Spread Footing Sizing Example

Problem Statement

Size a preliminary rectangular spread footing for a service column load of $360\ \mathrm{kip}$. The geotechnical report permits a gross allowable service pressure of $4.0\ \mathrm{ksf}$ for the proposed bearing material and footing size range. Use a 10 percent allowance for footing and supported soil weight and a trial length-to-width ratio of 1.25.

This example addresses plan area only. Settlement, eccentricity, sliding, uplift, and structural concrete design remain separate checks.

Step 1: Confirm The Design Basis

The load and resistance format is compatible: service load is compared with allowable service pressure. The pressure is stated as gross, so included foundation and backfill weight should be represented consistently.

The design load used for sizing is:

An allowance is useful for preliminary sizing, but final pressure should use calculated footing, pedestal, and soil weights rather than a percentage where those quantities are available.

Step 2: Calculate Required Area

This area satisfies the average gross service-pressure screen before practical rounding.

Step 3: Calculate Trial Dimensions

With $r=L/B=1.25$:

The mathematical minimum is approximately $8.90\ \mathrm{ft}$ by $11.13\ \mathrm{ft}$. Construction dimensions should be rounded up, not down.

Step 4: Select Practical Dimensions

Adopt a trial footing of $9.0\ \mathrm{ft}$ by $11.5\ \mathrm{ft}$:

The resulting average gross service pressure is:

The rounded footing passes the average-pressure screen because $3.83<4.0\ \mathrm{ksf}$.

Step 5: Identify The Remaining Checks

The conclusion is not "footing designed." It is "trial plan dimensions established." The next checks are:

• Resolve service and factored moments at footing-base elevation.
• Calculate all corner pressures and confirm full or partial contact.
• Verify settlement using the final width and ground profile.
• Check bearing resistance, sliding, overturning, uplift, and nearby excavation effects.
• Select footing thickness and reinforcement for punching shear, one-way shear, flexure, bearing, and development.
• Confirm founding elevation, groundwater, frost depth, durability, and construction access.

Sensitivity To Allowable Pressure

If the applicable allowable pressure were only $3.0\ \mathrm{ksf}$, the required area would become:

At the same aspect ratio, width would increase to about $10.28\ \mathrm{ft}$ and length to about $12.85\ \mathrm{ft}$. This illustrates why the pressure basis and settlement criterion should be verified before structural detailing advances.

Engineering Interpretation

The adopted geometry has about 4.5 percent more area than the mathematical minimum. That modest margin addresses dimension rounding, but it is not a substitute for uncertainty evaluation. If column moments are large, maximum corner pressure can exceed $4.0\ \mathrm{ksf}$ even though average pressure does not.

Use the Footing Size Calculator to reproduce the trial dimensions, then move to the Footing Contact Pressure Calculator with the simultaneous column load and moments.

References And Further Reading

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

FAQ

Why add 10 percent before sizing?

It is an illustrative preliminary allowance for footing and supported soil weight. Final design should calculate those weights and remain consistent with the report's net or gross pressure convention.

Why not adopt 9 by 11 feet?

That area is $99\ \mathrm{ft^2}$, exactly the calculated minimum with no rounding margin. The selected $9$ by $11.5$ foot trial stays above the required area.

Is the aspect ratio fixed by geotechnical design?

Not usually. It may be selected to suit column geometry, moment direction, property constraints, and structural efficiency, then checked for pressure and settlement.