Bonding with Masonry 2025 Q2

Words: David BiggsThis issue’s questions come from an Architect and an Engineer. What questions do you have? Send them to info@masonrymagazine.com, attention Technical Talk.

Q. An Architect asks about anchoring coping stones on a cavity wall with brick veneer and CMU backup. Specifically, she asked if it is appropriate to bear on both the brick veneer and the CMU backup. Don’t the bricks expand and the CMU shrink?

A. Good question! The answer to your question is yes, it is appropriate to bear on both, but only under specific conditions.

As you point out, the bricks will expand vertically, and the CMU will shrink. It is important to recognize the possible differential movement between the two materials.

a. For a small differential movement, the coping could be set on the CMU and brick. So, what is small? As an example, if we have an 18-foot tall brick veneer (above grade or from the closest shelf angle) and a 3-foot CMU parapet on the roof of a steel-framed building, the brick growth would be approximately 1/8” using TMS 402 criteria for irreversible moisture expansion and a 60°F thermal expansion. The CMU shrinkage and thermal would be about zero. The net result is a differential of 1/8-inch. That seems insignificant.

b. What is a large differential movement? If a three-story load-bearing building had no shelf angles, the brick expansion (over 40-foot height) would be approximately 1/4 inch. The movement of the 40-foot CMU would be approximately zero due to its offsetting shrinkage and thermal expansion. The net difference between the two materials would be 1/4 inch.

If thermal change is contraction rather than expansion, the numbers change to a differential of almost 3/8 inch. That could rock the coping and split the sealants. In this case, I would suggest not bearing the coping on both the veneer and the CMU.

There are details for both conditions. One good reference is the article “Improved Technique for Flashing and Anchoring Stone Coping” by Jeff Snyder in Storypole magazine, September/October 2003. The magazine does not exist any longer, but the article lives on digitally (https://www.masonpro.com/wp- content/themes/masonpro/pdfs/Improvedtechniques.pdf).

For the small differential movement solution, Jeff Snyder provides the following figure. In it, he shows the coping bedded in mortar but sealed around the edges.

Fig. B by Jeff Snyder from Storypole magazine, September/October 2003

The International Masonry Institute has an excellent masonry Detailing Series in SketchUp on the 3d Warehouse developed by Scott Conwell, FAIA. (https://3dwarehouse.sketchup.com/by/InternationalMasonryInstitute#models)

Figure 0900 Top of Wall by IMI on 3d Warehouse for Sketchup (courtesy of IMI)

Rather than bedding the coping in mortar, the IMI coping is set on shims that bear over the veneer and the CMU. The joints between the flashing and the coping are sealed and weeped for drainage.

For the large differential solution, Jeff Snyder provides a detail with a supplemental angle to support the coping over the cavity space, but not on the angle. There is a gap between the brick and the flashing.

Fig. E from Jeff Snyder in Storypole magazine, September/October 2003

The following detail is a modification of the IMI detail shown previously. Here, an angle is shown bolted to the CMU for added support of the coping over the cavity. The shims are located over the angle and the CMU, not the brick. The insulation and one coping stone are removed in the figure for clarity. In addition, the gap (see arrow) below the flashing allows differential vertical movement.

Summary:

Differential vertical movement between the brick veneer and CMU backup can affect several aspects of a wall, including coping.
Designers need to consider differential movements in selecting details.
The coping should be isolated from the veneer if there are large differential movements anticipated.

Q. An Engineer writes that he must design a large opening in a warehouse. The opening can have a pier for partial gravity support, but it is not adequate to take lateral load as well. His concern is how to distribute the lateral load above the openings.

A. This is a common problem with warehouses and large retail buildings.

One example is shown in the next photograph. The wall is load-bearing. The interior pier (shown in red box) supports gravity loads, but cannot support the out-of-plane lateral loads.

Warehouse wall with a large opening when ignoring the pier

The wall is designed for gravity loads using a portion of the masonry wall as a deep beam (see red shading in the next photograph). The entire wall was not needed as a deep beam as allowed by TMS 402 (see Figure CC-5.3-2).

In this building, the end piers support the vertical loads along with the interior pier. If the interior pier were eliminated, the end piers and the deep beam could be designed to support all the gravity loads.

Deep masonry beam

Figure CC-5.3-2 from TMS 402-22 (Courtesy of TMS)

For lateral support above the openings, the wall spans horizontally to the end piers; the interior pier does not participate. The deep beam horizontal reinforcement is supplemented with additional horizontal reinforcement over the height of the wall to span to the end piers.

This project has return walls that act as shear walls to resist the lateral loads on the wall above the opening. Typically, buildings have larger piers for this, like what is shown in Figure CC-5.3-2.

Summary:

If sufficiently large piers are not possible between openings to support both vertical and lateral loadings, a deep masonry beam might be the solution to span several openings to larger piers.
Narrow piers between openings can be designed for gravity loads only, provided the wall above can span to piers that support the full lateral loads.
Deep beams require horizontal reinforcement that can also be used to support lateral loads to the piers.

Thank you again for following this column. Remember, by bonding, we get stronger! Keep the questions coming. Send them and your comments to info@masonrymagazine.com, with attention to Technical Talk. If you have missed any of the previous articles, you can find them online for Technical Talk, Bonding with Masonry at Masonry Design magazine.
(https://www.masonrydesignmagazine.com/?s=biggs).

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David is a PE, SE with Biggs Consulting Engineering, Saratoga Springs, NY, USA (www.biggsconsulting.net), and an Honorary Associate Professor with the University of Auckland, NZ. He specializes in masonry design, historic preservation, forensic evaluations, and masonry product development.

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