Deep brick spandrels

Masonry Construction, May, 2005

Some masonry wall designs require brick spandrels to span over lengthy bands of window openings. For this condition, the designer should consider supporting the brick veneer on a girder truss. By utilizing this truss, the depth of the spandrel can be increased and its weight can be kept to a minimum.

The girder truss should be designed to resist all lateral wind loads. The maximum or vertical deflection permitted l/600 to l/720 of the girder span or depth. Steel support angles should be welded to the girder at 24-inch lateral horizontal intervals. The angles serve as bracing members for the brick veneer and should meet the same lateral deflection requirements as the girder truss.

The girder truss should be fabricated with slots to accommodate clip angle connections. The direction of the slots in the truss, clip angle, and shelf angle should be alternated to provide the adjustability required when the masons align and bolt the shelf angle to the girder truss. This adjustable connection should minimize or eliminate the need to shim the angle. Also, the shelf angle should be field welded to the girder truss after attachment to prevent bolt slippage due to vertical loading.

The girder truss should be protected from moisture transfer by applying a self-adhering flashing membrane over its exposed surface. The membrane should be tightly lapped and adhered to the support angles to create a moisture tight seal and to prohibit the migration of water under the flashing or through the lap joints. If self-adhering flashing is not available, a heavy coat of mastic should be applied along the shelf angle and concrete decking prior to setting the flashing membrane in place. Some wall designs require a water resilient sheathing fastened to the outer surface of the girder truss.

The brick veneer must be secured to the support angles with a two-piece adjustable anchor. The anchor should consist of a back plate and a wire loop tie. The back plate should be mechanically fastened through the flashing into the support angles with two heavily galvanized screws. The brick veneer is secured to the angles by connecting the loop tie to the back plate and embedding the tie into the brick veneer. This type of anchor provides the adjustability necessary to properly embed the anchor into the exterior wythe of brick and delivers the required lateral bracing, but does not inhibit in-plane movement of the outer brick wythe.

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Additional information on this general subject is available in the "Masonry and Steel Detailing Handbook" authored by Walter A. Laska. The price for the 218-page book is $51, and it can be ordered at www.wocbookstore.com.