A proper understanding of the transfer of stresses(generated due to applied loads) from columns to the footings or piles is crucial for the design of sound footings. Factored forces and moments at the base of columns are transferred to the supporting foundations by bearing on concrete and by the extension of longitudinal reinforcement into the footing or provision of dowels.
Either technique should be executed based on the specifications of the applicable design codes such as ACI 318-14. The detailing of reinforcement shall be executed adequately for proper distribution of stresses to footings and then further over the underlying soil layer.
How Stresses are Transferred from Columns to Footings?
1. Extending the Longitudinal Steel into the Footing
- Extended reinforcement shall be designed to transfer compressive forces that exceed the lesser of the concrete bearing strengths of either the supported member or the foundation, and any calculated tensile force across the column-footing interface.
- At least four bars should be extended.
- The stress-transfer bars should project into the base a sufficient compression-embedment distance to transfer the stress in the column bars to the base concrete.
- In any case, a minimum steel area of 0.5% of the column-area should be supplied for load transfer.
- Dowels shall be designed to transfer compressive forces that exceed the lesser of the concrete bearing strengths of either the supported member or the foundation, and any calculated tensile force across the column-footing interface.
- At least four dowels should be used.
- The dowel diameter should not exceed the column-bar diameter by more than 0.15 in.
- If the required dowel length is larger than the footing depth less 3 in, either smaller-diameter bars with the equivalent area should be used or a monolithic concrete cap should be added to increase the concrete depth.
- The dowels should provide at least one-quarter of the tension capacity of the column bars on each column face.
- The dowels should extend into the column a distance equal to that required for compression lapping of column bars.