Masonry Magazine September 1972 Page. 20
Loads to which
Foundation Walls
are Subjected
Part of the dead load on foundation walls is imparted by floors and roofs and is usually transmitted at some point between the center line of the wall and the inner surface, thus introducing a bending moment. Additional moment will be introduced at any point where flexural members are restrained by their connection to the wall. Usually these induced moments tend to counteract the moments from lateral earth pressures at the exterior face. Vertical compressive stresses from dead loads are usually assumed to reduce the vertical tensile stresses that develop in resisting bending loads.
The pressures from wind that ordinarily affect foundation design are those that are transmitted indirectly through the superstructure: compressive, uplift, shearing or racking loads. Stresses developed in foundations in resisting over-turning are not often critical except for light superstructures subject to high wind loads or for superstructures that have a high ratio of exposed area to depth in the direction of wind flow. Such buildings should be provided with good anchorage of the superstructure. Special attention should also be given to the tensile strength of the foundation and if necessary to the racking modulus or horizontal shearing resistance of the walls parallel to the wind flow.
Moments and shears from vertical or lateral loads on foundation walls are calculated by the usual methods.
Strength and Stability
The strength and stability that will be achieved in foundation walls depends on wall height and thickness, mortar bond strength, lateral support from crosswalls, pilasters or other wall stiffeners, stiffening provided by the first floor framing, and the vertical dead loads on the wall.
Recommended thicknesses of nonreinforced concrete masonry foundation walls backfilled on one side are given in Table 1 for residential construction and average soils. Where building officials approve, however, because soil conditions and local experience justify the practice, unbalanced heights of seven feet may be accepted for all of the combinations shown in the table.
If earth presures in the locality are known to be strong, block pilasters can be laid up with the wall to help strengthen it. Their length along the wall should be about one-tenth the distance between supports and they should project from the face of the interior wall about one-twelfth of the wall height. An alternative is to grout a Number 4 reinforcing bar in one core of the block for a full height of the wall to serve as a wall stiffener.