Steel grillage foundations consist of multiple tiers of steel beams laid perpendicular to each other and embedded in concrete. The beams are spaced using separators and the concrete fills the gaps but does not carry any load, instead keeping the beams in position and preventing corrosion. Construction involves excavating to the required depth, laying a concrete bed, placing the first tier of beams on it with grout, then adding the second tier perpendicular on top and filling with concrete. Grillage foundations can also consist of a single tier to support masonry walls on weak soils.

1. 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.

2. FIG. 3.10 TYPICAL GRILLAGE FOUNDATION FOR STEEL STANCHION.

3. FIG. 3.11 STEEL GRILLAGE FOUNDATIONS FOR WALLS.