Combined footings are constructed for two or more columns when they are close to each other and their foundations overlap. Design of combined footings with example is discussed.
Function: The function of a footing or a foundation is to transmit the load form the structure to the underlying soil. The choice of suitable type of footing depends on the depth at which the bearing strata lies, the soil condition and the type of superstructure.
Why provided:
1- When two columns are close together, causing overlap of adjacent isolated footings
2- Where soil bearing capacity is low, causing overlap of adjacent isolated footings
3- Proximity of building line or existing building or sewer, adjacent to a building column.
Steps for Design of Combined Footing (For Manual Calculations)
1- Locate the point of application of the column loads on the footing.
2- Proportion the footing such that the resultant of loads passes through the center of footing.
3- Compute the area of footing such that the allowable soil pressure is not exceeded.
4- Calculate the shear forces and bending moments at the salient points and hence draw SFD and BMD.
5- Fix the depth of footing from the maximum bending moment.
6- Calculate the transverse bending moment and design the transverse section for depth and reinforcement. Check for anchorage and shear.
7- Check the footing for longitudinal shear and hence design the longitudinal steel
8- Design the reinforcement for the longitudinal moment and place them in the appropriate positions.
9- Check the development length for longitudinal steel Curtail the longitudinal bars for economy
10- Draw and detail the reinforcement
11- Prepare the bar bending schedule
For Software calculations shown in figure.
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