Overstrength and Design Capacity of Yielding Links in the Presence of Axial Loads in Steel Eccentrically Braced Frames

Steel eccentrically braced frames (EBFs) have been used widely in steel construction since they were first proposed in the 1970s. In addition to conventional yielding links, replaceable shear links with wide-flange sections (W-sections), double-channel section (C-sections), and built-up tubular sections have been proposed for EBFs. More recently, cast steel replaceable modular yielding links (CMLs), which are designed to undergo distributed flexural yielding, have been proposed as yielding links in EBFs. Previous experimental studies have demonstrated that the presence of axial loads affects the shear capacity in yielding links. In particular, axial compression reduces the shear capacity, whereas axial tension increases the strength of links. This study first briefly reviews past research on EBFs and previously proposed yielding links, and highlights the circumstances under which axial loads could be present in yielding links. Simplified expressions are provided to quantify the axial load in yielding links in common EBF configurations. The effects of axial loads on the capacity and overstrength of shear and flexural yielding links are then discussed in detail. Simplified equations are derived and proposed to account for additional overstrength under axial tension and strength reduction under axial compression, for both shear and flexural yielding links. The equations for flexural links were verified using past experimental results of CMLs. The proposed equations for shear links were verified using results from experimentally verified finite-element analyses of W-sections and built-up hollow box shear links. In the end, a procedure is presented for considering axial load effects on the response of yielding links in the design of EBFs.