Footing Design: Why we need foundations?
In this tutorial, we will take a brief look at the process of designing a footing foundation.
Whether modern structures are made up of Reinforced concrete, steel, wood or any other material, they all require foundations to support them. As various types of loads like dead load, live load, wind load, earthquake load and snow load are acting on the structure, these loads are eventually transferred down to the foundation, which helps transmits them to the earth beneath. It is important to make the foundation strong in order to withstand these loads throughout the service lifespan of the structure.
What are footing foundations?
Depending upon depth we know that the foundation can be shallow or deep. A footing foundation is a shallow foundation type which can be made up of materials like brick masonry or concrete, they are mainly constructed right beneath the wall or column of the structure.
How does Load Transfer Mechanism work in structures?
In civil engineering it is important to know how loading system and load path works in the structure. In any structure the load is applied on the slab which get transferred through beams and in turn beams transfers these loads on to the column which are eventually transferred to the foundation. From here the loads ‘exit’ your structural system and are transferred to the earth or soil beneath it. The foundation must rest on hard strata, thus in most of the structural project the excavation is done in order to find the hard stratum which will help the foundation to rest easily on it without any settlement issues.
Footings support the dead (and other loads) to ensure the structure is static
How footing foundations are designed?
In older days the design of the structural elements like beams, columns, slabs were done manually using different set of methods to find shear force, bending moment and other various properties acting on these elements. But in today’s practice designing structural elements with manual calculation will take more effort and time and still it will be prone to human errors in calculations.
The design of the footing foundation is based on the combination of several processes, including:
Before designing foundation for any structure we need a soil investigation report, with which we know about some important characteristics about the soil beneath, characteristics like soil bearing capacity (SBC), different layers of soil type found beneath, all this information helps the engineer determine the type of foundation suited for the structure.
Various software exists to perform the structural analysis of the structure. It is mandatory to do a structural analysis in order to find various reactions, shear forces and bending moment forces acting on the structural elements, in particular at the supports. Let us assume a G+2, structure needs to be constructed, a structural design process needs to be followed to design each member of the structure. Any FEA Software can be used for doing modeling and structural analysis of the structure. After completing the structural analysis we need two types of data: (1) the reactions from the columns that will be connected with the footing foundations and (2) the column positions or coordinates.
After doing analysis and getting the end column reactions from FEA software, we need to design as per the requirements of our local standards. This art of the process can be hand calculated, or with the aid of Foundation Design Software (note: for a simplified calculator, try our free concrete footing calculator)
In the foundation design software, various values are inserted as input like type of foundation you want to design like isolated foundation, grade of concrete, grade of steel used and selection of the structural code to design as per country guidelines, for this case you can select ACI 318. Importing the column position and reaction data exported from the structural analysis software.
Some of the common design checks performed in the design of a concrete footing:
Overturning check is completed after finding overturning safety factor which is found by dividing the sum of resisting moments by the sum of overturning moment. Generally this factor should be greater or equal than 1.5.
Sliding check is completed after finding sliding safety factor which is found by coefficient of friction between the concrete and soil times the weight of the footing divide by the lateral forces acting on the footing. Generally this factor should be greater or equal than 1.5.
Structural Checks such as one/two way shear checks and flexure checks in both directions, to ensure the concrete structure is strong enough to support the forces being applied to it. These structural design calculations again depend on the design code (for instance the US use ACI 318).
After finding both safety factors of overturning sliding and knowing the coefficient of friction of soil and concrete, these values need to be inserted in design software to get the final design of footing foundation. From here, a competent engineer will try and reduce the amount of material used in the form of reducing the concrete and/or steel, while still maintaining the minimum requirements as set out by the design code.
Engineers can experiment with different sizes of the foundation, reinforcement arrangement and quantity required to find a result that makes a design more economical, without compromising the structure’s strength or safety. It is common to look at the governing result and determine why the structure is failing, then adjust some input (reinforcement, footing dimension) to improve the design.
The footing Foundation design process depends upon various structural processes. These include a soil investigation, running structural analysis of the model structure to get column reactions, design of the foundation and finally optimizing the design. Although, this is a very simplified explanation of the steps involved, it should give a good indication of the process.