Tie properties
The MSJC Specification says nothing regarding the type and properties of a brick tie. However, the MSJC Code requires the tie to be a minimum 22 gage or 0.8 mm (0.03 in.) in thickness and 22.2 mm (7⁄8 in.) in width. It also requires a corrugation wavelength and amplitude range of 7.6 to 12.7 mm (0.3 to 0.5 in.) and 1.5 to 2.5 mm (0.06 to 0.1 in.), respectively.
Studies have shown insufficient width, wavelength, and amplitude can contribute to decreased mortar pullout strength in tension. (See the article by James M. LaFave and Dziugas Reneckis, “Structural Behavior of Tie Connections for Residential Brick Veneer Construction,” published in The Masonry Society Journal in December 2005). Insufficient gage thickness can lead to the fastener punching through the tie in tension, as shown in Figure 4, as well as decreased compressive resistance due to buckling of the tie. It is the responsibility of the designer to specify a code-compliant brick tie as further discussed in Part 3 of the “Mandatory Requirements Checklist” within the MSJC Code.
Corrosion protection of corrugated brick ties is commonly provided by hot-dip galvanizing. While galvanizing is discussed and acceptable in both the MSJC Code and MSJC Specification, stainless steel brick ties are available; the marginal additional cost can extend the life of masonry veneer by decades. The additional cost of using stainless steel masonry anchors pales in comparison to the cost of replacing masonry veneer within the life of a building. The designer should specify stainless steel masonry anchors to increase the durability of masonry veneer construction. (See Clayford T. Grimm’s article, “Corrosion Protection of Metal Connectors in Masonry,” published in the December 2002 TMS Journal.)
Fastener properties
Similar to tie properties, the MSJC Specification does not discuss the fastener type to be used to secure the brick ties to the backing. The MSJC Code requires an 8d common nail or better as the minimum fastener requirement for brick ties. If all other variables are code-compliant, fastener pullout strength controls the tensile capacity of the brick tie connection.
A common error is to use roofing nails or a smaller 6d common nail. These nails have a smaller shank diameter and shorter length, which can significantly decrease the overall pullout strength of the fastener. Using a non-code compliant fastener can decrease the tie capacity up to 50 percent. (See the article by James M. LaFave and Dziugas Reneckis, “Structural Behavior of Tie Connections for Residential Brick Veneer Construction,” published in The Masonry Society Journal in December 2005). Figure 5 shows the size of code-compliant fasteners next to those erroneously used in practice.
Fastener-pullout capacity in wood depends on its moisture content, especially when smooth shank common nails are used. If the fasteners are installed to a wood backing that has a higher moisture content than it will have in service (e.g. rainy or wet conditions), studies recommend using ring-shank nails or screws. To ensure durable masonry veneer, the designer should specify a code-compliant fastener and consider the environmental conditions of the installation.
Bending the tie
Equally critical to the tie and fastener properties is the fastener’s location with respect to the tie. The MSJC Code requires the fastener be located within 12.7 mm (1⁄2 in.) of the 90-degree bend when using the prescriptive approach. Two errors can occur when this requirement is omitted. The first is the tie must be bent at 90 degrees to be MSJC Code-compliant. This becomes an issue when the location of the brick tie does not match up vertically with the location of the mortar joint; at that point, masons tend to adjust brick ties by bending them into a number of different non-code compliant configurations, as shown in Figure 6.
These profiles of tie installation decrease the stiffness of the connection significantly as the excess length of tie within the air space straightens out, resulting in significant out-of-plane deflections to the entire veneer before engaging the brick tie. In conjunction with the 90 degree bend requirement, the mason’s workmanship is essential to ensure the brick tie locations and bed joints line up to avoid inappropriate bending of the brick ties.
Fastener-to-bend location
The second requirement for placement of the fastener within 12.7 mm (1⁄2 in.) of the 90-degree bend is meant to decrease the eccentricity of the connection, which is the offset distance between the horizontal leg of the tie and the fastener above it. Fasteners with significant eccentricity allow for more displacement of the tie as it bends prior to engaging the fastener. The consequences for this mistake are similar to failing to provide a 90-degree bend in the tie: significant deflections at less than design loads, which can result in premature failure of the veneer.
Several studies have concluded the MSJC Code maximum of 12.7 mm (1⁄2 in.) of eccentricity may even be too big. (See the article by James M. LaFave and Dziugas Reneckis, “Structural Behavior of Tie Connections for Residential Brick Veneer Construction,” published in The Masonry Society Journal in December 2005). Figure 7 illustrates varying fastener-to-bend locations, with the middle and right configurations being ranges of code compliance, and the left being non-compliant (See H. Okail et al’s article, “Seismic Performance of Clay Masonry Veneer,” from the 14th World Conference on Earthquake Engineering, published in October 2008. Also, refer to the advisory provided by the Federal Emergency Management Agency (FEMA) in 2005 entitled, “Attachment of Brick Veneer in High-wind Regions”).
Air space
All masonry veneer anchors require a minimum air space (cavity width) and a maximum distance between the backing and the veneer. For corrugated brick ties, both the code minimum and maximum nominal values are the same: 25 mm (1 in.). The minimum air space allows enough space for moisture to drain down the weather resistive barrier. If the wall cavity is larger than 25 mm (1 in.), the capacity of the brick ties decreases. For wall sections necessitating a larger cavity width, such as those with continuous insulation, an alternative masonry anchor will be necessary. The designer must specify a 25-mm (1-in.) cavity width with brick ties to provide a functional drainage plane while maintaining the full capacity of the brick ties.
Recommendations
When specifying corrugated sheet metal anchors for masonry veneer, it is important for designers to convey all the code requirements to the contractor. In addition to referencing the MSJC Specification, designers must also include the following brick tie requirements in the contract documents if utilizing the prescriptive approach, as summarized in Figure 8:
- Use with wood-frame backing only.
- Use a code-compliant tie with sufficient gage thickness, width, corrugation wavelengths, and corrugation amplitude.
- Fasten with an 8d common nail or better.
- Provide a 90-degree bend in the brick tie.
- Place the fastener within 12.7 mm (1⁄2 in.) of the 90-degree bend in the brick tie.
- Provide a 25-mm (1-in.) cavity width.
Additionally, designers can extend the service life of masonry veneers by specifying stainless steel brick ties instead of the commonly used galvanized ones. By supplementing the MSJC Specification with these design-related decisions, specifiers can provide contractors with all the information necessary to construct a code-compliant masonry veneer. For conditions that cannot meet these requirements, designers can specify other masonry anchor types. By understanding and specifying a code-compliant masonry veneer, designers can improve the durability of masonry veneer construction. (The authors gratefully acknowledge the continuing support and leadership of David W. Fowler, PhD, PE—the faculty advisor for the research being performed at The Durability Lab, a testing center at the University of Texas at Austin).
Robert M. Chamra, EIT, is a project engineer with Building Diagnostics Inc., specializing in the investigation of problems with existing buildings, designing remedies for those problems, and monitoring the construction of the remedies. He participates in the research being performed at The Durability Lab—a testing center established by Building Diagnostics at the University of Texas at Austin. He can be reached by e-mail at rchamra@buildingdx.com.
Kyle R. Gagnon is a graduate student studying architectural engineering at the University of Texas at Austin. He serves as the graduate research assistant for The Durability Lab, which researches and tests the durability of building components, identifying factors causing premature failure. He can be contacted at kgagnon@buildingdx.com.
