When a column carries heavy structural loads which are to be transferred to the soil of low bearing capacity, grillage foundation is founded to be very suitable. this type of foundation avoids deep excavation. It is particularly useful for structure carrying heavy concentrated loads and hence are largely adopted for the foundation of building like workshops, factories, or town- halls, theatres etc, the total loads of the column is transmitted over very large area by means of rolled steel joist (RSJ)o tire the beam known as grillage which is provided in single or double tries.
In case of double, top tires Grillage foundations consist of a number of layers of beams usually laid at right angles to each other and used to disperse heavy point loads from the superstructure to an acceptable ground bearing pressure.
In case of a double tire, the top layer is laid of right angles to the bottom tires, a top tier may consist of 3 to 5 heavy girders and the bottom tires 6 to 10 smaller girders. The beams are suitably spaced so as to provide a facility for placing and compacting of cement concrete between them. In order to protect the beam from corrosion, it is entirely covered in concrete with a minimum cover of 10 cm.
Grillage foundation