COMPLETE DETAILS ABOUT TRAPEZOIDAL FOOTING  WITH SITE VISIT BY CIVIL GURUJI
Channel: Civil Engineers Training Institute
Date: 2020-05-19

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A special case of footings is the trapezoidal footing, which may be used to carry two columns of unequal loads when distance outside the column of the heaviest load is limited. In such case using a rectangular footing may lead to the resultant of loads dos not fall at the middle length of the footing.

In the primary design of footings or rafts, it is generally assumed that the contact pressure distribution is planar, whatever the type of model used in the analysis of the footing. Therefore, to achieve a desirable uniform contact stress distribution beneath the footing it is necessary to arrange the center of area of the footing directly beneath the center of gravity of the external loads. This may lead to irregular-shaped footing. If equal column loads are symmetrically disposed about the center of the footing, the contact pressure distribution will be uniform. In order to achieve a theoretically uniform contact pressure distribution, the footing can be extended so that the center of area of the footing coincides with the center of gravity of the external loads. This is easy to be done by rectangular footing.

A special case of footings is the trapezoidal footing, which may be used to carry two columns of unequal loads when distance outside the column of the heaviest load is limited. In such case using a rectangular footing may lead to the resultant of loads dos not fall at the middle length of the footing. To overcome this difficulty, a trapezoidal footing is used in such a way that the center of gravity of the footing lies under the resultant of the loads. Correspondingly, the distribution of contact pressure will be uniform.

The reasons of using trapezoid footing instead or rectangular are stated below:

Well the most important reason which is there for doing so is to save concrete. When we design the foundation the bending moments are critical at the face of the column and the depth requirement reduces as we go further away from it. Hence if we do not give slope we are using up concrete where it is not really required.
Also in colder regions where there is possibility of freezing if the water is retained over the footing and it freezes it can cause additional pressure and cracks in the foundation(freezing and thawing effect), hence there we keep a certain slope that the water which comes over the footing is drained easily.
Although it seems like a good practice you will see that many a times it is not used. That is due to the reason that there can be certain constraints to this and where it cannot be used. e.g. due to stability constraint where a certain dead load is required to avoid uplift.

Trapezoidal footing takes a little more time, labour-wise, and uses a little more material, resource-wise, but it results in a stronger, longer lasting, resilient, and integral footing. Rectangular footing has a tendency to simply break, chip, pancake, and or either upheave or droop and slump over time.

Trapezoidal footing helps in load distribution more effectively,wider over the base,saves concrete quantity but demerits for deeper base requirement (avoided in case of basement floor slabs)and cost of tapered formwork,more skill in finishing,and time required to handle trapezoidals.

The different types of footings used in construction are described below:
Continuous Wall Footing. The footing which supports a long masonry or RCC wall is known as a continuous footing. ...
Isolated Footing. ...
Combined Footing. ...
Strip Footing. ...
Strap Footing. ...
Raft Footing. ...
Pile Footing.
Isolated Spread Footing. This is the most common and simplest type of foundation as this is the most economical type of foundation. ...
Wall Footing or Strip footing. ...
Combined Footing. ...
Cantilever or Strap Footing. ...
Raft or Mat Foundation.
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