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.