Simplified nonlinear analysis for steel frames

Faisal Abdullah Al-Mashary, Purdue University

Abstract

Although the non-linear analysis of steel frames has been the subject of research for many years, the application of this type of analysis to engineering practice is not widespread. This is attributed in part to the complexity of this type of analysis and in part to the lack of clear understanding of the implications of these non-linear effects on the structural behavior. This study intends to fill the gap between theory and practice in the case of steel building frames. In this study, practical methods for elastic and plastic second-order analyses for rigidly and flexibly connected steel frames are presented. The second-order analysis is simplified here using the fact that the axial forces predicted by the first-order analysis are generally close to the exact values. To this end, a one-iteration procedure using the stability functions approach is proposed. The spread of plasticity in the members of a frame is modeled by a pseudo rotational spring with variable stiffness. The spring which is attached to the ends of all members is adjusted according to the plasticity index of the member-end. The spring stiffness decreases with the increase of plastification in the member until it reaches a zero stiffness at the full plasticity of the member-end. The non-linear joint flexibility is also considered in the present analysis using the same end-springs. In this case, the spring is assigned the lower value predicted by the plasticity consideration or by the connection stiffness model. The connection behavior is described by a three-parameter model that can be calculated in the absence of experimental data. A simple incremental method is used to trace the non-linear frame behavior for both the plasticity and the connection flexibility effects. These proposed simplified methods of analysis are checked against exact solutions to provide the final confirmation on the validity of the proposed methods. They are also compared with some applicable approximate methods such as the AISC/LRFD B$\sb1$-B$\sb2$ method for second-order elastic analysis and the alignment chart effective length factor. The proposed methods showed good agreement with exact methods over the available approximate methods.

Degree

Ph.D.

Advisors

Chen, Purdue University.

Subject Area

Civil engineering

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