A new improved fiber plastic hinge method accounting for lateral-torsional buckling of 3D steel frames

This paper presents a new advanced analysis method, specifically a new improved fiber plastic hinge method, for analyzing the nonlinear inelastic behavior of 3D steel frames accounting for lateral-torsional buckling. The second-order effects are considered by the use of the geometric stiffness matrix and stability functions obtained from the exact solution of beam-columns under axial force and bending moments at two ends. The spread of plasticity along the member length due to both residual stresses and the impact of axial force is considered by utilizing the Column Research Council (CRC) tangent modulus concept, while the gradual yielding due to flexure is represented by two fiber plastic hinges at the ends of the element. The lateral-torsional buckling stiffness matrix is established by the virtual work principle using the updated Lagrangian formulation. The generalized displacement control method is applied to solve the nonlinear equilibrium equations in an incremental-iterative scheme. The nonlinear load-displacement behavior and ultimate load results compare well with those of previous studies. It is concluded that accurately using only one element per member likely predicts the second-order inelastic behavior of 3D steel frames including the effect of lateral-torsional buckling.