Modification of Design Forces in order to Avoid Brace Yielding in Eccentrically Braced Steel Frames

Abstract eng:
In capacity-based design of eccentrically braced steel frames according to AISC-Seismic Provisions (2005), while the links are sized for the load combinations specified by the related code, the members outside of the links are designed to resist the forces generated by fully yielded and strain hardened links. However, after carrying out inelastic static and dynamic analyses of 18 model frames designed according to LRFD (1999) and ASCE 7-05, it is observed that plastic hinges can develop at the braces of some of these frames before the limit state condition of AISC-Seismic Provisions is reached. Both static and dynamic inelastic analyses are carried out with the DRAIN-2DX program. One of the reasons of brace yielding is found out to be the distribution of inertia forces along the building height which is different from the distribution of the equivalent lateral design forces for high-rise frames (higher mode effects). The other reason is the difference of the distributions of the member forces in the elastic and inelastic frame analyses which the former is utilised during the design procedure. In this paper, the design forces of braces of such frames are modified to avoid brace yielding.

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