The International Residential Code (IRC) has specific installation notes and details for laterally bracing residential structures. Portal frames are one of the bracing techniques specified, utilizing the garage door header as a component. The IRC details related to the header are often misunderstood and misapplied, leading to an installation with unintended structural consequences.
Portal frames are constructed with braced wall panels and full-length headers assembled in accordance with the International Residential Code (IRC) section 602.10.6, for use adjacent to window or door openings.
A portal frame is defined as two uprights connected at the top by a third, horizontal member (the header). Garage wall framing typically utilizes this technique encompassing the vehicle doors. In an attempt to construct portal frames, some garages are built with one continuous garage header extending over two or three garage door openings as shown in figure A.
Portal frame details within the IRC specify simple span headers, with a maximum finished opening width of 18'. These details are based on testing done by The Engineered Wood Association, that included only simple span headers. Additional information can be found on their website, within Technical Topic #TT-080. To our knowledge, no testing has been done to justify continuous headers over multiple garage door openings.
The single span limit ensures that the wall assembly has periodic full-length king studs to provide lateral support for the header. Without lateral support, there is potential for the header to buckle laterally and roll, over the intermediate support. Similar information is presented by the American Wood Council and can be seen on their website by clicking here and then scrolling down thru articles: Why are continuous portal frame headers not recommended?
The IRC DOES permit the use of multiple, single span portal frames as depicted in Figure B. Using single span beams eliminates the potential for compression edge buckling along the bottom of the beam.
Continuous headers spanning over multiple openings should be evaluated by a design professional for stability.
Additional info on this important stability issue can be found in STRUCTURE magazine - Lateral Support of Wood Beams in Residential Structures
For more information contact technical support.
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