Paul W. Richards

Design of Buckling-Restrained Braced Frames Using Nonlinear Time History Analysis and Optimization

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.

Estimating the Stiffness of Eccentrically Braced Frames

The stiffness of eccentrically braced frames (EBFs) is difficult to calculate by hand because they are indeterminate and have significant flexural, shear, and axial deformations in various members. EBF stiffness is important because it is used to compute story drifts and link rotations which have prescribed limits. This note presents an equation for the stiffness of an EBF story in terms of the design story shear, frame geometry, and beam depth. The equation is independent of specific member sizes, making it useful for determining appropriate geometry in early design. The equation is developed theoretically and validated with data from code compliant EBFs. One application of the equation is the direct calculation of the minimum beam depth required to ensure a link will satisfy inelastic rotation limits.

Efficiently Implementing Genetic Optimization with Nonlinear Response History Analysis of Taller Buildings

AbstractNonlinear response history analysis is an important tool for accurately determining the performance of tall buildings under severe earthquake loading. When a standard genetic algorithm is used in conjunction with nonlinear response history analysis, it is desirable to use smaller generation sizes because of the computational effort to analyze individual designs. A study was conducted to evaluate how different genetic algorithm techniques influence the reliability and efficiency of the algorithm when used with nonlinear response history analysis and small generation sizes. The system used in the study was a nine-story buckling restrained braced frame that was optimized to minimize brace areas under individual earthquake records. A baseline study showed that a typical genetic algorithm did not converge to the same best design for different random number sequences (seed numbers). Forced diversity improved the reliability of the algorithm such that it converged to the same optimum, regardless of initi...

Seismic performance of buckling-restrained braced frames with eccentric configurations

Braced frames are often used to resist lateral earthquake loads in steel buildings, but braces can interfere with architectural features. Eccentrically braced frames (EBFs) will accommodate windows, doors, and halls, but have performance limitations when link-to-column connections are required. An alternative to EBFs may be buckling-restrained braced frames with eccentric configurations (BRBF-Es). This paper introduces the concept of BRBF-Es and highlights design considerations. An analytical study was conducted that compares the performance and economy of BRBF-Es with EBFs. Results from non-linear time history analyses indicate that BRBF-Es will have greater residual drifts than comparable EBFs, but are less susceptible to failures at link-to-column connections. BRBF-Es require more steel than EBFs, but savings in design, fabrication, and erection may offset higher material costs.

Experimental and Numerical Investigation of Ductile Top-Flange Beam Splices for Improved Buckling-Restrained Braced Frame Behavior

Buckling-restrained braced frame performance at high drifts is improved by providing beam splices that reduce demands in the gusset regions. Existing experimental data only consider web splices without a slab present. An alternative top-flange splice, proposed by others, was investigated experimentally and numerically. Two full-scale top-flange beam splices from a prototype frame were tested using the qualifying buckling-restrained brace frame cyclic loading protocol. During experimental testing, the gusset connection regions remained essentially undamaged through multiple cycles at 0.06rad drift. The splice plates experienced low inelastic strains but fatigue analyses indicate they could withstand over fourteen similar loading histories without requiring replacement. Finite element models were used to investigate the influence of slabs on connections with web splices or top-flange splices. When slabs were considered, the top-flange splice transmitted over seventy-percent less moment than the web splice.

Demands on Reduced Beam Section Connections with Out-of-Plane Skew

AbstractSome architectural designs for steel buildings require skew at moment frame connections. Reduced beam section (RBS) moment connections with out-of-plane skew have not been tested experimentally, and it is unclear how much skew, if any, should be permitted. In this study, finite-element models were used to investigate the effect of out-of-plane skew on RBS moment connection rotation capacity. The parameters investigated were the column depth, amount of skew, and model configurations (boundary conditions). The models simulated inelastic buckling in the RBS and corresponding strength degradation under cyclic loading. A low-cycle fatigue damage model was used to evaluate the results to determine if out-of-plane skew made the RBS or weld regions more susceptible to fracture. It was found that out-of-plane skew resulted in increased column twisting and minor yielding at the column flange tips, but not lower connection rotation capacity or increased susceptibility to fracture.