Estimating the Required Axial Force for Designing Beams in Eccentrically Braced Frames

Abstract

One of the key factors affecting the shear capacity of a link beam, which is the ductile member of an eccentrically braced frame (EBF) system, is the axial force present in this beam. Due to reasons such as inadequate diaphragm models and simplified linear analyses, this axial force is often not accurately estimated, potentially leading to improper design and significant damage during earthquakes. Beams outside the link region also experience considerable axial forces, which are typically overlooked in design. Additionally, factors like the placement of the braced span in conventional capacity-based methods are not considered.

In this study, by distributing seismic loads along the frame and using equilibrium-based analysis of the ultimate capacity of beams, simplified relationships are proposed to estimate the axial force in both link beams and beams outside the link region. The foundations of link beams in EBFs with various permitted connection types are presented, and the accuracy of these methods is evaluated through linear and nonlinear static analyses. The results of the proposed approximate analyses show good agreement with nonlinear analysis results, with an average error of less than 10% compared to numerical analyses.

Factors such as frame geometry, braced span location, connection type, and cross-sections of link beams and braces are among the most influential variables in determining the axial forces in beams. For instance, as the braced span moves closer to the frame edges and connections become less restrained (e.g., simple connections), the axial forces in the beams increase significantly, highlighting the need for careful consideration during design.