Permanent bracing, what’s the big deal?
I was recently on a jobsite talking with the Superintendent. The topic quickly turned to the trusses overhead (imagine that!) and I asked if he had a set of the Truss Design Drawings that came with them. He said he did and that they were in the trailer. I asked him when they planned to install the permanent bracing that was (undoubtedly) required and he said, without hesitation, “…that they don’t do anything more after the trusses are up and sheathed-in.” I have to admit, I was not shocked to hear those words.
The trusses in question were not terribly long (about 40’), but they had a cantilever on each end of around three feet. There was a little stubby vertical over the bearing with a diagonal web coming from the top chord panel point down to that little vertical. Experience says that under almost any gravity loads case, that diagonal (or kicker as I call them) is going to need at least one and possibly two lateral restraints (along with the associated diagonal bracing periodically to resolve the forces) to have the truss perform as the sealing Truss Design Engineer intended. I explained that when the truss went under load (snow loading being rather prevalent up here in the upper Midwest) that web would see considerable compression load, so much such that it would want to buckle out of plane. The Super was having none of that kind of talk. He informed me that he had been in the building trades since the beginning of time and that he had never seen a web bow the way I was describing. This was going nowhere at an alarming rate! I asked him if I could have a few more minutes of his time and he said sure. I was pretty sure that he figured that he was going to need it to teach me a thing or two.
I went out to my vehicle, pulled out my laptop, and met him back at his trailer. Once there he produced the Truss Design Drawings. Sure enough, the kickers on either end of these trusses showed two continuous lateral restraints (CLRs). Surprised, he asked me how I knew that before looking at the drawings. Just a good guess, I said, before explaining the flow of loads through a truss under both gravity and uplift. I also explained that, although they may never see this kind of bowing under construction and ‘sunshine loads,’ they were very real in the built environment. I proceeded to show him on my laptop several situations where webs had bowed significantly to prove my point. I then pulled up the latest edition of the industry’s BCSI Book and showed him graphically the lateral restraint and diagonal bracing necessary. He again stated that he had never installed that kind of bracing unless he was told to do something more by the architect, engineer, or Building Official. He said that his framers didn’t consider it necessary as the trusses were up, plumb, and sheathed. He even pointed out to me that they always run a single flat 2x4 along the top edge of the bottom chord to maintain the truss spacing. He said that as though they were doing something ‘above and beyond.’ When I pointed out that the truss bottom chords should be restrained (and diagonally braced) at a maximum of 15’ on-center for Temporary Bracing and at no more than 10’ on-center for Permanent Bracing, he said that he’d “never seen that done anywhere.” We went to the back of his set of Truss Design Drawings and sure enough, there were the B-Series Supplement sheets showing exactly what we were talking about!
We went on to have a conversation regarding design responsibilities, especially those referenced in his contract documents. Lo and behold, the architect has specified that the trusses be per ANSI/TPI 1. He said he had never heard of that and had no idea what it was. When I pulled that up on the laptop, he was ready to go back outside. He said that wasn’t his job and that if I could send him a link to that (and BCSI) he’d talk to his boss about it. That was pretty much the end of the meeting that day. I offered to come back to review the Truss Design Drawings with him to highlight the other locations where restraints were assumed by the Truss Designer and he said that wouldn’t be necessary.
It was a very friendly visit and we all shook hands before we left. The Territory Manager I was travelling with said I need not worry as he was good friends with the EOR and that he’d be sure that the EOR paid a visit to the site with permanent truss bracing on his mind. Perhaps this one would end happily after all.
My question is this: How many other buildings are being built without regard for the required Permanent Bracing being installed on the wood trusses? We have a lot of additional education to do with everyone involved in the process including the Building Designers (Architects, Engineers, AIBDs, etc.), Building Officials, as well as those doing the physical construction. The performance and integrity of the structure depends on it.
Thanks for reading, SS.