Response Gradients for Nonlinear Beam-Column Elements under Large Displacements

Accurate and efficient response gradient computations for nonlinear geometry are required in structural reliability, optimization, system identification, and response sensitivity analysis of frame structures that undergo large displacements. In this paper, the exact response gradient of beam-column finite elements under large displacements is derived considering uncertain material and geometric parameters. The element response formulation takes place in a corotational reference frame that displaces and rotates with the element, thus permitting the separation of nonlinear material from nonlinear geometric effects in the computation of the response, as well as of the gradient of the response. Relative to the corotational reference frame, small deformation theory suffices for all structural engineering applications. Thus, the proposed response gradient computations are applicable to most beam-column element formulations available in the literature, including force-based and mixed formulations.