Cyclic Elastoplastic Analysis and Stability Evaluation of Steel Braces of Hollow Section

Abstract

This paper deals with the cyclic elastoplastic analysis and stability evaluation of steel braces of hollow sections subjected to axial tension and compression. The inelastic cyclic performance of cold-formed steel braces of circular and box hollow sections is examined through finite element analysis using the commercial computer program ABAQUS. First some of the most important parameters considered in the practical design and ductility evaluation of steel braces of tubular hollow sections are presented. Then the details of finite element modeling and numerical analysis are described. Later the accuracy of the analytical model employed in the analysis is substantiated by comparing the analytical results with the available test data in the literature. Finally the effects of some important structural and material parameters on cyclic inelastic behavior of steel braces are discussed and evaluated. INTRODUCTION Steel braced frames are one of the most commonly used structural systems because of their structural efficiency in providing significant lateral strength and stiffness. The steel braces contribute to seismic energy dissipation by deforming inelastically during an earthquake. The use of this type of construction indeed avoids the brittle fracture found in beam-to-column connections in moment resisting steel frames that occurred in the Northridge earthquake in 1994 and the Kobe earthquake in 1995 (ASCE 2000, IGNTSDSS 1996). However, careful design of steel braced frames is necessary to avoid possible catastrophic failure by brace rupture in the event of a severe seismic loading. The current capacity design procedure adopted in most seismic design steel specifications (AISC 1997, CAN-CSAS16.1 1989), for concentrically braced frames requires yielding in the braces as primary members, whereas the secondary members of the frame should remain elastic and hence carry forces induced by the yielding members. The transition from current perspective seismic codes to performance-based design specifications requires accurate predictions of inelastic limit states up to structural collapse. The cyclic behavior of steel brace members is complex due to the influence of various parameters such as, material nonlinearity, structural nonlinearity, boundary condition, and loading history. The material nonlinearity includes structural steel characteristics such as, residual stresses, yield plateau, strain hardening and Bauschinger effect. The structural © 2008 ASCE 18th Analysis and Computation Specialty Conference Copyright ASCE 2008 18th Analysis and Computation Specialty Conference nonlinearity includes parameters such as, brace slenderness parameter, cross-section slenderness, width-to-thickness ratio of the cross-section’s component elements (or radius-to-thickness ratio of circular hollow sections), and initial out of straightness of the brace. The complex behavior results in various physical phenomena, such as, yielding in tension, buckling in compression, postbuckling deterioration of compressive load capacity, deterioration of axial stiffness with cycling, and low cycle fatigue fractures at plastic regions. Steel braces can be designed to resist only tensile forces, or to resist both tensile and compressive axial forces. Recent earthquakes and experiments have shown that the tensioncompression braces provide better performance under cyclic loading (during an earthquake) as compared with the tension-only braces having almost no compressive strength. Under severe earthquakes, the braces are subjected to cyclic axial forces and they are allowed to undergo compression buckling or tensile yield to dissipate the imposed energy while columns and collector beams respond elastically. Therefore, understanding the behavior of the bracing members under idealized cyclic loading is an important step in the careful design of steel braced frames. This paper deals with the cyclic elastoplastic analysis of steel braces. The most important parameters considered in the practical design and ductility evaluation of steel braces of tubular hollow sections are presented. The cyclic elastoplastic performance of steel braces is examined through finite element analysis using the computer program ABAQUS. The accuracy of the analytical model employed in the analysis is substantiated by comparing the analytical results with the available test data in the literature. The effects of some important structural and material parameters on inelastic cyclic behavior of steel braces are discussed and evaluated.