Code-compliant designs

Emory Point in Atlanta includes three buildings, one with five stories of Type III-A wood-frame construction over slab-on-grade, and two with four stories of Type V-A wood construction over a Type I-A post-tensioned concrete podium. Architects: Cooper Carry and The Preston Partnership; Structural engineers: Ellinwood + Muchado LLC and Pruitt Eberly Stone Inc. Photo: Josh Meister, courtesy Cooper Carry

Options for brick veneer on mid-rise wood-frame buildings.

With growing interest in taller wood-frame buildings — many with five stories of wood on podiums with wood-frame mezzanines — there has also been interest in the use of brick veneer at greater heights.

For designers interested in brick veneer as an exterior finish, some publications and design guides reference using steel studs and non-combustible supports. However, there are in fact code-compliant methods for using brick veneer over the entire height of a mid-rise wood-frame structure. Options include a prescriptive approach for the use of brick veneer up to 30 feet in height and an alternate design approach for its use above 30 feet.

A publication by the Brick Industry Association (BIA; Reference 1) gives direct guidance for the application of brick veneer on wood backing above the 30-foot prescriptive height limit. As this paper explains, one approach is to stack the brick veneer at full height off the foundation without shelf angles or intermediate support by the wood framing. Another is to support the brick veneer off shelf angles that are attached to the wood framing at desired intervals. Both require the use of Section 12.2.1, Alternative design of anchored masonry veneer in the masonry code (Reference 2).

This excerpt of a WoodWorks technical paper focuses on these approaches. For engineers considering the use of brick veneer on a wood-frame building, the complete paper, available at www.woodworks.org/options-brick-veneer, also includes design considerations.

Prescriptive requirements

The masonry code’s prescriptive height limitations for brick veneer on wood construction allow veneer up to 30 feet above the veneer support, which could be interpreted as a foundation or an alternate location of support. This is based on Section 12.2.2.3.1.2, which states: “Anchored veneer with a backing of wood framing shall not exceed 30 feet, or 38 feet at a gable, in height above the location where the veneer is supported.”

However, this is followed by Section 12.2.2.3.1, which also requires that the weight of the veneer be supported on concrete or masonry foundations or some other non-combustible construction. In a wood-frame building, a steel shelf angle is often cited as this non-combustible support.

Under prescriptive requirements of Section 12.2.2.3.1.3, the code says that, when anchored veneer exceeds 30 feet or 38 feet at a gable, in height above the location where the veneer is supported, the weight of the veneer can be supported by cold-framed steel studs at each story above the 30-foot height limit. This prescriptive provision is the reason many code officials struggle with the use of wood-frame structures supporting brick veneer.

However, further inspection of Section 12.2.2.3.1.5 and its commentary reveals an exception to that prescriptive provision which explicitly allows veneer to be supported on and by wood construction provided the installed weight is 40 psf or less and the supported height is equal to or less than 12 feet:

12.2.2.3.1.5 Exterior masonry veneer having an installed weight of 40 psf or less and height of no more than 12 feet shall be permitted to be supported on wood construction. A vertical movement joint in the masonry veneer shall be used to isolate the veneer supported by wood construction from that supported by the foundation. Masonry shall be designed and constructed so that masonry is not in direct contact with wood. The horizontally spanning element supporting the masonry veneer shall be designed so that deflection due to dead plus live loads does not exceed l/600 or 0.3 inch.

This series of code sections has been used to allow the framing details shown in Figure 1 and support condition shown in Figure 2 on projects across the U.S. Details published in a BIA technical note (Reference 3) confirm that multiple support conditions are permitted. These details and others (Reference 4) are allowed provided their design is based on a rational analysis and complies with the requirements of the Alternative design of anchored masonry veneer of Section 12.2.1.

Brick veneer applications exceeding 30 feet in height exist all over the country. One example of brick veneer applied on wood framing beyond the 30-foot height limitation can be referenced in a WoodWorks case study of the University of Washington West Campus Student Housing project (Reference 5).

“The code allows you to go up 30 feet [prescriptively] but we did not go that high,” said Chris Duvall, with structural engineering firm, Coughlin Porter Lundeen. “We isolated the brick panels at each level by using veneer ledger angles hung from the rim board at each floor above the podium. So, the first story of brick sits on top of the concrete foundation; it is re-supported at the concrete podium slab and then at every level of wood floor framing above. We then detailed the brick to allow the wood framing to shrink behind it while the brick veneer panel moves with the building.”

Alternative design requirements

Common practice when exceeding the prescriptive 30-foot height limitation for brick veneer on wood framing has been to follow Section 12.2.1 of the masonry code. This approach requires that the veneer support system be based on a rational engineered solution. Code commentary notes that there are no rational design provisions for anchored veneer stipulated in any code or standard. However, some guidance is provided in this document and in Designing Anchored Brick Veneer above 30 Feet with a Backing of Wood Framing, published by the BIA. The intent of Section 12.2.1 is to allow the designer to use alternative means of supporting and anchoring masonry veneer. Section 12.2.1 requirements are as follows:

a. The forces applied to the veneer are distributed through the veneer to the anchors and the backing using the principles of mechanics.

b. Out-of-plane deflection of the backing is limited to maintain the stability of the veneer.

c. The veneer is not subject to either the flexural tensile stress provisions of Section 8.2, or the nominal flexural tensile strength provisions of Section 9.1.9.2.

d. The veneer must meet the General veneer provisions Section 12.1, the prescriptive requirements for stack bond Section 12.2.2.9, and the prescriptive requirements for higher seismic areas Section 12.2.2.10.

Conventional brick veneer

According to Section 1.6 of the masonry code, conventional brick veneer is plain unreinforced masonry (see Figure 3). Veneer is not allowed to be load bearing and only supports self-weight and out-of-plane loads. Out-of-plane bending loads are transferred through the masonry ties into the backing material or studs, where it is resisted.

Section 12.2.1(c) of the masonry code states that the veneer is not allowed to be subjected to the flexural tension stress provisions of Section 8.2 or Section 9.1.9.2. Contrary to Section 12.2.1(c), commentary states that the designer may choose not to consider stresses in the veneer or may limit the flexural stresses to ASD values, the anticipated cracking stress or some other limiting condition. Flexural tension stresses and cracking can be limited by controlling the deflection of the backing material. Current deflection limits for walls supporting brick veneer are subjective, with recommendations varying from L/180 to L/720, or greater, based on limiting the crack width (Reference 6). The IBC prescribes a minimum of L/240 for brittle finishes. The BIA recommends a limit of L/360.7

As part of the design, the designer should also consider the following:

• Provide horizontal in-plane, out-of-plane, and vertical support for the veneer.
• Control deflection of the backing material.
• Allow for differential movement between the veneer and the wood framing, including vertical shrinkage and lateral drift.
• Develop anchor loads through the connections into the backing, providing adequate strength and stiffness.
• Account for water penetration expected through the brick veneer in the building envelope system.
• Account for air and vapor transmission expected through the brick veneer in the building envelope system.

Reinforced brick veneer

Reinforced brick veneer provides an alternate option for an exterior veneer system or cladding. This brick veneer system follows the definition in Section 1.6 in that it is non-load bearing and only supports self-weight and out-of-plane loads. It does not replace the wood-frame walls or lateral-resisting elements.

The structural brick veneer (Reference 8) is hollow, similar to concrete masonry units (CMU blocks), which allows for the installation of vertical and horizontal reinforcement (see Figure 3). The veneer thus becomes a structural façade or cladding panel, which allows it to span beyond the 12-foot limitation of Section 12.2.2.3.1.5. Reinforced brick eliminates the need for steel lintels at openings and also allows sloping window sills, brick soffits, and precast concrete bands.

Pros:

• Out-of-plane loads are resisted by the reinforced brick panels, which span between floor levels, and are not transferred into the backing studs, thereby eliminating deflection limitation requirements of the backing studs.
• Intermediate brick ties or anchors are reduced or eliminated.
• Reinforcement helps reduce cracking and brick expansion due to temperature and moisture.

Cons:

• The reinforced brick units and reinforcing are more expensive.
• Additional coordination with the contractor is required.
• More detailing is required for the anchor connections.

For more information, WoodWorks provides free technical support related to the design of commercial and multi-family wood buildings. To discuss the needs of a specific project, visit www.woodworks.org to contact a regional director or email at help@woodworks.org.    

References

1. 12th North American Masonry Conference, Designing Anchored Brick Veneer above 30 Feet with a Backing of Wood Framing, Brick Industry Association, Charles B. Clark, Jr., Leroy Danforth, Jr., and Jim Bryja.
2. Building Code Requirements for Masonry Structures, ACI 530-13/ASCE 5-13/ TMS 402-13.
3. Brick Veneer/Wood Stud Walls, Technical Note TN-28, Brick Industry Association.
4. Ibid (End Note 2).
5. University of Washington West Campus Student Housing, WoodWorks case study.
6. Deflection limits for Wood Studs Backing Brick Veneer, Wood Design Focus Magazine, Fall 2007, Harold Sprague.
7. Ibid (End Note 3); Analysis of the Behavior of Anchored Brick Veneer on Metal Stud Systems Subjected to Wind and Earthquake Forces, Computech Engineering Services, Berkeley, Calif., 1989, T. Kelly, M. Goodson, R. Mayes, and J.Asher.
8. Design Guide for Structural Brick Veneer, Western States Clay Products Association, KPFF Consulting Engineers.

Terry Malone, P.E., S.E., is senior technical director, WoodWorks – Wood Products Council (www.woodworks.org).