A two-phase research program was undertaken numerically to assess the behavior of the beam outside of the link that is designed for overstrength of the link in eccentrically braced frames (EBFs). In the first phase, software was developed to conduct a statistical analysis of the typical cases designed according to the AISC Seismic Provisions for Structural Steel Buildings. In this analysis, it was noticed that most of the statistically analyzed cases do not satisfy the code requirement provided for overstrength factor. Furthermore, the analyses results revealed that troublesome designs are highly influenced by normalized link length and slenderness of the beam. In this phase, redistribution of forces between beam and brace after the yielding of beam was also studied and it was observed that the forces not carried by the yielded beam are taken by the brace. In second phase, a total of 91 problematic designs were analyzed on finite element program to investigate the effective parameters on the overstrength issue, and overall and local stability of the beam outside of the links. According to analysis results, it was observed that unbraced beam length and flange slenderness are responsible for the stability of the system. Based on these results, the boundary values were suggested to prevent lateral torsional buckling of the beam and local buckling of the brace connection panel separately. Moreover, the overstrength factor specified by code was found conservative for the intermediate and long links although it is fit for the short links.

Seismic-resistant steel eccentrically braced frames (EBFs) are designed so that that yielding during earthquake loading is restricted primarily to the ductile links. To achieve this behavior, all members other than the link are designed to be stronger than the link, i.e. to develop the capacity of the link. However, satisfying these capacity design requirements for the beam segment outside of the link can be difficult in the overall design process of an EBF. In some cases, it may be necessary to make significant changes to the configuration of the EBF in order to satisfy beam design requirements. If this discovery is made late in the design process, such changes can be costly. The overall goal of this research was to develop guidelines for preliminary design of EBFs that will result in configurations where the beam is likely to satisfy capacity design requirements. Simplified approximate equations were developed to predict the axial force and moment in the beam segment outside of the link when link ultimate strength is developed. These equations, although approximate, provided significant insight into variables that affect capacity design of the beam. These equations were then used to conduct an extensive series of parametric studies on a wide variety of EBF configurations. The results of these studies show that the most important variables affecting beam design are 1) the nondimensional link length, 2) the ratio of web area to total area for the wide flange section used for the beam and link, 3) the angle between the brace and the beam, and 4) the flexural stiffness of the brace relative to the beam. Recommendations are provided for selection of values for these variables in preliminary design. / text