Bruce A. Feldmann, PE
Perhaps you have designed a post-frame structure that used metal plate connected wood trusses and wondered why the trusses were much larger than the wood trusses used on other commercial and residential projects. Or perhaps you’ve installed wood trusses on a post-frame structure and are curious why more attention is given to the permanent bracing in a post-frame structure compared with the bracing in residential projects you’ve worked on. Welcome to the world of post-frame trusses, where proper design and installation play a much larger role in the performance of the structure than they do in a typical commercial or residential project, where the trusses are spaced on 24-inch centers or less.
A post-frame truss by loose definition is a metal plate connected wood truss used in a post-frame structure that will be spaced on greater than 24-inch centers and supported by, and attached to, solid sawn or laminated posts. The profile of the typical post-frame truss may be similar to the truss in a home or office building, but the span and supported load are often considerably greater. For this reason, post-frame trusses require increased member sizes and connections, as well as special design considerations to account for the impact of the larger members on the truss modeling and analysis.
Post-frame trusses are often still referred to as ag trusses or pole barn trusses, terms used back in the days when these trusses were used primarily, if not solely, in structures for agricultural purposes and the supporting members were round utility poles. Today these structures are widely used for commercial applications such as retail buildings, schools, and houses of worship. Though agricultural structures are still a common application for post frame today, they make up a smaller percentage of the structures post-frame trusses are used in compared with 30–40 years ago.
For this reason, greater attention is required for the design, fabrication, and installation of post-frame trusses. The wide use of post-frame structures for commercial applications requires that these structures and the wood trusses be code-compliant. The design and analysis of the wood trusses must meet code-referenced standards such as the National Design Standard for Wood Construction, ASCE7’s Minimum Design Loads for Buildings and Other Structures, and the Truss Plate Institute’s National Design Standard for Metal Plate Connected Wood Trusses. In addition, the trusses must be installed and braced in accordance with the Building Component Safety Information document published jointly by the Structural Building Components Association and the Truss Plate Institute.
These issues and others will be discussed during the “Using Metal Plate Connected Wood Trusses in Post-Frame Buildings” session at the 2013 National Frame Building Association Expo in Memphis (February 20–22).
Bruce A. Feldmann, PE, is chief engineer at ITW Building Components Group, Inc., Earth City, MO.