Q: If the purpose of these ties is to support the veneer during winds, why does it make any difference what backup is used?
A: The codes and industry standards that permit the use of corrugated anchors are prescriptive. In other words, they include a detailed description of the type, spacing, and placement of these anchors without requiring engineering load calculations. The 2010 Building Code Requirements and Specifications for Masonry Structures reported by the Masonry Standards Joint Committee (MSJC code and specification), in Section 126.96.36.199.1.2 of the code, permits the use of anchored veneer attached to wood framing, provided that the height above the noncombustible foundation does not exceed 30 feet at the height of the plate or 38 feet at the height of the gable. The prescriptive requirements are as follows:
Section 188.8.131.52.1.1 of the code states that "Corrugated sheet-metal anchors shall be at least 7/8 in. (22 mm) wide, have a base metal thickness of at least 0.03 in. (0.8 mm), and shall have corrugations with a wavelength of 0.3 to 0.5 in. (7.6 to 12.7 mm) and an amplitude of 0.06 to 0.10 in. (1.5 to 2.5 mm). In Section 184.108.40.206.6.1, this code states that there should be "one anchor for every 2.67 ft2 (0.25 m2) of wall area."
- Section 220.127.116.11.2 of the code states, "Attach each anchor to wood studs or wood framing with a corrosion-resistant 8d common nail, or with a fastener having equivalent or greater pullout strength. For corrugated sheet-metal anchors, locate the nail or fastener within 1/2 in. (12.7 mm) of the 90-degree bend in the anchor."
- Section 2.4 D.2 of the specification requires that sheet metal anchors and ties conform to ASTM A1008/A1008 M.
Similar language is contained in the International Residential Code (IRC) and in Brick Industry Association (BIA) Technical Note 28, Anchored Brick Veneer, Wood Frame Construction.
There are also requirements concerning the width of the airspace when corrugated anchors are used. Section 18.104.22.168.3 of the MSJC code and BIA Technical Note 44B, Wall Ties for Brick Masonry state that "when corrugated sheet metal anchors are used, a maximum distance between the inside face of the veneer and outside face of the solid sheathing of 1 in. shall be specified." It is interesting to note that these same documents also require that the air space between the veneer and the backing be a minimum of 1 inch. Therefore, corrugated anchors can only be used for 1 inch air spaces. Air spaces greater than 1 inch require adjustable anchors meeting the requirements of Section 22.214.171.124.5.
Critical loads acting on corrugated anchors are generally wind suction loads. In most cases, the magnitude of wind suction loads is greater than wind pressure loads and these anchors generally have greater displacement and lower capacity in tension than in compression. Corrugated ties can fail in tension due to nail pullout and straightening of ties with significant eccentricity (see Figure A). An 8d nail with 2 inches of penetration (2-1/2 inches less 1/2 inch sheathing) has an allowable withdrawal strength of 42 to 84 pounds and an ultimate withdrawal strength of 235 to 460 pounds (depending on the species of wood ranging from Spruce-Pine-Fir to Southern Yellow Pine). With a half inch offset, a corrugated anchor made of 22 gage, Grade 60 steel has a capacity of approximately 18 lbs at 75 percent of yield.
ASTM A1008, Standard Specification for Steel, Sheet, Cold-Rolled, Carbon, Structural, High-Strength Low-Alloy, High-Strength Low-Alloy with Improved Formability, Solution Hardened, and Bake Hardenable permits the use of steel with less than half the yield strength of Grade 60, which would reduce the capacity proportionally. If ties are provided to support 2.67 square feet of masonry veneer, the maximum allowable negative wind load that can be handled by veneer supported only by the corrugated wall anchors is in the range of 16 to 31 psf (for the case where the nail is positioned at the bend in the tie) to 7 psf (for the case where the nail is installed 1/2 inch from the bend). Common design wind loads for residential structures away from areas subject to hurricanes range from 10 to 20 psf depending on the location and exposure. Therefore, when the tie is not bent directly at the nail, the anchor will have a tendency to straighten out. This results in excessive movement and potential cracking or wall failure.
Another significant shortcoming of corrugated anchors is limited corrosion resistance. Corrugated anchors are wide and made from thin sheet metal. As a result, the ratio of surface area to cross sectional area is high. The wide flat surface can hold water and in many cases some of the zinc galvanizing can be chipped off when the ties are bent. For these reasons, corrugated anchors are more susceptible to corrosion than galvanized wire ties.
The MSJC code, the IRC, and BIA Technical Note 28B, Brick Veneer/Steel Stud Walls require adjustable anchors that have greater capacity and require additional anchors at critical areas for brick veneer anchored to steel stud backup walls. Corrugated anchors are not permitted with steel stud back-up. In addition to requiring one anchor for every 2-2/3 square feet of wall area, BIA Technical Note 28B recommends that the anchors closest to the shelf angle must be designed to support over 30 percent of the lateral loads. BIA Technical Note 44B contains additional recommendations regarding adjustable ties for brick veneer walls over steel stud backups. This document limits the maximum deflection of the adjustable ties to less than 0.05 inches when tested under an axial load of 100 pounds in tension or compression.