Strap Footing Design Guidelines

A strap footing is an element of a building’s foundation. It is a type of combined footing and comprises of two or more column footings which are attached with a concrete beam.

This type of beam is known as a strap beam. It facilitates to allocate the weight of either heavily or eccentrically loaded column footings to adjoining footings.

A strap footing normally supports two columns. When a property line subsists at or adjacent to the edge of an external column, a normal isolated footing is arranged eccentrically under this column and it has a tendency to tilt. The issue can be resolved by attaching this footing with the adjoining interior footing through a strap concrete beam.

It is suitable where the span among columns is greater and a normal combined footing can’t be provided because of the required large excavation. To simplify the construction process, normally the bottom surfaces of the exterior footing, the strap beam, and the interior footing are kept at equivalent elevation, but the thickness of each element varies based on the strength requirements.

The purpose of a strap footing is to attach an eccentrically loaded column footing to an internal column. The strap can transfer the moment resulting from an eccentricity to the internal column footing in order that a consistent soil pressure is developed under both footing.

The strap footing can be utilized in place of a rectangular or trapezoidal combined footing when the distance among the column is greater and/or permitted soil pressure is considerably higher to avoid supplementary footing area.

Also Read: Download Reinforced concrete Strip Footing Design Spreadsheet

This type of footing is ideal for two columns. The two columns are set by two individual footings attached with a inflexible beam known as ‘strap beam’.

The following basic concepts are required for making the design of strap footing :-

Strap should be inflexible – possibly Istrap/Ifooting > 2. This inflexibility is required to manage rotation of the exterior footing.

The footing should be uniform for maintaining roughly equivalent soil pressure and getting rid of greater variations in B to minimize differential settlement.

Strap should not be in touch with soil in order that there are no soil reactions.

Normally, the strap weight is not taken into consideration in the design. The strap should be sufficiently connected with both the column and the footing through dowels in order that the footing and the strap function as unit.

The footing is exposed to one-way bending.

The strap beam is reinforced with prime reinforcement at top among the columns and at bottom under the interior footing.

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