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A procedure for the design of buckling-restrained braced frames is presented. The procedure uses nonlinear time history analysis and optimization. Results from this procedure are compared to those from the commonly used equivalent lateral force procedure. Results are used to develop design curves and formulas for rapid determination of cross-sectional areas for buckling-restrained braces in each story. One interesting result is that the optimum variation of area from story to story is nearly linear for most cases studied. This result is not predicted by the equivalent lateral force procedure, and can only be discovered through optimization and nonlinear time history analysis. DOI: 10.1061/(ASCE)ST.1943-541X.0000007 CE Database subject headings: Bracing; Frames; Buckling; Optimization; Algorithms; Time factors; Nonlinear analysis.

In ductile braced frames, column seismic demands depend on the strength of ductile elements and yielding patterns that occur during earthquake loading. Column demands were investigated in buckling restrained braced frames (BRBFs), special concentrically braced frames (SCBFs), and eccentrically braced frames (EBFs) with different heights (3, 9, and 18 story) and strength levels using nonlinear time history analysis. For columns at the base of 9- and 18-story BRBFs and EBFs, axial demands observed from the analyses were 55-70% of demands commonly used in design, indicating potential cost savings on columns, anchor rods, base plates, and foundations in tall buildings; however, in low-rise SCBFs with braces in the 2-story X-configuration, column axial demands were up to 100% greater than those commonly used in design because of force redistribution that occurs after brace buckling. Column rotations in all frames were less than 0.03 rad. These rotation demands are lower than rotation capacities that have been demonstrated in other work. DOI: 10.1061/(ASCE)0733-9445(2009)135:1(33) CE Database subject headings: Steel frames; Steel columns; Seismic design; Seismic analysis; Nonlinear analysis; Bracing; Foundations.

The inelastic rotation capacity of links in eccentrically braced frames (EBFs) is dependent on testing loading history or sequence. Current design rotations for links are based on experiments from the 1980s performed with somewhat arbitrary loading protocols. An AISC link loading protocol, derived from the standard moment frame loading protocol, has been used for link testing since 2002. This protocol results in lower link rotation capacities. Since no previous or current link protocols are based on EBF analyses, nonlinear time-history analyses were conducted to investigate cumulative rotation demands for links in EBFs. Results indicate the AISC link loading protocol is overly conservative, having 1.5 times the cumulative rotation demand and a much higher percentage of large cycles than is justifiable. A rational loading protocol for future link testing was developed based on cumulative and maximum link rotation demands observed from the time-history analyses. Experimental tests performed in a parallel study with this new protocol indicate that link design rotation capacities in the AISC Seismic Provisions are satisfactory. DOI: 10.1061/(ASCE)0733-9445(2006) 132:8(1183) CE Database subject headings: Steel frames; Seismic design; Seismic analysis; Nonlinear analysis; Cyclic loads; Bracing.

An analytical study was conducted Io investigate the influence of the flange width-thickness ratio on the rotation capacity of links in eccentrically braced frames. Finite element models, verified using experimental data, were used to investigate the performance of 112 links. Models simulated strength degradation associated with flange and web local buckling that may occur during cyclic loading. The models provided data for short, intermediate, and long links with a range of flange width thickness ratios. Results indicate that a relaxation of the flange width thickness ratio requirement in current AISC seismic design provisions is justified. It was also found, however, that the provisions specify nonconservative design rotations for some intermediate links. This is shown to be related to web stiffener spacing and not the flange width thickness ratio. DOI: 10.1061/(ASCE)0733-9445(2005)131:10(1546) CE Database subject headings: Flanges: Width: Thickness: Rotation: Frames: Finite element method.