This article presents the formulation and solution of the equations of moti
on for distributed parameter nonlinear structural systems in state space. T
he essence of the state-space approach (SSA) is to formulate the behavior o
f nonlinear structural elements by differential equations involving a set o
f variables that describe the state of each element and to solve them in ti
me simultaneously with the global equations of motion. The global second-or
der differential equations of dynamic equilibrium are reduced to first-orde
r systems by using the generalized displacements and velocities of nodal de
grees of freedom as global state variables. In this framework, the existenc
e of a global stiffness matrix and its update in nonlinear behavior a corne
rstone of the conventional analysis procedures, become unnecessary as means
of representing the nodal restoring forces. The proposed formulation overc
omes the limitations on the use of state-space models for both static and d
ynamic systems with quasi-static degrees of freedom. The differential algeb
raic equations (DAE) of the system are integrated by special methods that h
ave become available in recent years. The nonlinear behavior of structural
elements is formulated using a flexibility-based beam macro element with sp
read plasticity developed in the framework of state-space solutions. The ma
cro-element formulation is based on force-interpolation functions and an in
trinsic time constitutive macro model. The integrated system including mult
iple elements is assembled, and a numerical example is used to illustrate t
he response of a simple structure subjected to quasi-static and dynamic-typ
e excitations. The results offer convincing evidence of the potential of pe
rforming nonlinear frame analyses using the state-space approach as an alte
rnative to conventional methods.