2ND-ORDER REFINED PLASTIC-HINGE ANALYSIS FOR FRAME DESIGN .1.

This paper discusses the issues involved in using simplified second-order inelastic analysis for design of steel-frame structures. In particular the elastic-plastic hinge method is examined and the assumptions inherent in its use are outlined. It is shown that this method does not always lead to an accurate representation of inelastic behavior, and that it does not furnish enough information for assessment of the ultimate strength of individual members. An alternative approach called the refined plastic-hinge method is introduced, and its design implications are discussed. The refined plastic-hinge approach accounts for inelastic stiffness degradation, and a column tangent-modulus expression is utilized to represent the effective stiffness of the element when it is loaded with large axial loads. This paper highlights and illustrates the qualities and limitations of these two inelastic analysis methods with regard to their acceptability for use in predicting the ultimate strengths of steel frames and their members. Case studies and recommendations for use of inelastic analysis for direct frame design are presented in a companion paper.