The evolving rules for wall bracing have become perhaps the most complicated part of the prescriptive International Residential Code. Some builders point out that you can simplify your life by viewing the choices in a black-and-white way: Either stick to small houses with simple, boxy shapes (and keep your doors and windows away from the corners); or, hire an engineer.
When your design wind speeds are above 110 mph, you have no choice: Engineered design is mandatory (although there are some "deemed-to-comply" guidebooks available that let the engineers follow simple recipes). And for homes in wind speed zones below 110 mph, the prescriptive code does actually offer a lot of flexibility. But if you want to play around with window locations or sheathing types in the prescriptive IRC, there's a lot to learn.
Let's take a look at just one example. The 2009 IRC will allow builders to mix wall bracing types on a single building -- so for example, one wall might have plywood sheathing on the whole wall, while another wall might have let-in bracing and foam sheathing. But the wall that's fully sheathed needs to be anchored at the ends. One way to anchor those ends is with a short, 24-inch plywood or OSB-sheathed return wall at the corner.
The code books provide a plan-view drawing that explains how that corner has to be constructed.
Against moderate wind force, that corner is enough to hold down the end of the wall to prevent overturning or uplift.
But suppose that wall around the corner isn't sheathed, or suppose you want a door or window at that corner, closer than 24 inches from the wall end? Now you don't have the requisite anchoring. So the code gives you another option: You can place an 800-pound-capacity hold-down — an engineered anchor that is embedded into the concrete and fastened to the side of a wall stud — at the specified location, to replace the anchoring power of the framed corner.
Under certain circumstances, you don't have to place that hold-down right at the wall end. You can also put the hold-down at other places in the braced wall line.
In the 2009 version of the IRC, the value of those 800-pound hold-downs is recognized in another way. For fully sheathed buildings, adding the hold-downs allows you to reduce the required amount of wall bracing in a braced wall line — because the presence of the hold-downs increases the power of short plywood or OSB-sheathed wall segments to stiffen the building.
It's not simple. It's complicated, and it's fussy. It's also easy to make mistakes — move a door or a window half a foot, and you've gone over the line and you might fail the plan review. Builders, especially if they build large, complicated, expensive houses, might reasonably conclude that it's worth it to just bite the bullet from the outset and hire an engineer, then use engineered design with formal shearwalls to meet their bracing needs.
On the other hand, if you take the time and trouble to learn the complicated rules, you may find many cases where meeting the prescriptive standard in the IRC saves time and money as compared to the engineered solution. Here's another resource for learning about the rules: a PowerPoint PDF posted on the website of Henrico, Virginia. (Virginia has already incorporated the 2007 IRC supplement, which includes most of the upgrades coming in the 2009 IRC, into their statewide building code.)
Even building inspectors don't find the new rules easy to manage. Simpson Strong-Tie engineer Randy Shackelford has been traveling the country educating building department officials on the bracing rules. On March 23 and April 26-29, 2009, Shackelford will be in Massachusetts teaching building officials (the state has only recently implemented the 2006 IRC). In a recent phone interview, Shackelford said most Simpson hold-downs have capacities well beyond the 800 pounds required for the augmenting braced wall panels. The advantage of the new 800-pound requirement, he points out, is that it simplifies the attachment point to the concrete foundation. "It's such a light-duty connection," he says, "that the anchorage to the foundation is easier. It doesn't take an epoxy adhesive. In a lot of the country, they're not going to know where these things go until the framers lay out the walls, and so they are going to want to retrofit the anchorage. And for that, we have our new Titen HD's, where you can just drill the hole and screw them in. It's not like an expansion anchor that might have special problems right next to the edge of the concrete, and it's not like you need any special tools, like for injecting epoxy — since the capacity we need is so low."
Shackelford also suggested builders take a look at a new Simpson light-duty hold-down, the DTT2Z. The new connector was originally designed to provide a through-bolted attachment for connecting deck posts to deck frame joists, but Shackelford says it also works well for holding down braced wall segment ends or house corners.
The large hole in the base of Simpson's new DTT2Z connector receives a concrete anchor bolt. The smaller holes are for attaching the hardware to a wall stud, for which Simpson specifies it's load-rated "Strong Drive" hex-head screws.
