Foundation Wall Geometry
After 20 years of effort, the American Concrete Institute has published its first residential concrete code.
Not all design conditions are easily addressed or recognized by building codes. In the residential concrete industry, for example, are the common issues of footing discontinuity and reduced wall thickness. Extreme changes in site elevation and transitions from frost walls to basement walls (for example, a garage to a basement) result in designed wall steps at varying intervals. At these locations, vertical footings serve no structural purpose. Still other conditions exist where the designed wall must account for footings that are spanning trenches or poor soil, or that are interrupted by penetrations. ACI 332-04 provides support for these field conditions with sound design guidance. These transitions are considered discontinuities in the footing design –the foundation wall must therefore be designed to perform as a beam spanning from the end of the footing above to the beginning of the footing below. ACI 332 states:
"Where a wall footing is discontinuous due to an abrupt elevation change, the maximum horizontal discontinuity of the wall footing shall be 4 feet."
The code recognizes the need for and the historical success of footing discontinuity, such as at abrupt elevation changes in the foundation. ACI 332-04 limits the horizontal discontinuity, the distance between the footing ends, to 4 feet and requires a minimum of two No. 4 horizontal reinforcing bars, one at the top and the other at the bottom of the wall, in addition to other required wall reinforcement when the step is greater than twice the footing thickness. By providing reinforcement as stated above, the foundation wall spans the footing discontinuity and effectively serves as a structural beam, transferring the vertical load within that span to the areas of the wall directly over the two footing sections.
Equally important to today's builder and foundation contractor is the need to address brick ledges in the design of foundation walls. Brick ledges are reduced thickness regions along the tops of foundation walls deep enough to support the above-grade brick façade. The foundation wall carries not only the vertical loads of the framed home above, but the additional dead load of the entire brick façade. The code again responds to existing construction methods in a practical way, placing limitations on the depth of this reduced thickness and controlling the integrity of the foundation wall. ACI 332 states:
"The thickness of the top of a foundation wall shall be permitted to be reduced. The height of the reduced thickness section shall not exceed 24 inches."
This section of the code goes on to state that the thickness of the top of the foundation wall shall not be less than 3-1/2 inches –to allow a 4-inch recess in a nominal 8-inch wall. The code also states that when a foundation wall is 4 inches or less in thickness, it shall be reinforced vertically with No. 4 bars at 24 inches on center. By recognizing acceptable performance for reduced sections, ACI 332-04 allows greater flexibility in the economical use of 8-inch walls. Beyond 24-inch-high recesses, the walls would have to be thickened to provide a minimum 5-1/2-inch wall thickness.