Contests where researchers and practitioners blindly predict the response of structural systems have produced some rather interesting results. And by “interesting”, I mean “all over the place”. So much so, that in an effort to protect the contestants, the contest organizers rarely make the results publicly available.
Nonlinear structural analysis is hard though. Even with detailed structural drawings, material properties, and loading protocols, it’s easy to run into problems with units, dimensions, element formulations, constitutive models, numerical solution algorithms, and boundary conditions.
So, let’s take a step back and determine how well we can predict, or rather, agree upon, the linear response of a structural model that’s pretty well defined. A colleague in Eastchester studied the experimental behavior and numerical simulation of strongback frame systems during their Ph.D. Braced frames offer a multitude of teaching moments for linear and nonlinear structural analysis, so I’ve integrated my colleague’s strongback models in my courses. One such model is shown below.
The model shown here assumes infinitesimal joints, a simplification of the model with rigid offsets presented in this paper. All members are steel (=200,000 MPa), except the BRB, which we’ll assume has an effective of 300,000 MPa. Note that the column members on the right side of the frame are turned for weak axis bending. Also note that the left support is fixed while the right support is pinned.
So, what is the horizontal displacement (units=mm) of the joint indicated in the figure? The total horizontal load is only 1 kN, putting the frame response well within the linear-elastic, small displacement range.
E-mail me your answer, or if you are brave, post your answer in the comments section below. You don’t have to use OpenSees, use whatever software you want or whatever back of an envelope you can find. Let me know if you have any questions.
This will be fun! I look forward to your responses. I will share anonymized results in another post.
Update (March 6, 2020): The shear tabs are intended to be moment releases.