Concrete Masonry Unit (CMU) Foundation Walls

By Dan Martel, P.E. | September 29, 2021

Concrete masonry units (CMUs), still often and affectionately referred to as cinder blocks, are a common building construction material. They are economical and are often the best solution for structural elements like commercial building walls and elevator shafts. They can also double as fire protection, so they make a great choice for structural walls between units in multi-unit residential and commercial buildings.

Cast-in-place concrete walls are far and away the most common foundation types for homes built today, but CMUs are still permissible by the Residential Code and often used. In the mid-1900’s, CMU foundation walls were very common, and unfortunately so are their shortcomings. This CMU wall foundation supporting a simple 2-story house is a great example.

 

We were called to assess horizontal cracks and wall movement. What we found is very common: this house was built in the 1970’s, and the CMU cells are not grouted, and there is no steel reinforcing inside the cells either. The walls are “pinned” at the bottom by the concrete slab, and since all four sides of the foundation are evenly backfilled, the walls are pinned at the top by the first floor diaphragm. The walls are bulging at the middle, one masonry course below the exterior grade, coincident with the highest point of lateral earth pressure. Without grout and reinforcing, the wall lacks the capacity to resist those lateral loads as its relying only on the grout in the horizontal joints between blocks – clearly inadequate. To mitigate, we designed a solution using a wood stud wall built inside the existing and engaging the floors above and below, which avoided reconstructing the foundation and costly excavation.

Concrete masonry units are still very much a viable material to build foundations – so long as they are designed correctly!

 

 

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