The first time I had to deal with a statics problem was first semester physics. It wasn’t too crazy – something like levers to introduce the concept of balance of forces and moments. Later, we would enjoy an entire semester-long course dedicated to statics. Beam analysis, trusses, multiple point loads, concentrated moments and other tricks Professor Meyer threw at us during his infamous Friday morning pop quizzes. It was a 7 am class, so quizzes were on Friday to make sure we showed up.
A few weeks back, we were developing anchor reactions for the new SJC steel-joist connectors when used in a kicker application. We wanted to publish anchor reactions for the given connector allowable loads so engineers could skip that step in the calculations.
First step was a lot like statics class. Draw a free body diagram with loads and resolve the reactions. A simple model ignoring eccentricity gave us a load we knew was too small. Adding eccentricity and prying forces gave us a load that seemed way (way!!) too large. We used finite element models to better understand the forces in the connection.
Of course, there is no substitute for physical testing. So we also designed a test setup to capture the anchor tension forces directly. Eliminating as much friction from the setup as possible required some precision machining, several rounds of trial and error and a lot of patience from the lab technicians building everything. The mechanics of the final setup are fairly straightforward. The anchor rods attach to the blue load cells, which measure the tension forces in the rod directly.
The test results correlated very well with the original FEA models. I still marvel at the number of tests that go into creating one number for a load table. Of course, even knowing the anchor forces, we still ran a series of tests in metal deck – just to be sure.
Let us know what you think about the testing in the comments below.