days for permanent foundations except for very heavy loads. However their prefabricated form can prove very useful for temporary works particularly where re-use of the foundations is required (see Fig. 9.59).
The grillage beam can be in any material, the most usual being either steel, precast concrete or timber. In some permanent situations, however, where unusual circumstances exist, such as an abundance of durable timber or the possible re-use of existing rolled steel sections, the grillage can prove both successful and economic. In permanent conditions durability becomes an important design factor and protection and/or the selection of suitable materials is a major part of the design. In the case of steel grillage below ground this is usually achieved by encasing the grillage in concrete. The concrete for average ground conditions would usually require to provide a minimum cover to the steel of 100 mm. In the case of timber grillages the selection of a suitable species of timber and/or suitable preservation protection is crucial to the design, in a similar way to that for timber piles.
Fig. 9.58 Grillage foundation.
The design of the grillage is carried out by calculating the loads and moments applied from the superstructure and determining the required base area using a suitable allowable ground bearing pressure for the condition involved.
From this area, the number and size of each grillage layer can be decided. The layers are then designed to cantilever from the edge of the layer above, which determines the beam sizes required to resist the applied bending moments and shear forces (see Fig. 9.60).
If the grillage is encased in concrete and the sequence and method of construction and loading is compatible with the design requirements, the composite action of the beam and concrete can be exploited.
Fig. 9.59 Grillage foundation – temporary works.
Fig. 9.60 Grillage foundation – bending and shear diagrams.