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questions we made up about ... Masonry Question: Recent changes were made to ASTM C90 related to the minimum web requirements for loadbearing concrete masonry units (CMU). What, if any, impact do these changes have on the design of concrete masonry assemblies?

Answer

For the vast majority of loadbearing concrete masonry construction, there is little difference in the resulting design methodologies or assumptions when using CMU meeting the new ASTM C90, Standard Specification for Loadbearing Concrete Masonry Units. However, there are some nuances that structural engineers should be cognizant of if opting to specify alternative unit configurations now permitted under the new ASTM C90 standard. The Table summarizes the new minimum unit configuration requirements contained in ASTM C90. Compared to historical versions of this standard, the minimum web Changing Masonry Standards thickness for all unit sizes is now 0.75 inches where previously it varied Answer provided by Jason from 0.75 inches to 1.125 inches, dependThompson, Vice President of ing upon the nominal thickness of the unit. Engineering for the National Additionally, the equivalent web thickness Concrete Masonry Association. (which is the summation of the thickness of Mr. Thompson is responsible for each web in a unit normalized per unit length) overseeing the technical activities, has been replaced with a normalized web area, services, and research for the which cannot be less than 6.5 in.2/ft2 for all Association. He is also a Fellow units. The drawback of the historical equivalent of the Masonry Society. web thickness requirement was that it did not capture possible variations in the height of the web, which is commonly reduced or notched for various reasons. The normalized web area sets a minimum value for the web to connect the face shells of a unit. It is important to stress that the revisions to ASTM C90 do not require unit configurations to be changed; instead, they permit more flexibility in unit configuration to meet evolving market-driven demands. Any unit configuration that met historical ASTM C90 requirements will continue to comply with contemporary versions of this standard. Of primary importance to structural engineers, new unit configurations used in reinforced and grouted construction will still be structurally modeled and designed as they have been in the past, taking into account the appropriate section properties to reflect the cells containing grout and reinforcement. However, for unreinforced/ungrouted masonry, the thinner webs permitted under the latest ASTM C90 standard can impact the resulting section properties of the assembly, and therefore the resulting assembly design strength. Since unreinforced, loadbearing masonry construction is rarely used any longer in the United States, these change will have little effect structurally. However, for those engineers still designing with unreinforced masonry, such structural impacts should be considered. So why change the iconic configuration of the concrete masonry unit? The short answer is that this change has already occurred in the marketplace, resulting in unit configurations such as H-Block, bond beam units, lintel units, and multi-purpose units that have evolved to meet specific project needs. Depending upon one’s perspective, these changes to ASTM C90 offer several potential benefits, including reducing unit weight (reducing structural dead load and increased construction productivity) as well as substantially increasing the thermal R-value of concrete masonry construction.

New ASTM C90 Requirements for Loadbearing Concrete Masonry Units. Minimum Face Shells and Web RequirementsA

Webs

Nominal Width (W) of Units, in. (mm) Face Shell Thickness (tts), min. in. (mm)B,C Web ThicknessC (tw), min. in. (mm)

Normalized Web Area (A nw), min. in.2/ft2 (mm2/m2)D 3 (76.2) and 4 (102) 3/4 (19) 3/4 (19) 6.5 (45,140) 6 (152) 1 (25) 3/4 (19) 6.5 (45,140) 8 (203) and greater 11/4 (32) 3/4 (19) 6.5 (45,140)

A Average of measurements on a minimum of 3 units when measured as described in Test Methods C140. B When this standard is used for units having split surfaces, a maximum of 10% of the split surface is permitted to have thickness less than those shown, but not less than 3/4 in. (19.1 mm). When the units are to be solid grouted, the 10% limit does not apply and Footnote C establishes a thickness requirement for the entire faceshell. C When the units are to be solid grouted, minimum face shell and web thickness shall be not less than 5/8 in. (16 mm). D Minimum normalized web area does not apply to the portion of the unit to be filled with grout. The length of that portion shall be deducted from the overall length of the unit for the calculation of the minimum web cross-sectional area.

Mike Mota, Ph.D., P.E., F. ASCE is stepping down as a member of the STRUCTURE® magazine Editorial Board. Mike joined the Editorial Board in the spring of 2008 as a concrete industry representative. He is the Atlantic Region Manager for the Concrete Reinforcing Steel Institute (CRSI). Mike is an Adjunct at Drexel University and is an active member of several ACI and ASCE committees and serves on the Board of Directors of the Concrete Industry Board of New York City/NYC ACI. Jon Schmidt, Chair of the STRUCTURE magazine Editorial Board, had this to say on Mike’s departure: “Mike has been a valuable member of the Editorial Board for the last five years. I wish him well as he continues to serve CRSI and the structural engineering profession.” Regarding his tenure on the Board, Mike commented, “I would like to take this opportunity to say farewell to the fellow members of the Editorial Board of STRUCTURE magazine and to thank my employer (CRSI) for the opportunity afforded to me to volunteer during the past five years. This time has been extremely gratifying both personally and professionally. I am pleased that Amy Trygestad of Chase Engineering has accepted to represent the concrete industry on the Editorial Board. The Board will be well served and the Concrete industry well represented. I will continue doing what I do best which is to provide technical assistance to structural engineers on all areas of reinforced concrete design and construction.” Amy M. R. Trygestad, P.E. will replace Mr. Mota. Ms. Trygestad manages her own structural engineering consulting firm near Minneapolis, Minnesota. She is a licensed Professional Engineer with over 17 years of experience in the structural design and construction industry. Her experience has touched many facets of the engineering and construction industry, including engineering consulting, program management, concrete subcontracting, ready-mixed concrete production, and strategic marketing and technical enhancement of concrete construction. She previously worked for the Portland Cement Association as their Building and Special Structure’s Regional Engineering Manager for the Central United States.

Mike Mota, Ph.D., P.E., F.ASCE Amy M. R. Trygestad, P.E.

Her area of expertise is post-tensioned concrete design and construction, and she actively serves on the American Concrete Institute committees for Parking Structures, Prestressed Concrete, Reinforced Slabs, and Formwork. Jon Schmidt said this about Ms. Trygestad’s appointment: “I am pleased to welcome Amy to the Editorial Board. She is well-positioned to serve as an outstanding representative of the concrete industry and provide useful content for our readers.” Please join the STRUCTURE magazine Editorial Board in welcoming Amy Trygestad.

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