NCMA Weighs in on Structural Engineering Q and A

There are some code-imposed height limits on CMU construction, but they are very tall and only start to kick in around 15-20 stories depending upon the seismic risk, building use, etc.

In 2011 when we (NCMA) changed ASTM C90 to permit alternative unit configurations, we concurrently had to check and revise some of the design equations for masonry to account for the wide array of unit configurations that previously did not fall under ASTM C90. I would not expect these design checks to result in substantial differences in the design strength of Omni versus a standard CMU assembly, but there are some differences. In order to have a ‘code-compliant’ structural design using Omni, an engineer would need to use the 2013 edition of TMS 402 (the structural design standard for masonry) that captures all this cross-standard coordination.

The 2013 edition of TMS 402 allows Omni Block to be ungrouted (or partially grouted), but the design checks may be different…but there are literally hundreds of factors that trigger different checks for different applications making it impossible to state that the design strength of a standard CMU assembly and an Omni Block assembly would be the same for every circumstance. They will be close, but there can be differences – if for no other reason than the difference in the cross-sections.

Structural design is more complicated as an engineer needs to consider both the impact to the net cross-sectional properties as well as the average cross-sectional properties. (The former is used for strength determination, the latter used for calculating deflections.)

We are in the process now of updating our structural design TEK to reflect the changes incorporated into the 2013 edition of TMS 402 (where these additional design checks were incorporated). The relevant requirements are shown below. Essentially an engineer would need to check the shear stresses in the webs of the units if the normalized web area is less than 27 in.^2/ft^2 (which corresponds roughly to a conventional three web unit). In the case of Omni Block, the critical web shear plane would need to be determined (which is likely to coincide with the location of the insulation inserts where the web area is minimum) and the corresponding web shear stresses calculated for the design loads.




Jason Thompson
Vice President of Engineering
National Concrete Masonry Association
13750 Sunrise Valley Drive
Herndon, VA 20171-4662