Self Supporting Brick Without Shelf Angles

By Charles Ostrander, PE

Cavity Walls are often successfully used in mid-rise and high-rise construction.  Buildings in excess of 40 stories have utilized cavity walls with a structural concrete frame.

There are two methods of support the exterior brick wythe for cavity walls cladded to concrete frame structures.  One is by means of shelf angles ; the other is to bear the total weight of the brick directly on the slab edge.  Each system has advantages and disadvantages.

Shelf angles create a continuous horizontal break within the cavity, obstructing the vertical flow of moisture through the air space.  If special attention is not given to the detailing and installation of the shelf angles, moisture infiltration, efflorescence and brick spalling can occur.  A reduction of shelf angles will eliminate the number of horizontal expansion joints required.  The further apart the expansion joints are spaced, the wider each joint needs to be.  Most horizontal expansion joints need to be initially 3/4 inch to 1 inch wide.  After differential movement occurs, the joint will compress to approximately 5/8 inch to 3/4 inch.  The width of an expansion joint is based on the desired width of the joint, its spacing, the compressibility of the sealant selected, the thermal and moisture expansion of the brick, and frame shortening. 

Exposed slabs are susceptible to moisture infiltration at the cavity wall base.  Special details must be developed to prevent wind-driven rain from penetrating beneath the flashing and into the building.  Flashing must be set in a continuous bed of mastic, or a self-adhering flashing with stainless steel drip edges must be used to prevent wind driven rain from penetrating beneath the flashing.

A hybrid use of brick with loadbearing concrete masonry backup or brick as a veneer bypassing a cast-in-place concrete wall has been used extensively for some time.  Within the last couple of years, many knowledgeable architects and structural engineers have eliminated steel shelf angles by designing the brick to be self-supporting from the foundation to the top of the building.  Before such a design application of brick is considered for use in a project, a number of caveats must be followed:

1. Brick should be self-supported by the concrete foundation walls.
2. Backup necessitates the use of non-combustible construction, whether cast-in -place, hollow-core precast concrete wall panels with veneer dovetail slots cast in at 16" o.c.  or concrete masonry used as infill between the structural members and floors of the exterior wall system.
3. You cannot have arbitrarily large or non-repetetive window spacing in an elevation.  These cause larger, irregularly concentrated loads placed on the brick veneer.  The use of adjustable, hot-dipped joint reinforcing or adjustable hot -dipped ties is necessary to provide for the additional thermal and moisture movements that will occur in the exterior wythe during annual heating and cooling cycles.
4. When limited windows are used in the exterior of the building, the windows must be anchored to the concrete block backup system, not the brick veneer.  Also, at least a 3/8 inch to 1/2 inch expansion joint should be detailed all the way around the enclosed windows.  A number of projects have used this application successfully.
5. Vertical expansion joints of 3/8 to 1/2 inch width at 30 feet on center are also recommended.
6. The cap of the  building must allow for the anticipated vertical movement of the brick

With the above being accounted for, another issue is that people will arbitrarily try to apply this system to a brick veneer / steel stud system.  The concern here is that in a fire situation, even though steel studs are considered noncombustible, they can melt.  As part of the structural and lateral support for the brick veneer, a melted and deformed steel stud / brick veneer system could cause additional structural failures and life safety concerns for the firefighters and public, from falling masonry.  The structural stability and integrity of self-supporting brick must be maintained.  Therefore, using shelf angles at every floor is recommended for use with steel stud backup / brick veneer exterior wall systems.  Multistory fire testing with brick veneer and steel studs is limited.

Another concern when using a self-supporting brick veneer from the foundation up is if the building occupancy changes or the use of the building is revised, the structural engineer of record of the newer structure should be aware of the fact that greater care will have to be exercised in cutting or removing some of the exterior facade for potential alternatives due to the design of the wall system.

Shown here are some applications and wall sections for consideration.  Consider using a shelf angle at every two or three floors with a large enough horizontal soft joint underneath the angles to avoid distress due to frame shrinkage (creep) and masonry's thermal expansion.  It is important that the drainage plane and the flashing / weep holes are installed properly with end dams.