Cyclic stability and capacity design of steel eccentrically braced frames

The successful performance of steel eccentrically braced frames (EBFs) under seismic loading depends on stable inelastic rotation of links and the ability of other frame members to facilitate these link rotations. This dissertation addresses critical issues relating to link rotation demands and capacities that were not addressed in initial development of the system and new EBF issues raised by changes in the steel industry and design provisions over the past decade.; In response to unexpected failures of recently tested links, experimental data and finite element models were used to quantify the effect of the testing loading protocol on short link rotation capacity. Past protocols were evaluated in light of maximum and cumulative link rotation demands determined by analyzing nine EBFs with a suite of 20 design level earthquake ground motions. Experimental loading protocols for past links studies inappropriately represent cyclic demands. A new loading protocol is proposed.; Current design provisions require link sections to satisfy the same flange width-thickness requirement as beams in steel moment frames in order to delay flange local buckling and ensure stable inelastic rotation. This significantly impacts EBF economy since capacity-based design dictates heavier braces and columns to resist the ultimate strengths developed in the oversized links. Link finite element models that simulate strength degradation associated with web and flange local buckling were developed in a nonlinear finite element analysis program (ABAQUS) and verified with experimental data to investigate the impact of flange width-thickness on link stability. A parametric study of more that 100 links indicates a relaxation of the current flange width-thickness requirements is justified.; The ultimate strength of EBF links must be known to safely design beams, braces, and columns to remain elastic. A procedure to compute ultimate link shear strength, which accounts for shear that can be carried by link flanges in the ultimate state, was developed and verified using experimental and finite element data.; Capacity-based design of columns in EBFs is challenging since links do not yield and harden uniformly along the height of a building. Results from EBF frame analyses were used to quantify reasonable combinations of link forces for computing column demands along the frame height.