Will the Top Chord Buckle? Exploring Truss Bracing Alternatives
On the BCMC show floor last week, SBCA and the National Framers Council (NFC) constructed two roof truss systems comprised of five identical 50-foot trusses. One truss system was laterally restrained and diagonally braced according to the guidance provided in the BCSI booklet. The other truss system had lateral restraint and diagonal bracing within the web-member plane only.
The key concept discussed during tours of the two systems was which one would you rather restrain and brace?
Restraining and bracing the web member plane (left side above) took a framing crew of two people one hour to install. Restraining and bracing to BCSI (right side above) took a crew of four people two-and-a-half hours to install, and had much higher potential for an accident to happen as work was done on the top chords of the trusses.
It should be noted, these times don’t take into account the process of removing the top chord temporary lateral restraint and diagonal bracing in order to sheath the assembly. Contrast this with the web member plane restraint and bracing, which does not have to be touched again once installed, because it serves as both temporary and permanent restraint/bracing and can be immediately sheathed.
To put this in perspective, BCSI temporary lateral restraint and diagonal bracing was created using three 250 pound concentrated loads to theoretically design the optimal lateral restraint and diagonal bracing. This approach resulted in the top chord bracing guidance currently found in BCSI, as seen below.
The BCMC demonstration used 50 foot trusses, which would follow the ‘up to 60 feet’ approach in the BCSI illustration.
BCMC attendees (including several veteran industry engineers pictured below) had an opportunity to walk on the system and get a feel for the truss stability of each roof system bracing approach. The primary concern is prohibiting the truss top chords from buckling when bearing the load of three 250-pound people. In the photo below, seven people applied non-symmetrical loading to the bottom chord of the web member retrained/braced trusses.
The traffic through the BCSI bracing demonstration was continuous and sparked a number of valuable conversations. It showed that web-member-plane-only restraint/bracing has the potential to save time and provide better installer safety. In addition, this approach to bracing presents framers like NFC executive director Chris Tatge with several practical benefits in the field. This timelapse video shows the installation process. In 2018, SBCA and NFC will undertake testing to develop a more framer-friendly approach to bracing/restraint that is easy to install, safe and reliable. If a new approach is easier and makes sense to framers, as well as provides a solution that can act as both temporary and permanent bracing, the belief is more framers will be inclined do it.
Finally, the video below provides an excellent illustration of the effectiveness of the web-member-plane-only restraint/bracing. Eight people stood on one truss and even had a little fun as one of the engineers energetically loaded the bottom chord by adding a bit of bounce to the applied loading conditions. As you can see in the video, the trusses survived even these dance movements well!