Large Trial Displacement

[salehi]

Hello Frank,

I have developed a force-based finite-length joint element and built a whole OpenSees project having it implemented in. In order to verify the element's response, I am trying to perform a displacement-control pushover analysis on a model including only my element as a cantilever beam-column with the length of 80 mm and square section of 400 mm depth. The problem is that after the axial static load is applied (say as gravity load), the axial component of the trial displacements got from the top node (theNodes[1]->getTrialDisp()) is very large (about 10000 times larger than the last committed step's counterpart), although pushover displacement increment is laterally imposed. I tried to see if I face the same problem while using forceBeamColumn with the same length and sections, and it led to the same issue. Could I possibly know why it happens and how I may resolve this problem? I tried to figure out how displacementControl integrator works but I could not understand it completely.

Thanks so much in advance...

[fmk]

your model is possibly incorrect; the axial stifness is way to small or it is yielding on application of lateral load. replace the element with an elastic element to see what happens to the displacements.

[salehi]

Thanks for your reply, it gave me an insight into the potential issue, but my model is correct and its axial strength is way larger to be yielded at the start of lateral loads' application.

Now I am trying another model which is more complex and comprises my own implemented element, which is a force-based, finite-length, 3-integration point, joint element and includes a friction hysteretic model at the middle integration point. When I try applying a lateral load to the top of a short cantilever including my element which connects two elastic beam-column elements, after a short time that sliding happens in the friction model, again I see that the trial basic displacements which are related to the lateral deformation of my element (two end rotations in the basic reference system) get too large (in the order of 10 Rad or larger)... By the way, in the model there also exist two post-tensioning tendons connecting the top elastic segment to the base and now I am using HHT integrator to numerically add damping to the system... Is the reason behind this the too small friction stiffness as sliding initiates (1.0E-4)? How may I tackle this issue? Does it have to do with the integrator? It is worth noting that my element's stiffness matrix is reasonable.

Thanks so much in advance.

[salehi]

I found out what the problem was and solved it... the problem was existence of a number of gap elements in the model which were of very large stiffness; these elements would cause the numerical solution mess up. I reduced those elements' stiffness value and now there is no problem in terms of trial displacements.