Rectangular Column Footing: A Short Guide to Design
Being a vertical support structure, columns need proper footing that disperse the load of the column and the supported superstructure. For this, each column needs a dedicated or shared supporting foundation structure located below ground level; this is called a Footing.
In most cases, the horizontal cross-section of the footing is kept as round, square or rectangle; which leads to our topic today - designing a Rectangular Column Footing. This kinds of footings are supposed to hold a single column.
Purpose of a column footing:
a) Supporting the upper structure; i.e. the column in this case
b) Transfering the moments and loads to the soil/bed safely
c) Resisting the shearing and bending forces developed in the soil or bed
While designing a rectangular column footing, the following points need to be kept in mind:
1. Ensure that the footing has adequate load-bearing capacity.
2. If soil is soft, the shrinkage and swelling due to weather effects need to be considered and the depth of the locus of the footing must be below the region of such movement.
3. The footing must penetrate below the maximum scouring depth.
4. Since these sort of footings are generally made from cheap vulnerable materials, you need to make sure that they do not come often in contact with organic or corrosive materials in soil. For example, don’t dump garbage at the base of the column.
The footings placed independently underneath columns are called isolated footings. They consist of a thick slab; this may be flat (or pad), stepped or sloped. These may be affected by soil swelling or shrinkage.
In that case they can bend vertically and thus destabilize the structure above. To prevent this, two sets of bars are placed on top of each other near the bottom of the footing.
The rectangular footing has a long side and a short side. The longer side should be supported by reinforcement bars. This effectively transfers the loads as the shorter side does. Some part of the total reinforcement on the shorter side is placed in the midsection of the longer direction, which is referred to as the band.
These sorts of footings are generally well-preferred since they are easy to build, they do not require specialized substances (made of standard concrete), and they do not need skilled labour in construction.
Needless to say they are cheap as well. However, they can be easily subjected to tension or torsion forces if present in ground, and they will have settlement and these do not have much load-bearing capacity. For these reasons, these footings are mostly used in light urban constructions mostly.