Designing Brick Walls for High-Wind Regions
Brick walls may look sturdy, strong, and ready to take on the fiercest storms that Mother Nature can conjure up, but brick construction has proven to be vulnerable during high-wind storms, especially hurricanes.
This was immediately evident in the wake of hurricanes Katrina and Rita, when dozens of brick-clad buildings, both residential and nonresidential, sustained damage as a result of brick veneer that crumbled and collapsed amid excessive wind pressure. The failure of exterior brick walls not only puts building occupants in harm’s way, but also leaves the structures exposed to wind-driven rain and wind-borne debris, causing interior damage.
How can a wall structure that appears to be so rock-solid under normal circumstances fail during windstorms?
Poor construction techniques and improper material selection are the most common causes for failure of brick veneer walls, according to an advisory report published by the Federal Emergency Management Agency Mitigation Assessment Team. The report, which summarizes the team’s post-Katrina assessment of brick veneer buildings on the Gulf Coast, identifies five main deficiencies that lead to failure:
• Brick tie fastener pullout
• Poor bonding between the ties and mortar
• Low-quality mortar
• Failure of masons to embed ties into the mortar
• Brick tie corrosion
Misalignment of brick ties is another common mistake, and can lead to one or more of the deficiencies listed above. Because brick ties are usually installed before the bricklaying begins, they’re often improperly placed above or below the mortar joints. When misaligned, the ties must be angled up or down in order for the ties to be embedded into the mortar joints. This not only reduces the embedment depth, but also minimizes the effectiveness of the ties because the wind forces do not act parallel to the ties themselves.
The FEMA advisory offers a number of prescriptive recommendations for designing and building brick veneer walls to withstand hurricanes and other high-wind events:
• To avoid the misalignment of brick ties, make sure the brick masons install the ties as the brick is laid. This will ensure that the ties line up with the mortar joints.
• Specify two-piece adjustable ties over traditional corrugated ties for light commercial applications that utilize wood-frame construction. Two-piece ties provide significantly greater compressive strength than corrugated ties. If corrugated ties must be used, then make sure to specify zinc-coated, 22-gauge ties that are at least six inches long and 7/8 inches wide to comply with standards ASTM A 366 and ASTM A 153.
• Space studs 16 inches on center to ensure that the ties are anchored at the ideal spacing.
• For tie fasteners, use ring-shank nails in lieu of smooth-shank nails. A minimum embedment depth of two inches is recommended.
Brick ties should be spaced according to American Society of Civil Engineers’ ASCE 7-02, Minimum Design Loads for Buildings and Other Structures (see table, opposite). Note: Veneer tie locations for 24-inch stud spacing are included for applications that involve repairing damaged brick veneer on existing buildings that have wider stud spacing.
• Locate ties within eight inches of door and window openings and within 12 inches of the top of veneer sections.
• Bend ties at a 90-degree angle at the nail head in order to minimize tie flexing when the ties are loaded in tension or compression (see illustration, above left).
• Embed ties in joints so that mortar completely encapsulates the ties. Embed a minimum of 11/2 inches into the bed joint, with a minimum mortar cover of 5/8 inch to the outside face of wall (see illustration, above right).
For more information, see FEMA 549, “Mitigation Assessment Team Report, Hurricane Katrina in the Gulf Coast,” and appendix E. www.fema.gov/library/viewRecord.do?id=1857
Specifying the proper tie spacing for brick veneer construction
|(mph; three-second peak gusts)||(psf)||16-inch stud spacing||24-inch stud spacing|
|Wind speed||Wind pressure||Maximum vertical spacing for ties|
Notes: 1. The tie spacing is based on wind loads derived from method 1 of ASCE 7-02 for the corner area of buildings up to 30 feet high, located in Exposure B with an importance factor of 1.0 and no topographic influence. For other heights, exposure, or importance factor, an engineered design is recommended.
2. Fastener strength is for wall framing with a Specific Gravity G=0.55 with moisture contents less than 19% and the following adjustment factors: Ct=0.8 and CD, CM, Ceg, and Ctn=1.0.
3. Nail embedment depth of two inches for 21/2-inch-long 8d common (0.131-inch-diameter) ring-shank fasteners
a Maximum spacing allowed by ACI 530-05
b 24-inch stud spacing exceeds the maximum horizontal tie spacing of ACI 530-05 prescribed for wind speeds over 100 mph