Can standard veneer anchors handle the loads of larger cavities?
Mason contractors across the country face similar challenges when out in the field. With the help of Hohmann and Barnard’s director of technical services, Jeremy Douglas, Masonry delivers answers to some of your most technical, complicated questions.
Q. We’ve been seeing larger brick veneer wall cavity spans over the last several years, but the veneer anchors we have been using are still the same. Can standard veneer anchors handle the loads they need to within these larger cavities?
A. Sure, I address this issue pretty much every day of the week in one way or another. This has become a major concern for several reasons.
First, while the TMS-402 code only requires a 1-inch air cavity, most designers and masonry entities recognize that a larger air cavity is more desirable. Because construction tolerances of the backup and veneer walls typically run at ±1/4 inch, they potentially jeopardize 50 percent of that airspace should they run outboard and inboard of those tolerances, respectively. Additionally, the need for higher R-values in our exterior continuous insulation has increased the span, as has the expanded use of mineral wool insulation to more easily create NFPA 285-compliant assemblies. We also see larger airspaces today to help to better achieve pressure equalization between the air in the cavity and the outside.
The same TMS-402 referenced above states that a 4 1/2-inch maximum distance between the inside face of the veneer and the steel framing, masonry, or concrete backing, with a minimum of 1 inch of air space, shall be specified. This means that, currently, we must provide engineering calculations for any cavity spans larger than 4 1/2 inches. What the code doesn’t suggest is any specific performance criteria for any anchors to be held to in order to be considered adequate.
The closest thing the masonry industry has to a performance standard regarding anchor load capacities would be Tech Note 44B as published by the Brick Industry Association. 44B suggests we should be working only with ties that show maximum deflections of less than 0.05 inch (1.2 mm) when tested at an axial load of 100 lbs. in tension or compression. Sounds pretty straightforward, right? Well, not exactly.
Two-part masonry anchors often consist of a portion of the anchor fixed to the backup wall and an adjustable tie, usually formed from 3/16-inch wire to be embedded in the veneer joint. If we’re considering a performance standard as I suggested above, that standard must apply to the anchor/tie combination everywhere within its plane of adjustability. As an example, the TMS-402 code allows for 3/16-inch pintle hook anchors to be offset no more than 1 1/4 inches. In evaluating how a standard round wire pintle performs across this 1 1/4 inches of adjustability, testing a standard wall configuration inside the 4 1/2 inches shows us a nearly 90 percent reduction in strength from the point where the pintle and anchor are completely engaged to where the pintle is offset 1 1/4 inches. This hasn’t even taken into account changes in pintle size or larger airspaces, brick corbelling, or other scenarios where the airspace can potentially become larger than 3 inches.
So, to answer the question, a standard masonry anchor may or may not be able to handle the expanded cavity. A lot of factors need to be taken into consideration, which is why the code calls for engineering calculations beyond 4 1/2 inches. You may also see anchors referred to as “high strength” or as being specifically manufactured for “wide cavity walls” to address some of the concerns mentioned above. For now, the only way to be certain is for engineering personnel to evaluate testing that shows the load performance of the anchor throughout its adjustability and specific to the conditions in which it will exist.
About the Author
Masonry, the official publication of the Mason Contractors Association of America, covers every aspect of the mason contractor profession - equipment and techniques, building codes and standards, business planning, promoting your business, legal issues and more. Read or subscribe to Masonry magazine at www.masonrymagazine.com.