Concrete pad footing is the most common shallow foundation you will meet in any building construction. Pad footings are also known as isolated foundations, meaning they serve as separate column foundations.
Different types of pad footings
You can differentiate concrete pad footings according to their shape and design. See the following concrete pad footings of different shapes.
Footing shape – Square/ Rectangular/ Circular
Footing cross section – Simple/ Stepped/ Sloped
Also, in designing, we consider uniform and non-uniform pad foundations according to the loading arrangement.
When the center of loads coincides with the center of the area of the footing and the overturning moment is negligible or the foundation is analyzed as pin support, we may have a uniform bearing pressure from the soil.
When the loading is eccentric or an overturning moment is present, the case will be different. You have to consider the additional pressure acting on the pad due to the moment. In designing this type of pad footings are called non-uniform or eccentric type pad footings
How to cast a concrete pad footing
Concrete pad footings are easy to cast. First, you have to do the setting out to mark the location for excavation.
Manual methods or machinery are adaptable for excavation works. You may need some shoring to retain the soil.
After you remove the soil in the pit, first thing is to level the surface with lean concrete. This is normally known as the screed. Generally C10 strength concrete is enough for screed. This stops the mixing of soil and mud with footing concrete and acts as a leveling concrete.
When the screed is hardened, mark the exact locations of the column. Do the formwork around the footing and start building the rebar cage. If you need you can embed few rebar pieces into the screed. These will come in handy when supporting the forms, keep in mind that these rebars should not allow embedding into the footing concrete.
In most cases, bottom reinforcement is enough for concrete pad footings. However when the thickness increases you have to add top reinforcement for thermal and shrinkage actions.
Design of concrete pad footing
The design of pad footing is governed by two cases. We need to satisfy safe load transfer through the concrete as well as the soil. soil bearing capacity and structural capacity of the footing are them.
First, the bearing capacity will decide the footing size. The total pressure of self-weight and the serviceable load on the soil should be below the allowable bearing capacity of the soil to retain the soil under the load safely.
The thickness and the reinforcement of the footing will be decided by the shear force and bending moments in the footing. This is mainly the concrete design. All the factors need to ensure the safety of the concrete, consider here.
It is important to take the serviceable loads in sizing the footing and ultimate loads when doing structural design. This is because that serviceable load is the force acting for a longer duration compared to sudden ultimate loads. These sudden loads will not have much effect on the soil however, they can cause concrete to fail. So keep in mind to use ultimate loads during structural designing of footing.
In the structural design following parameters are considered.
Bending moment at the column face
Line shear (beam shear) at column face, d, and 2d distance from the column face
Punching shear at column perimeter and perimeters at d and 2d distance from the column.
The general reinforcement detail of the pad foundation is common to most of the footings. At the bottom, bending reinforcement is placed in both directions. These rebars can be bent up at the 4 sides to provide additional thermal and shrinkage reinforcement for thick pads. Also, you can predict the influence of hydration heat using the concreteWorks tool for thick footings. This will help to calculate the thermal stress on the footing during hardening.
Pad footing is acting as an inverted cantilever under soil pressure when loaded. So, there won’t be a reinforcement requirement on the top side. However, you have to consider the thermal and shrinkage action of the concrete. Foundations are the last thing you expect of failure. So, if the thickness is beyond 1.5 ft, it’s better to add at least minimum reinforcement.
Influence zone of pad footings.
This is the most critical part that I believe, everyone should have an idea about concrete pad footings. When column load is acting on the pad, it transfers the load safely to the soil. This pressure, Pushes down the soil to a certain depth causing settlement. The subsoil below the footing stressed corresponding to that settlement. The bubble of subsoil stressed just under the pad footing is called the influence zone. Under any circumstances, disturbing this soil volume is bad for the footing.
Generally, the influence zone spread down 2 times the width of the footing. Beyond that the soil stress is negligible. See the following diagram for a better understanding.
One of the common questions about pad footings is, whether these can consider as a pinned support or fixed support. Actually, it’s depending on the way you analyze your structure and tie-beam layout you are using. Generally, the pad footing is considered as pinned supports.