The rules for bracing against wind pressures have gotten complicated. Way complicated. The 2006 IRC devoted a scant eight pages to wall bracing, and by 2009, that section of the code (R602.10) had ballooned to 30 pages. An effort to simplify the 2012 version still left Section R602.10 with 27 pages of wall bracing rules.
Those rules are limited in scope: The methods they prescribe are allowed only if the design wind speed is less than 110 mph and the building is three stories or less. For taller buildings or higher wind speeds, you'll need an engineer. But if you learn the ins and outs of the IRC wall bracing rules — complicated as they are — you should be able to build homes in most parts of the country without an engineer's help.
For this article, JLC spoke with Brian Foley, a building official with Fairfax County, Va. (population 1,118,602). Foley is a structural engineer who directs the engineering office at the Fairfax building department. He was also a member of the special International Code Council committee that updated and revised the wall bracing rules for the 2009 and 2012 editions of the IRC. (The publications available from Fairfax County on wind bracing are as good as any you'll find on the topic:www.fairfaxcounty.gov/dpwes/publications/wind_bracing.) Virginia is far ahead of many states in adopting the new codes, and its experience in both coastal and inland terrain is giving builders in states that have yet to adopt the new code an early chance to learn hard-won lessons.
Wind vs. Seismic
Why did the IRC change the wall bracing rules after 2006? Simple, says Foley: If you're building in wind country and not in earthquake country, the old rules were wrong. The bracing rules in the 2003 IRC were brought in from the old Council of American Building Officials (CABO) One and Two Family Dwelling Code, and carried over into the 2006 IRC. "But all of the provisions in the 2003 and 2006 code were based on seismic loads only," says Foley. "And the way we analyze a structure for seismic is 180 degrees different from how you analyze a building for wind."
In earthquakes, Foley explains, the stress on a wall is caused by the mass of the wall reacting against the ground movement under the house — the earth moves, and the house, because of its inertia, wants to stay still. Because longer walls have more mass to react against the earth's motion, they experience greater forces in an earthquake, and need more bracing. Conversely, shorter walls have less mass, so they need less bracing — in an earthquake.
But a windstorm affects walls the opposite way, explains Foley. Walls that are facing the wind feel the wind pressure, but it's the walls parallel to the wind direction that do the work of resisting the force.
For example, take a typical colonial that's just a simple 25-by-50-foot box (see "Walls Act Like Sails in the Wind," above). When the wind blows against the 50-foot wall, it's the two short gable-end walls that have to handle the load; when the wind blows against the 25-foot wall, the bracing in the 50-foot walls comes into play. "With wind, the shorter wall has to resist more loads than the longer wall," says Foley. "And so shorter walls need more bracing than long walls in a wind situation — just the opposite of what they need for an earthquake." This means that builders who want to have a lot of glass on the short gable-end wall of a shoe-box-shaped building just might come up short in the bracing department.
So the new code has different bracing rules for seismic conditions and for wind conditions. But Foley says there's another reason the wall bracing rules needed updating: They just weren't keeping up with the times. The old code was based on historic behavior of traditional buildings — the traditional two-story colonial or one-story ranch, for example, with walls between each room. "But if today's plans don't include walls between the living room and the dining room, and there is no more foyer, and it's all a big open concept," Foley says, "then a lot of that rigidity that was the basis of the historical experience is gone."
A prime example is that the old code required a 4-foot braced panel at every corner and one every 25 feet on-center, and if you wanted a window at the corner, you had to hire an engineer. In the 2009 and later code versions, however, the requirement for a 4-foot solid wall section at every corner is gone. Now, the nearest bracing panel can fall as far as 10 feet away from the corner (see "Locating Braced Wall Panels," page 36).
The Downside: Complexity
Builders were at the table when the rules were revised, and they influenced the result. In some ways, the latest code offers builders more flexibility than ever for creative modern designs — but at a price. While the old code was restrictive but simple, the new code is, Foley admits, "exceedingly complex."
In Foley's experience, engineers are no more likely than builders to get the answers right. He notes that the Fairfax County building department routinely rejects engineer-stamped plans as well as builder-submitted plans. That's one reason Foley teaches an eight-hour class on wall bracing and the IRC; his students include not just builders and architects, but also engineers and code officials. "A lot of engineers just sit in one of my classes and then they use the prescriptive requirements," he says. "It makes me wonder why on earth a builder would hire them to do that." Foley believes that there are a lot of smart builders who, once they sit through the training, are able to manipulate their design to accommodate some of the flexibility built into the code.
One reason the new IRC Section R602.10 is so long is that, in an effort to be inclusive, it covers a lot of wall bracing methods that most builders don't use. Specifically, it covers traditional let-in bracing, stucco walls, diagonal wood boards, hardboard panel siding, and structural fiberboard, along with two different approaches to using wood structural panels (plywood or OSB), plus a slew of special narrow-wall options developed by the structural panel industry. So if you're a builder who uses insulating foam sheathing with just a few pieces of OSB or some metal straps in each wall for bracing, you can use one or another approach in Section R602.10 to accomplish that. In fact, the code will let you combine different methods in one house or even in one wall — teaming up let-in bracing on one end of a braced wall line with OSB panels on the other end, for example.
But if you're a builder who believes that simpler is better, your best bet is to stick with OSB or plywood, especially if you need high capacity. Compared with the other traditional methods, wood structural panels can supply more bracing in fewer linear feet of wall. They're most effective when used as continuous sheathing across the whole wall (if you don't cover the entire wall with sheathing, you need wider braced-wall panels at the points where you do sheathe it).
As an introduction to the topic, we'll look at a case study to examine how the required bracing is calculated in varying wind conditions and with different building options (see "Calculating the Required Length of Bracing," pages 32–34). But first, to understand this case study, it's necessary to be clear on a few key concepts.
