ASDIP FOUNDATION is a structural engineering software for footing design. It includes the design of concrete strap footings based on the latest ACI 318 provisions. This article provides engineering background on the design of strap footings when subjected to a combination of vertical and horizontal loads, and bending moments.
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What is a strap footing?
A strap footing usually supports two columns, so it’s a special type of combined footing. If a property line exists at or near the edge of an exterior column, a normal isolated footing would be placed eccentrically under this column and it would tend to tilt. This problem may be prevented by connecting this footing with the adjacent interior footing with a strap concrete beam. The use of a strap footing may be justifiable where the distance between columns is long and a regular combined footing is impractical due to the required large excavation.
The images below show the general case of a strap footing with two eccentric columns. For construction simplicity, it is common practice that the bottom surfaces of the exterior footing, the strap beam, and the interior footing be at the same elevation, but the thickness of each element may be different, depending on the strength requirements.
How do you calculate the soil pressures under a strap footing?
In the design of a strap footing the system is assumed infinitely rigid, and the longitudinal bending moments are resisted entirely by the strap beam. As a result, the bearing pressures under the footings are always uniformly distributed. The calculation of the bearing pressure implies the calculation of the upward reaction force at the center of the footing by taking moments about the other column. Note that by doing this the system actually works as a unit and some loads could be transferred from one footing to the other, for example the exterior reaction might not necessarily equal the exterior loads.
Once the service reactions are calculated, the two footings may be sized so that the resulting bearing pressures are under the allowable limits established in the soils report. ASDIP FOUNDATION accurately calculates the soil bearing pressures under the exterior and interior footings for the applied service loads, as shown in the picture below.
How are the longitudinal stresses calculated?
A strap footing is usually analyzed as a beam in the longitudinal direction. The factored shear and moment diagrams help to visualize the variation of the internal forces along the strap footing, and therefore the required strengths. The first step is the calculation of the reactions and bearing pressures under the two footings for the applied factored loads. The bearing under the strap beam is neglected. The shear and moment diagrams are then generated as the internal forces required to maintain the equilibrium along the system.
The image below shows the shear and moment diagrams generated by ASDIP FOUNDATION. Note that the strap beam is subject to a constant shear and a linearly varying moment. For this reason this system is called either strap footing or cantilever footing. The moment is mostly negative along the beam, therefore the main reinforcing steel should be placed at the top of the beam.
Calculation of the shears and transverse bending
In the transverse direction the two footings are designed as spread footings. The shear stresses should be resisted by the concrete, otherwise a special and expensive shear reinforcement would be necessary. Two separate checks are required by the ACI: one recognizes that the footing may fail in shear as a wide beam along a critical section at a distance d from each column face in each direction. The second check considers that the columns may penetrate, or punch, the footing at a distance d/2 all around the columns.
Out-of-plane applied moments, if they exist, will not affect the strap beam design. The resulting upward bearing pressure will produce transverse bending moments on the footings, being the critical section the face of the columns, similar to the bending design of a spread footing. Once the bending moments are found, the reinforcing steel may be designed per the concrete design theory.
It should be noted that the bearing pressures are calculated using service loads, but both shear forces and bending moments must be calculated by applying the factored loads. The images below show the typical reinforcement in a strap footing.
Strap footing are useful when one footing is eccentric and it needs to be connected with adjacent footing. The design of such a system may be cumbersome and time-consuming, and the calculation of bearing pressures, shears, and moments in both directions may become complex. ASDIP FOUNDATION includes the design of strap footings, with multiple options to optimize the design in minutes.
For a discussion of the bearing pressures calculation, see Strap Footing: How to Calculate the Bearing Pressures. For a footing design example, please see the blog post Strap Footing Design Example Using ASDIP FOUNDATION. For our collection of blog posts about foundation design please visit Structural Footing Design.
Detailed information is available about this structural engineering software by visiting ASDIP FOUNDATION. You are invited to download the Free 15-day Software Trial, or go ahead and Place your Order.
Javier Encinas, PE
ASDIP Structural Software