Select the INCORRECT statement from the following with regard to grillage footing in a steel structure.
🔬 Understanding Grillage Foundations
A grillage foundation is a special type of shallow foundation designed to support heavily loaded steel columns. It is essentially a "raft" made of steel beams, used to spread a concentrated column load over a very wide area.
Primary Purpose: The main reason to use a grillage foundation is when you have a combination of extremely heavy column loads and a soil with low bearing capacity. A standard footing would be impractically large, so the high bending strength of the steel beams is used to create a wide, strong base in a limited depth.
⚖️ Detailed Analysis of the Statements
The question asks us to identify the statement that is false.
(a) It consists of steel beams encased in concrete.
TRUE. A grillage foundation is constructed with one or more tiers of steel I-beams or channels placed at right angles to each other. The entire assembly is then fully encased in dense concrete to provide corrosion protection and to hold the beams in place, ensuring they act as a single unit.
(b) It is provided when the loads on the column are extremely heavy.
TRUE. This is a primary condition for selecting a grillage foundation. They are an economical solution for supporting columns with very high axial loads where a standard reinforced concrete footing would become excessively thick and heavily reinforced.
(d) The column is placed over a base plate.
TRUE. The steel column is not placed directly on the grillage beams. The load is first transferred from the column to a thick steel base plate. This base plate is then bolted to the top tier of the grillage beams, ensuring the concentrated load is spread evenly before being distributed through the beam layers.
(c) The bearing capacity of soils should be very high to place a grillage foundation.
FALSE. This statement is the exact opposite of the truth. If the soil had a very high bearing capacity, a simple and much cheaper isolated footing would be sufficient. The entire purpose of a grillage foundation is to create a large bearing area to safely transfer heavy loads to a soil with low bearing capacity without causing excessive settlement.