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This paper presents an analytical study on seismic performance of special concentrically braced frames (SCBFs) with and without brace buckling. It demonstrates that the collapse prevention goal needs attention in modern buildings using SCBFs as main seismic-force-resisting systems, and an simple and low-cost solution is possible to achieve this goal. Typical 6-story buildings using two-story X-braced frames with strong and weak braced-intersected beams and inverted V braced frames with and without brace buckling were subjected to a set of 15 earthquake ground motions, and resulting seismic responses were discussed in terms of seismic strength and deformation demands on braces, beams and columns. The study finds that the braces in SCBFs often fracture prior to 2% story drift ratio response, particularly in the popular two-story X-braced frames with weak beams, mainly due to premature yielding of the beams. The buckling-controlled braced frames (BCBFs) are shown to be a cost-effective system to improve the seismic performance of SCBFs. The analysis indicates that the buckling-controlled braces may substantially reduce story drift response, eliminate weak beam yielding, and prevent braces from fracturing.

Ductile concentrically braced frames (CBFs) with circular hollow shapes (CHS) have been one of the most common structural systems in seismic regions and are expected to undergo significant inelastic deformations mainly through post-buckling or yielding deformations in their bracing members. However, a comprehensive survey of the experimentally tested CHS specimens carried out in the course of this study indicated that there exists a discrepancy between the expected demand and capacity of conventional CHS bracings. To overcome this quantitatively identified inconsistency, which might result in a poor performance under severe ground shakings, a new and simple performance-enhancing method is proposed and examined. The developed improvement technique, which can be implemented in both new and existing steel buildings, is employed by encasing a CHS bracing in two channels connected with welded stitches. Experimental and numerical investigations into the effectiveness of the developed system showed that the developed channel-encased bracings are promising in terms of improving the cyclic stability and cumulative energy dissipation capability compared with the conventional bracings.

Reliably predicting brace fracture is crucial in seismic evaluation of post-fracture performance of concentrically braced frames. This paper evaluates existing fracture models, proposes a new fracture initiation model based on a large amount of experimental data, and compares all models with the test results. The existing ductility-related empirical models and strain-related fiber models are evaluated by experimental tests. A comprehensive database of previously tested square hollow structural sections from different research programs is established to conduct a regression analysis leading to development of a fracture initiation model. The study concludes that the existing fracture models are unable to predict fracture events observed during physical tests. Furthermore, the existing strain-related fracture models, accessible to all users in OpenSees, fail to simulate the cyclic response of braces beyond initial inelastic deformation. The fracture initiation model proposed by this study depends on the reliable measure of ductility, and it has consistently predicted fracture initiations observed during the tests. The fracturing process concept, along with the proposed fracture initiation model, is able to simulate the cyclic response of the braces, whether they are tested as single brace members or in actual braced frames, throughout inelastic deformation, fracture initiation, fracture propagation, and terminal fracture stages, providing a much-improved tool for numerical simulation of the post-fracture response of concentrically braced frames.

A study on the seismic demand on braced-intersected beams in two-story X-braced frames is presented to address a major concern: whether or not the beam is likely to become inelastic during the design earthquake ground motion, and if so, how such inelastic deformation affects the seismic behavior of the frames as a whole. Typical 6- and 12-story buildings using two-story X-braced frames with strong and weak braced-intersected beams were subjected to a set of 20 earthquake ground motions, and resulting seismic responses were discussed in terms of seismic strength and deformation demands on beams and the impact of inelastic deformation in weak beams on critical components in two-story X-braced frames. In addition, the 6- and 12-story buildings with inverted V-braced frames were also studied for comparison. The study concludes that brace-intersected beams in two-story X-braced frames designed with the minimum possible required strength permitted by the current design provisions would undergo significant vertical inelastic deformation when the frames experience the expected 0.02–0.04 story drift ratio response, and the inelastic deformation in the middle spans of brace-intersected beams substantially increases ductility demands on braces and beam-to-column connections. This study also finds that the use of story drift ratio as sole deformation response index fails to reveal actual seismic response of critical components in two-story X-braced frames when beams experience inelastic deformation within their own spans.

Steel moment frames have gone through intensive investigation to understand the cause of the damage since the 1994 Northridge–California earthquake. A research conducted by the SAC Joint Venture has helped to improve the current code requirements for steel special moment frame design. One major outcome of these research efforts was on the enhanced design requirements for column splices. For example, AISC 341 Seismic Provisions require that column splices in intermediate and special moment frames, when not made using complete joint penetration welds, be designed to develop the expected flexural strength of the smaller connected column and the shear demand associated with flexural hinging at the top and bottom of the spliced column. This paper focuses on the effect of various span lengths on seismic demands on column splices in steel moment-resisting frames. A comprehensive nonlinear analytic investigation was undertaken to evaluate the seismic responses of 4- and 9-story moment-resisting frames with constant and various span lengths subject to an ensemble of 20 strong ground motions. The results indicate that the structural response is sensitive to the span length and variable span length within the frame might significantly increase the seismic demand on column splices.

Structural performance in ductile concentrically braced frames (CBFs) can be generally associated with the plastic deformation capability of the bracing members. Considering the popularity of ductile CBFs incorporating square hollow structural shapes (HSS) in seismic areas, a simple and effective performance-enhancing technique for existing CBFs is developed. This paper discusses the hysteretic behavior of the developed channel-encased braces through finite element (FE) simulations and assesses the impact of the developed performance-enhancing technique on the seismic response of CBFs. For this purpose, first, the hysteretic stability of the braces has been examined through non-linear FE simulations. Then, the strain demands on the enhanced braces are compared with the conventional square HSS. Subsequently, a set of ductile CBFs designed in accordance with the current design codes are subjected to an ensemble of ground motion records to recognize the effect on the overall structural response. Our results point out that the developed enhanced braces are promising in terms of capability to improve cyclic stability, and consequently energy dissipation capacity of HSS by mitigating possibility of the potential failure modes that might take place in conventional HSS.

Given the lack of comprehensive research on seismic demands on column splices in special concentrically braced frames (SCBFs), this study presents an inclusive investigation on splice demands in SCBFs to address whether the seismic design requirements for column splices in AISC Seismic Provisions are conservative or not. The study numerically evaluates the nonlinear response of 5-story and 13-story SCBFs with common two-story X-bracing configuration under an ensemble of strong earthquake ground motions. For each SCBF, two structural models were considered to examine the impact of shifting column splice locations on splice demands. The study concludes that flexural demands on column splices exceed the minimum required flexural design strength stipulated in the seismic provisions for the column splices in SCBFs. The likelihood of reaching the nominal capacity of the smaller column is very high for column splices under combined effects of axial force and bending moment, indicating an unsatisfactory margin of safety against the excessive inelastic deformations expected to take place in ductile braced frames.