What is the thickness requirement for a plain masonry foundation wall that is 8 feet tall with an unbalanced backfill height of 5 feet 6 inches in Seismic Design Category C?

For a plain masonry foundation wall that is 8 feet tall with an unbalanced backfill height of 5 feet 6 inches in Seismic Design Category C, the thickness requirement is 10 inches. This requirement takes into consideration the lateral earth pressure exerted by the unbalanced backfill, as well as additional seismic forces that may act on the structure.

In seismic design categories, structural elements must have additional strength and stability to resist both static loads and dynamic forces induced by seismic activity. A foundation wall of this height and backfill condition necessitates increased thickness to ensure structural integrity and support.

The thickness of 10 inches is thus specified in building codes for walls subject to significant pressures. This thickness provides the necessary resistance against the lateral loads while also accommodating the additional safety factors associated with seismic conditions. Therefore, when determining the appropriate thickness for foundation walls in earthquake-prone areas, the design codes require a minimum of 10 inches to ensure safety and stability for the structure in question.