Friday, October 11th, 2013

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Subtle Considerations in Design of CMU Construction

by John McDonald

Masonry structures have been constructed for several thousand years, and, as a material has been used worldwide for building and for ornamental purposes. About a century ago concrete block masonry units (CMU) were introduced in the building community; and it was not until around the 1940s that reinforcing steel began to be placed in masonry construction.

The four principal materials used in reinforced masonry are the masonry units, mortar, grout and the reinforcing steel. Masonry units are available in a variety of sizes, shapes, colors, and textures. The masonry unit, mortar & grout are combined to form the ‘masonry assembly’ whose compressive strength is given the symbol f’m. CMU has an impressive compressive strength (think pyramids of Egypt, Great Wall of China, Monadnock Building in Chicago, etc.) and this is the characteristic by which masonry is generally known, and used in design. While it does have defensible flexural capacities, masonry is known for its ability to carry compressive loads. While you may see several elevated two-way concrete slabs (another material known for its compressive carrying ability), you won’t see too many elevated masonry slabs.

Through analysis, the structural engineer will determine the size of the block, and the specified or required compressive strength, f’m. Often the Architect will be keenly interested in the color and texture of the concrete masonry units, as well as the actual size. At first blush, the color of the actual block would not appear to have any effect on the structural design. However, as a general rule, the lighter colored CMU blocks are often limited in their compressive strength. If you are pushing the structural design to the limits of the masonry material, the reduced capacity due to a light color may make a difference in the viability of meeting the demands. Always check with the local manufacturer or supplier for the properties and availability of the desired units.

And keep in mind, that the strength developed in CMU assemblies can depend on many factors, including materials and workmanship.

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