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

This paper discusses the issues involved in using simplified second-or der inelastic analysis for design of steel-frame structures. In partic ular the elastic-plastic hinge method is examined and the assumptions inherent in its use are outlined. It is shown that this method does no t always lead to an accurate representation of inelastic behavior, and that it does not furnish enough information for assessment of the ult imate strength of individual members. An alternative approach called t he refined plastic-hinge method is introduced, and its design implicat ions are discussed. The refined plastic-hinge approach accounts for in elastic 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 illustr ates the qualities and limitations of these two inelastic analysis met hods with regard to their acceptability for use in predicting the ulti mate strengths of steel frames and their members. Case studies and rec ommendations for use of inelastic analysis for direct frame design are presented in a companion paper.