When there is heavy load on steel columns and the soil is of low bearing capacity, grillage foundations can be provided. These consist of rolled steel joists in one or two layers that are encased In continue with the overall dimensions being such that the soil pressure is within the allowable bearing capacity. Adequate gap must be provided between the flanges of the girders to enable the concrete to flow and fill the gaps. However, it should not be more than 1.5 to 2 times the flange width in order that the concrete and steel should act together. Concrete is not intended as a structural element but is used to keep the beams in position and prevent its corrosion. To prevent movement during concreting, the joists are connected through the webs by bolts with spacers. This type of foundation is expensive on account of the large amount of steel and is now rarely used.
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