Steel Grillage Foundation - Method of Construction.

Steel grillage foundation is constructed of steel beams, structurally known as rolled steel joists(RSJ.), provided in two or more tiers. In the case of double tier grillage(which is commonly provided), the top tier of grillage beams is laid at right angles to the bottom tier. The joists or beams of each tier are held in position by 20 mm diameter spacer bars wtth 25 mm diameter pipe separators. Fig. 3.10 shows the plan and section of such a foundation. The grillage beams are embedded in concrete. Generally, a minimum clearance of 8 cm is kept betwen the grillage beams so that concrete can be easily poured and properly compacted. However, the distance between flanges should not exceed 30 cm  or 1 ½  times the flange width (whichever is small) so that the filled concrete acts monolithically with the beams. It should be noted that the concrete filling does not carry any load; it simply keeps the beams in position and prevents their corrosion. A minimum concrete cover of 10 cm is kept on the outer sides of the external beams, as weIl as upper flanges of top tier. The depth of concrete below the lower tier should atleast be 15 cm.

Method of construction. The foundation is excavated to the desired depth. Generally, the depth of foundation is shallow, just sufficient to accommodate the two tiers of grillage beams and the gusset plates etc. connecting the stanchion to the base. Howeer this depth should not be less than 90 cm in any case. After leveling the foundation base, rich concrete is poured and compacted, so that the formed thickness is not less than 15 cm. Compaction should be done properly so that the layer of concrete becomes an impervious bed.

This would protect the steel joists against ground water. After levelling the concrete bed, first layer of grillage beams of designed sizes are laid over it, at proper distances, with the help of separaters. The upper surface of all the beams should lie in one horizontal plane. Rich cement grout is then poured all around the lower flanges of the beams so that they are secured to the concrete bed. Cement concrete is then poured betwecn and around the beams of the first tier. The second tier of beams is then placed at right angles to the first tier and over the top flanges of the beams of the first tier. They are properly spaced with the help of separators. Concrete is then poured between and around the steel beams. The steel stanchion is then connected to the upper tier with the help of a base plate, side angles and gusset plate. These connecting elements are also embedded in the concrete so that joint becomes rigid..

Steel grillage foundation may also be provided for a masonry wall on soils of low bearing capacity. The grillage foundation for such a case consists of only on tier, though in some circumstances when the wall is wider and it carries heavy loads, two tiers may also be provided. Fig. 3.11 shows the details for both the cases.



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