Thickness of Poured Concrete Foundation Walls
DEAR TIM: How thick should a poured concrete foundation wall be? Is it a function of the size of the house? Bob McKnight, Hagerstown, MD
DEAR BOB: The answer, although fairly intuitive, is not as crystal clear as you might think. As a poured concrete wall gets taller, it needs to be thicker. But there are many other variables that control the thickness of the wall. A structural engineer would consider lateral loads as well as loads from the structure above. Even concentrated loads from columns and beam pockets within a wall must be considered.
To help visualize what is really happening with foundation walls, especially those that are buried in the ground, think about simple retaining walls. Perhaps you have seen a retaining wall that has tipped over, was leaning or has cracked. The soil on the other side of a basement wall exerts a powerful force and this needs to be taken into consideration when designing and building a foundation wall.
To add further confusion, you also have to factor in reinforcing steel. Steel bars that are incorporated into poured concrete add enormous strength to the wall system. The placement of the steel is critical depending upon how you are trying to strengthen the wall.
For example, if soil loads are significant and a house is built into a hillside, vertical reinforcing steel is a must. The pressure of the soil creeping down the hill can cause a foundation wall to develop a horizontal crack much like when you snap a saltine cracker in half with your fingers. Vertical steel bars of a given thickness spaced a distinct distance apart and placed at a precise place within the wall can help ensure the wall will not fail. Structural engineers know exactly where the steel needs to be and how much to include.
There are some basic rules of thumb for foundation wall thickness and these are outlined in many modern building codes. Poured concrete foundation walls that are less than 8 feet tall and have soil outside that is 6 or 7 feet deep against the wall can often be 8 inches thick and function quite well. As soon as you go higher or have greater depths of soil pushing against the wall, you need to increase the thickness to 10 inches.
Don't forget that the concrete comes in different strengths depending upon how much cement is added at the ready-mix plant. I would use a minimum mix of 3,500 pounds per square inch(psi) mix. You can upgrade to 4,000 psi concrete if you desire, but I would only do this if it was specified by the structural engineer.
One last point: Remember that the specifications in the building code are minimum standards. You can always improve upon the guidelines you see in the code.