Homes in high-wind regions must be able to resist several kinds of wind forces, including uplift suction on the roof, as well as lateral pressures against the walls. So wall systems have to serve multiple purposes: transmitting uplift forces from the roof system through the wall down to the foundation, and also resisting the racking or shear force applied when the wall is bracing the structure against the wind's sideways pressure. There's more than one way to approach that design problem. Some situations require structural hold-downs and metal strapping, but in other cases, all the loads can be handled using ordinary anchor bolts and well-nailed plywood or OSB sheathing. Increasingly, engineers and builders are learning to be proficient with both approaches, selecting whichever solution seems best suited to the case at hand. In a recent paper for the American Society of Civil Engineers (ASCE), engineers Borjen Yeh, Tom Williamson, and Ed Keith, from APA - the Engineered Wood Association, describe recent research into the use of wood structural panels to resist "combined shear and uplift" loads. Their report, "Combined Shear and Wind Uplift Resistance of Wood Structural Panel Shearwalls," presented at the ASCE's 2009 Structures Conference, is available for $25.
This test apparatus at an APA facility in Washington State lets engineers apply simultaneous uplift and shear forces to a shearwall assembly, measure the force required to damage or destroy the wall, and observe the mechanism of failure. APA has used the test data to refine its instructions for building house walls capable of withstanding uplift wind suction along with shear forces ("racking"), as permitted by the most recent International Building Code. The report describes recent testing at an APA engineering facility to validate APA's design method for wood-frame walls loaded in both shear and uplift. According to the engineers, the new experimental data confirm the design values found in APA guidance documents for the method, and also provide new information allowing builders to fine-tune the spacing of the anchor bolts and three-inch steel washers required to lock walls down to the concrete foundations. APA has revised that technical guidance to reflect the new testing; the current document, System Report SR-101B, " Design for Combined Shear and Uplift from Wind," is a free download from the APA site (registration required). To use the APA method, the engineer first designs the wall as a shearwall to resist the loads applied on the structure by the lateral pressure of wind. Depending on the required uplift capacity, that structure may also have enough capacity to resist the upward suction on the roof system. If not, it may be possible to add enough sheathing nails at the wall plates to provide the extra uplift capacity that's needed. In layman's terms, Florida engineer Bryan Murray explained to Coastal Connection, "you are just apportioning the total strength of the panel between shear and uplift." Murray provides design engineering services to Florida builders ranging from small custom outfits to large production builders. In today's tough market, he says, "They're looking for any way to save a buck, and we're trying to find methods that can do that." Using the wall sheathing to handle uplift as well as shear is one attractive option, he says: "It's much more economical, not only from the materials standpoint, but from labor standpoint as well. Because you've already got a guy tacking sheathing on the wall. Now, he's just adding more nails, and you're maybe paying a little more attention to what he's doing. But you're eliminating an entire crew that used to come in to install all the metal." But applying the combined shear and uplift technique — or any economical framing method — requires a suitable overall house design, Murray says. And that means paying attention to structural issues early in the design process. But it's hard for non-engineers to get familiar with the structural issues, and to get a good feel for when the choices they make for esthetic or functional reasons are going to have expensive structural repercussions. Says Murray, "The draftsman would need to have an idea of how we're going to design the structural components, so that he doesn't put too many windows or doors in a wall so that we have to have an unusual shear panel, or very close nail spacing with really large holddowns. But that is hard to teach a draftsman to do — almost impossible, really. They would have to work as an engineer for a few years, to get a really good grasp of how to do that." It's particularly tough in a state like Florida, with its multiple wind zones. You can't reduce the problem to a cookbook, says Murray: "Just here in our area we have three different wind speeds, and we have three exposures, and those things determine the total wind load on the house. So you can't really tell a designer, 'Give me twelve foot of clear wall at the front of the house, the back of the house, the left, and the right' — because that might work for one house, say a one-story home in a 110-mph exposure B; but maybe it wouldn't work in a two-story building in a 120-mph zone sitting on the river, which is an exposure C." One production builder Murray works with sends rough plans to him for an initial review. "It's really just so that they can figure out their costs and set a price," explains Murray. "But that's also the stage at which we make recommendations." Take optional features such as sliding glass doors and bay window bump-outs, for example: "We might tell them that if the homeowner were to select those two options, then there would be no shearwalls at the back of the house, and it's would cost them some dollars to put in a Simpson steel shearwall panel. And then they might make modifications to the plans based on that feedback." Even if they don't make the design change, the early engineering feedback helps the builder estimate the true cost of those options — including the required structural measures, not just the cost for the doors or windows themselves. The combined shear and uplift strategy is affected by those same kind of big-picture design choices, says Murray. "Say you have a ten-foot blank wall where the sheathing alone, with close nailing, could ordinarily handle the shear and uplift loads combined. But if you put a five-foot bay window in that wall, now you increase the shear load per line foot on the remaining sheathed areas. So the remaining strength in those sections that is available to resist the uplift loads may not be adequate. Now you may have to strap that whole wall instead. And in fact, the same thing could happen to a longer wall, a 40-foot or 80-foot wall — just by putting in one opening instead of a sheathed area, you may end up having to strap that whole wall." It's a good idea to involve an engineer early in the design process, says Murray; but he says that in his experience, that's a rarity. More typically, he says, "We'll work for a custom home builder who's building a three story home, and he doesn't even look at the plans until he's pouring the slab, and then he realizes he has to put in great big cast-in-place one-inch anchor bolts — and then he calls us and says, 'Hey, what's going on, why did we have to do that?' "Well,' we tell him, 'you put four garage doors down one side of the house, and you left us with about three feet of wall.' But at the same time, if we call the builder up before that and say, 'Hey, you know, you have this thing coming up,' he'll ask, 'Well, how can I fix it?' — and if we say, 'You have to get rid of this bank of windows or that garage door,' they just laugh at us. You know, they've been through such a process to get to that point, they're not going to backtrack and talk to the homeowner about that now."