by Jason Fitch, Engineer in Training at RedBuilt
(Editor’s Note: There are three levels of open web certification. Level I is the introductory understanding of how to use software, and read and understand the output. Level II provides the understanding of how to manipulate designs within the software for things like fixed panels, header clips, etc. Most designers are Level II certified. Level III certification is the highest level of certification and entails designing custom trusses and profiles by hand. Level III certified designers are the Yoda’s of open web truss design, and there are only a handful of them. Jason is providing a series of blogs about his experience in attaining Level II certification. We are looking forward to the day he can blog about his Yoda [Level III] certification!)
I am an EIT at the Boise Engineering office and have been working on my Open Web Level II certification. In order to get my certification, I had to design, build and test a truss. Since the Boise Design Center is not a manufacturing site, I was sent to our Open Web plant in Hillsboro, OR for the build/break part of my training. This was very exciting for me because getting a chance to travel to the plant and not only see, but to also participate in the making of a truss is not something I get the opportunity to experience very often. As someone who started their career in this industry as a framer, I was looking forward to being able to physically put something together, and of course, I was equally excited to test (code for break) what I make. As a former little boy, I still have this excitement and urge to break things, and this time it was not only okay but it was required that I break it, and I was paid to do it, too!
The first step was designing my truss. Rex Flegel, who has been in charge of my Open Web Level II training, came up with some ideas. With the help of Dave Vanderzanden from the Hillsboro Office, we compiled a “good” truss design. I should qualify the “good” design. It was not good from a manufacturing or efficiency of design aspect, but it was good as far as a teaching tool. This truss was designed to have multiple aspects that tend to make manufacturing difficult. In this case, a tapered truss with a very shallow end depth was selected. There were also some tight web to web angles (at the limits), and it also included extra hardware that had to be squeezed into a rout.
Once the truss was designed, I had to then predict the failure. The design output stated that there would be a pin failure, or a Hankinson failure [Wood has an allowable stress both perpendicular to grain and parallel to grain. Hankinson’s formula is used to find the allowable stress at an angle. Hankinson failure would result in pin bearing force at an angle which exceeds the allowable based on Hankinson’s formula.]. Knowing a couple of things like:
- The design was based on 2700 fb MSR (machine stress rated) solid sawn lumber chords but the plant uses 2850 fb MSR
- Wood is pretty good for short-term load durations like the truss would experience in a test
I decided that the wood would not fail first. This eliminated the panel and pin controls. Also knowing that slender steel members do well in tension and not as well in compression, I made my prediction that a steel web would fail in compression. And not just any web, but the longest, lightest gauge compression web would fail by buckling. As a result of this predication, I estimated the ultimate load on the truss to be 328.4 plf (or 10,345# total of both reactions).
Now that I had all my preparations made, and everything was ready, I made my trip out to Hillsboro to build/break my truss design.
Read about the next part of my training, Putting Together the Pieces.