Wall bracing for lateral loads is a complicated topic in the codes. In the West, the most extreme lateral loads are associated with earthquakes, while on the East Coast, it's hurricane winds that drive the engineering. Often, the same generic structural assemblies can address both types of situation. But not always. Sometimes earthquake and wind forces are different -- and so are the structural solutions.
Here's an example of how the difference between earthquake and wind can play out in the building code, or in an engineering analysis. One of the permitted wall bracing solutions in the IRC, "Method PFH Portal Frame With Holdowns," has just been through a complicated series of re-assessments. But in the end, things are pretty much the same as the way they were to begin with.
Simpson Strong-Tie engineer Randy Shackelford explained the situation in a blog post on June 12 (see " New Holdown Requirements for the IRC® and IBC® Portal Frame Bracing Method," by Randy Shackelford). Wrote Shackelford, "Method PFH Portal Frame with Holddowns ... relies on low-deflection holdown anchorage at the bottom, and substantial nailing at the overlap of the sheathing and the header at the top to prevent overturning of the narrow panel. An identical method for use as wall bracing is in the Conventional Construction section in Chapter 23 of the IBC."
The portal frame concept was originally tested and proven using a test assembly designed to mimic the holdown performance of strap anchors embedded in concrete. The test didn't use the actual anchors or an actual concrete foundation, though — only a nailed strap designed to resist the same load. And that became an issue later, when the national standard for concrete construction, ACI 318, got revised, and when the organization that sets standards for construction components and hardware, the ICC Evaluation Service, responded by revising its approved test method for strap anchors embedded in concrete.
The ACI 318 revision, and the ICC Evaluation Service change, were based on earthquake risk. The concern was that in an earthquake, the concrete footings or walls where the holdown straps were anchored might crack, and that the attachment of the holdown to the concrete might then be compromised. The upshot was that Simpson Strong-Tie's embedded strap anchor, STHD14, was down-graded by the code to a rated capacity well below 4,200 pounds, the capacity assumed in the portal frame design testing. And that threw the whole capacity of the code-permitted portal frame into doubt.
The response in the engineering community was a rare event: Simpson Strong-Tie, the hardware manufacturer, cooperated with APA-The Engineered Wood Association, who had originally conceived the portal frame solution, to re-test the portal frame concept using the new, lower-capacity holdown values.
The result? Writes Shackelford, "... both the old tests with a simulated 4,200 lb. holdown and new tests with a simulated 3,500 lb. holdown were rerun... The tests showed little to no effect of reducing the holdown from 4,200 lbs. to 3,500 lbs. allowable load."
APA then submitted code change proposals to allow the relevant portal frame methods to continue to be permitted in both the International Residential Code (IRC) and the International Building Code (IBC). "With support from Simpson Strong-Tie, both of the proposals were approved," writes Shackelford. "So in the 2015 IRC, bracing method PFH will require an embedded strap-type holdown with a minimum capacity of 3,500 lbs. instead of 4,200 lbs. The same will hold true for the Alternate Braced Wall Panel Adjacent to a Door or Window Opening bracing method in the 2015 IBC. APA also re-tested the portal frames with only two sill plates instead of three. This will allow the use of a 5/8" by 8" Titen HD® anchor as a retrofit anchor bolt."
The whole tempest in a teapot is history now — problem solved. But code issues take a long time to work their way into the real world, and it's possible that some code officials, or some engineers, may have heard that there's a problem with the portal frame solution, but might not know the details. If that happens to you, stick to your guns: Fairfax County, Virginia, building department engineer Brian Foley confirms, "I do not believe this has any impact on coastal regions." And while the new testing of the portal frame was tailored for seismic loads, Foley says, "assemblies tested under it would perform well for wind loads as well."