EVALUATION OF NONLINEAR FRAME FINITE-ELEMENT MODELS

In recent years nonlinear dynamic analysis of three-dimensional struct ural models is used more and more in the assessment of existing struct ures in zones of high seismic risk and in the development of appropria te retrofit strategies. In this framework beam finite-element models o f various degrees of sophistication are used in the description of the hysteretic behavior of structural components under a predominantly un iaxial state of strain and stress. These models are commonly derived w ith the displacement method of analysis, but recent studies have highl ighted the benefits of frame models that are based on force interpolat ion functions (flexibility approach). These benefits derive from the f act that models with force interpolation functions that reproduce the variation of internal element forces in a strict sense yield the exact solution of the governing equations in the absence of geometric nonli nearity. While the numerical implementation of force-based models at f irst appears cumbersome, simple examples of nonlinear analysis in this paper offer conclusive proof of the numerical and computational super iority of these models on account of the smaller number of model degre es of freedom for the same degree of accuracy in the global and local response. A numerical implementation that bypasses the iterative natur e of the element state determination in recent force-based elements is also introduced, thus further expanding the benefits of flexibility-b ased nonlinear frame models.