The plans give a total load of 2,400 pounds per lineal foot on a continuous footing and a presumptive soil load-bearing value of 1,500 pounds per square foot. The crew has formed the footing 16 inches wide. What is the correct finding?
- AThe footing is adequate as formed, since 16 inches of width is already twice the nominal thickness of the foundation wall standing on top of it.
Twice the wall thickness is a familiar rule of thumb for footing projection, and it says nothing about the soil. Width has to be checked against the load per foot of wall and the bearing value of the soil, which is the calculation here.
- BThe footing must be 24 inches wide, because a footing is always three times the thickness of the foundation wall standing above it.
Three times the wall thickness is a proportion someone remembered, not a code requirement. It happens to be conservative on strong soil and unconservative on weak soil, because it never looks at the load or at the bearing capacity.
- CThe footing is adequate as formed, provided the mix design is increased by 1,000 psi to carry the additional soil pressure.
Concrete strength governs the footing's own bending and shear, not how much soil it needs beneath it. A stronger mix in a 16-inch footing still delivers the same load onto the same area of ground.
- The footing is undersized, because roughly 19.2 inches of bearing width is needed to keep soil pressure within 1,500 pounds per square foot.
Why D is correct
Required width equals load per foot divided by allowable soil pressure: 2,400 divided by 1,500 gives 1.6 feet, or 19.2 inches, so the practical minimum is 20 inches. The 16-inch form leaves only 1.33 square feet of bearing per foot of wall and pushes soil pressure to roughly 1,800 pounds per square foot, above the value the design relied on.
What this question is testing
Running the basic bearing calculation in the direction an inspector actually needs it, from a known load and a stated soil value to a required width, and then comparing that width to what is formed on the ground. It also tests recognizing that material strength and bearing width solve different problems.
On the job
Soil bearing shows up in inspection when a plan is based on a value the site does not deliver, or when the crew simply forms to the trencher width they own. If the plan lists a presumptive value and the trench bottom is obviously soft, wet, or filled, that is the moment to require a geotechnical evaluation rather than accept the number on paper. Widening a footing before concrete is a half-hour of shovel work; underpinning a settled corner is a structural repair.
Memory technique
Pounds per foot over pounds per square foot leaves feet: the units hand you the width.
Exam tip
Load divided by soil value gives square feet per foot of wall, which reads directly as feet of width; convert to inches last.
Where to look it up
IRC Chapter 4, footing sizing, and the presumptive load-bearing values table for soil classes.