Strip, Combined, and Strap Footings

Why These Footings Are Used

Continuous, combined, and strap footings solve load-distribution problems that isolated pads cannot handle efficiently. They are particularly useful below bearing walls, closely spaced columns, and exterior columns near property lines.

These systems share soil-structure interaction across more than one reaction. That can improve load distribution, but it also makes stiffness, differential settlement, and construction sequence more important.

Continuous Strip Footings

A strip footing supports a wall or a row of reactions and is commonly designed per unit length. For a centered service line load $w$ and allowable service pressure $q_{\mathrm{allow}}$:

This relation assumes reasonably uniform pressure and does not include local concentrated loads, wall openings, changes in grade, pilasters, or stiffness transitions. The structural footing behaves as a beam on soil, and local shear and flexure may govern near concentrated reactions.

Strip footings are common under masonry and concrete walls. They can also support repetitive columns when the longitudinal footing has enough stiffness and reinforcement to distribute load.

Combined Footings

A combined footing supports two or more columns on one concrete element. It is useful where separate footings would overlap or where an exterior column cannot be centered on an isolated pad.

For uniform service pressure, the centroid of the effective footing area should align with the resultant of the supported vertical loads. Measured from a reference axis:

A rectangular combined footing works when the required area centroid can be aligned with the resultant using a rectangular plan. A trapezoidal plan can shift the area centroid where column loads or boundary constraints make a rectangle inefficient.

The soil reaction, column loads, and footing self-weight form a continuous beam problem. Longitudinal shear and moment vary between columns and toward the footing ends. Two-way effects, punching around columns, transverse cantilever action, and reinforcement anchorage still require checking.

Strap Footings

A strap footing uses separate column pads connected by a stiff strap beam. The strap transfers moment from an eccentrically loaded exterior pad to an interior pad, allowing the soil pressures to be brought into a more acceptable range.

In the common idealization, the strap beam is designed not to bear on soil. A compressible void, formed gap, or other detail may be needed if unintended soil reaction would change the load path. If the strap is intentionally supported by soil, it should be modeled as a grade beam or continuous foundation rather than using the no-bearing assumption.

The strap must have enough stiffness and strength to transfer shear and moment. Connection zones at the pads deserve careful anchorage and construction detailing.

Geotechnical Limitations

• Uniform soil pressure is an idealization; actual pressure depends on footing stiffness and spatially varying subgrade response.
• A long strip or combined footing can cross more variable ground than one isolated pad.
• Differential support, utility trenches, old foundation excavations, and localized fill can create concentrated demands.
• Bearing pressure and settlement should be checked at relevant sections, not only as total load divided by total area.
• Exterior excavations and adjacent foundations can limit available passive resistance and alter stress distribution.

Selecting Among The Three

Use a strip footing for continuous wall loads or a regular line of reactions where longitudinal distribution is beneficial. Use a combined footing where two or more pads would overlap or where the load resultant can be aligned with one practical slab. Use a strap footing where separate pads are desirable but one pad is eccentric because of a boundary.

A mat may be more efficient when combined footings proliferate or occupy much of the footprint. Deep foundations may be needed when these wider elements still cannot satisfy settlement or resistance criteria.

Design Coordination Checklist

• Provide unfactored and factored column reactions at consistent reference elevations.
• Include column moments, shear, uplift, and load combinations.
• Define whether the strap beam bears on soil.
• Coordinate property lines, utilities, pits, sumps, walls, and waterproofing.
• Check settlement compatibility between connected supports.
• Specify bearing-surface preparation and observation requirements.
• Revisit load distribution if column stiffness or construction sequence changes.

References And Further Reading

• FHWA GEC 6, Shallow Foundations.
• USACE EM 1110-1-1905, Bearing Capacity of Soils.
• Current governing structural concrete standard.
• Project-specific geotechnical report and foundation recommendations.

FAQ

Is a strap beam the same as a grade beam?

Not necessarily. A classic strap beam transfers load between pads and is idealized as not bearing on soil. A grade beam may be designed for soil support, to span between supports, or for a combination of actions.

Why use a trapezoidal combined footing?

The varying width moves the centroid of footing area toward the larger end, helping align soil reaction with an unequal or constrained column-load resultant.

Can one allowable pressure be used along a long strip footing?

Only if the geotechnical recommendation reasonably represents the materials and conditions along its full length. Significant variability may require zoning, treatment, or a more explicit analysis.