Strap footing
An exterior column at a property line, on an eccentric footing, tied by a strap beam to an interior footing. The strap holds the exterior footing level, so its reaction acts at its center: more than the column load, by what the interior footing is relieved. Use it where a combined footing would be too long.
ACI 318-19 has no section on straps. The strap is a beam by Chapter 9, each footing is by Chapter 13, and the rigid body statics are the method in the textbooks.
Inputs of the strap footing #
- Column spacing, footing sizes and offsets. Column 1 is the exterior one. e1 runs from column 1 to the center of the exterior footing, toward column 2. e2 runs from column 2 to the center of the interior footing, usually 0.
- Strap width and depth. The strap is drawn with its top at the top of the footings.
- Columns: sizes along the strap and across, and their concrete strength.
- Column loads: per column and load type, unfactored, at the top of the footing. M acts along the strap, and positive moves the resultant toward the interior column. M across acts out of the plane of the strap, and tilts the pressure across the footing.
- Soil: cover, unit weights, allowable pressure as gross or net.
- Reinforcement: strap bars, hooked or straight at their far ends, stirrups, and the bars of each footing both ways.
- Size effect: neglected for the footings by default (13.2.6.2). The strap uses Table 22.5.5.1 with stirrups.
Auto-design varies the footing widths (1/2-ft steps), thicknesses and strap depth (2-in. steps), the strap bars, the stirrup spacing and the footing bars. The column spacing, footing lengths and offsets are kept: they set where the footing sits at the property line.
Checks performed #
| Check | Basis |
|---|---|
| Reactions | Rigid body statics, about the center of each footing, with the weights of footings, soil and strap |
| Uplift, resultant within the footing | 13.2.6.1 at every ASD combination |
| Soil bearing, each footing | 13.3.1.1, uniform along the strap, with the moment across it |
| Strap, top and bottom flexure | 22.2, 9.6.1.2. Moments along the whole length under the factored column loads, the net reactions and the strap's own weight |
| Strap, shear with stirrups, section limit, stirrup spacing, minimum stirrups | 22.5, 9.6.3, 9.7.6.2.2. At d from the column faces |
| Exterior footing, flexure and one-way shear across the strap | 13.3.3, 13.2.7.2. A cantilever from the strap face |
| Exterior footing, bars along the strap | 7.6.1.1, 7.7.2.3. Minimum and spacing |
| Interior footing, flexure and one-way shear both ways | 13.3.3, 13.2.7.2. From the column faces |
| Interior footing, two-way shear | 22.6, 8.4.4.2. The section takes the soil reaction outside it |
| Bearing of the columns | 22.8, without the frustum increase |
| Development of strap and footing bars | 25.4 |
| Strap stiffer than the footing | Advisory: the strap's I over the exterior footing's I above 2, a textbook practice and not ACI |
Limitations #
- The rigid strap is the model. It takes no soil pressure, and each footing's reaction stays at its center along the strap. Whether your strap is rigid enough, and how to keep it from bearing on the soil, is your judgment; the advisory gives the usual ratio.
- Column 1 sits at the end of the strap, over the exterior footing. Its load goes to the strap, so it has no two-way shear check here. A column much wider than the strap needs your own check of that load path.
- At column 2 the two-way section takes the soil outside it, by equilibrium, because the strap carries its force to the column region. This is a reasoned reading, not a code clause: check it if a reference disagrees.
- The strap section is used alone inside the footings for moment and shear, which is conservative where the footing would help.
- Lateral forces on the footings, sliding and transverse beams at the exterior footing are not covered. Enter shears at the top of the footing as moments if you need their effect.
- Dowels are stated, not designed.
Verification examples #
Verification status: 4 values compared in 1 example (1 published, 0 hand calculations from the cited equations): 0 within tolerance, 4 recorded deviations. Thin: only 1 published example. None of the compared values is within tolerance; the reasons are below. Code basis: partial. Only some provisions were read in the target edition; the rest rests on older editions, examples or summaries.
The textbook examples (Bowles, Foundation Analysis and Design; Wight, Reinforced Concrete: Mechanics and Design) were not on hand. One free example was found, and it is only a partial check:
| Example | Source | Values |
|---|---|---|
| Exterior column at a property line, service pressures | ASDIP, Strap Footing Example Using ASDIP FOUNDATION (2024), a software output read from its screenshots | Pressures 3.2 / 2.3 and 3.5 / 2.5 ksf. Differences documented below. The spread across the footing agrees |
The post does not state the soil weight and shows the water table at the surface, so the weights are not known and the pressures differ, by up to 6 %. Those are documented differences, not fitted. The split of its live, roof and wind loads is not stated either, so its factored strap diagrams cannot be run.
Everything else is a hand calculation in the tests: the reactions, the strap moment under the footing, the footing moments and shears, the moment across the strap, uplift, and the net bearing. A published, complete strap example is still needed.