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Multitiered steel concentrically braced frames (CBFs) are commonly used to provide lateral resistance for tall single-story commercial, performing arts, sports, and industrial buildings. The seismic response of these frames is studied in this paper. A set of seven special concentrically braced frames (SCBFs), ranging from 9 to 30 m tall with two to six tiers, located in a high seismic area was designed according to the 2010 AISC Seismic Provisions. Fundamental behavior of the two- and four-tiered frames was investigated using three-dimensional (3D) finite element models with shell elements, with particular focus on the buckling response of the columns. The seismic frame response and column stability were then studied more broadly for all frames using more computationally efficient 3D finite element models with fiber-based beam-column elements, which were validated against the shell element models. Multitiered CBFs designed according to current multistory CBF procedures are shown to develop drift concentration in a single tier and high in-plane column bending demand, which in some cases leads to flexural yielding and column instability. As potential solutions to this problem, alternate design strategies were studied and their seismic performance is also presented. Designing for higher seismic forces did not appreciably improve column stability, but use of fixed column bases or buckling-restrained braces provided improved distribution of drift over multiple tiers and reduced the occurrence of column instability. Unlike multistory braced frame seismic design, column flexural demands are more important in multitiered braced frames and must be considered in seismic design. [ABSTRACT FROM AUTHOR]

A seismic design strategy, which is intended to be implemented within the framework of the U.S. seismic design provisions for steel structures, is presented for single-story steel concentrically braced frames that are divided into two tiers. In this method, the columns are designed to resist the axial loads acting in combination with the in-plane flexural demand resulting from uneven distribution of brace inelastic deformations over the frame height. This design procedure, which establishes enhanced requirements beyond the 2010 edition of the U.S. seismic design provisions, prevents concentration of deformation in one tier and causes frame nonlinear deformation to be distributed between the tiers. The column bending moments depend on the story shear resistance that develops in each tier when the bracing members are at buckling and in the postbuckling range. The method also aims to control tier drifts to protect the bracing members from excessive inelastic demand, which could cause brace fracture. Nonlinear static and dynamic analyses are performed to validate the proposed design procedure. [ABSTRACT FROM AUTHOR]

This article investigates the seismic design and response of steel multi-tiered concentrically braced frames (MT-BFs) in which braces meet at columns between diaphragms. The seismic design provisions of CSA S16-14 are described and illustrated for three-tiered Type MD (moderately ductile) and five-tiered Type LD (limited ductile) braced frames. Analysis methods are proposed to evaluate the in-plane flexural demand on columns. The seismic response of the frames is examined through nonlinear response history analysis. As assumed in design, inelastic deformations tend to concentrate in one tier over the frame height, causing non-uniform drift demands and in-plane bending moments in the columns. CSA S16 provisions predicted well the frame in-plane flexural response and result in acceptable ductility demands on the braces. An extended seismic analysis and design approach that accounts for vertical distribution of brace tension yielding along the frame height is proposed for frames that exceed the limits prescribed in CSA S16. [ABSTRACT FROM AUTHOR]

The self-centering energy-dissipative (SCED) brace is an innovative cross brace for buildings that provides a nonlinear response with good energy dissipation and postyield stiffness while minimizing residual drift after an earthquake. This provides a high level of seismic performance by allowing structures to remain operational even after major seismic events. Recently, the SCED brace has been improved through the design and experimental evaluation of a high-capacity SCED (HC-SCED) that has an axial capacity similar to some of the largest available conventional cross braces for buildings. This prototype HC-SCED satisfied testing protocols for buckling-restrained braces and exhibited full self-centering behavior during cycles up to 1.5% drift. To characterize the hysteretic response of the brace in detail, a new analytical approach is developed. This new approach is necessary because simplified stiffness estimates do not provide good predictions of the low-amplitude displacement response and initial effective stiffness that was measured in the full-scale experiments. The proposed analytical approach includes the effects of fabrication tolerances, which have been identified as the main reason for incorrect low-amplitude displacement predictions that result from the simplified stiffness estimates. Using the results from the HC-SCED tests, the new analytical approach provided good estimates of the initial stiffness of the braces and also was able to predict the behavior of the brace well under a larger fabrication tolerance scenario. These improved predictions may be used to improve the characterization of the effective hysteretic behavior of actual SCED braces for use in nonlinear time history analyses. [ABSTRACT FROM AUTHOR]

SUMMARY The self-centering energy dissipative (SCED) brace is a new steel bracing member that provides both damping to the structure and a re-centering capability. The goal of this study was to confirm the behavior of SCED braces within complete structural systems and to confirm the ability to model these systems with both a state-of-the-art computer model as well as a simplified model that would be useful to practicing engineers. To these ends, a three-story SCED-braced frame was designed and constructed for testing on a shake table. Two concurrent computer models of the entire frame were constructed: one using the opensees nonlinear dynamic modeling software, and a simplified model using the commercial structural analysis software sap2000. The frame specimen was subjected to 12 significant earthquakes without any adjustment or modification between the tests. The SCED braces prevented residual drifts in the frame, as designed, and did not show any significant degradation due to wear. Both numerical models were able to predict the drifts, story shears, and column forces well. Peak story accelerations were overestimated in the models; this effect was found to be caused by the absence of transitions at stiffness changes in the hysteretic model of the braces. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The global seismic response of 3- and 8-story eccentrically braced frames (EBFs), designed for western and eastern North American locations, is studied using nonlinear time history analysis. Analytical models were built in three computer programs. Similar maximum forces were obtained, but the inelastic deformation predictions at the element and global structural levels showed sensitivity to the modeling employed. This feature should be considered in seismic performance assessment or design. Current design methods failed to predict interstory drifts and plastic link rotations, but the study confirmed the strong correlation between the two parameters associated with rigid-plastic behavior. The influence of yielding and flexural buckling of frame members, other than the links on the global frame performance, was evaluated using the OpenSees model. It was confirmed that the flexural yielding of outer beams is acceptable for EBFs with short and intermediate links, if the combined flexural strength of the braces and beams is at least equal to the expected end-link moments and the braces can resist the associated axial force-bending moment demand. Inelastic response of braces and columns in such frames did not negatively impact the overall frame behavior. [ABSTRACT FROM AUTHOR]

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Braces with Intentional Eccentricity (BIEs) have been proposed to overcome some of the shortcomings of Concentrically Braced Frames (CBFs), namely the implications of their inherently stiff nature, their limited post-yielding stiffness and the susceptibility of Hollow Structural Sections (HSSs) to premature local buckling and fracture, and the excessive overstrength that may result from the design codes limits on the local and global slenderness. However, the application of BIEs for use in buildings has not yet been attempted, nor has their implementation in a global design approach yet been addressed. In this paper, a procedure based on the Direct Displacement Based Design (DDBD) method is employed in the seismic design of Frames with Intentionally Eccentric Braces (FIEBs). Buildings of 4, 8 and 12 storeys are designed as FIEBs with HSS brace members, with target drift ratios of 1.5% and 2.5%, and as Special CBFs for comparison purposes. The performance of the resulting buildings is assessed through Non-Linear Response-History Analysis. The results show that the employed design procedure is well suited to FIEBs, that their seismic performance is satisfactory and complies with the proposed performance objectives, and that they can constitute an economically advantageous alternative to conventional CBFs. • Frames with Intentionally Eccentric Braces – novel Seismic Force Resisting System • Frames with Intentionally Eccentric Braces offer better control of dynamic response. • Braces with Intentional Eccentricity address shortcomings of conventional braces. • Displacement-Based Design suitable for Frames with Intentionally Eccentric Braces • Frames with Intentionally Eccentric Braces potentially safe and economic [ABSTRACT FROM AUTHOR]

This paper examines the possibility of mobilizing gravity columns in the resistance of in-plane bending moments imposed on the columns of multi-tiered steel braced frames subjected to seismic loading. This would be the case when horizontal struts are used to connect gravity columns to braced frames at every tier level, as is often seen along exterior walls. A seismic design strategy based on the AISC Seismic Provisions is presented for four-tiered prototype steel concentrically braced frames. Three different approaches are proposed for the design of the braced frame and gravity columns. A set of 12 four-tiered X-braced frames, ranging from 15 to 30 m in height and located in a high seismic area were designed based on the proposed design approaches. The seismic behavior of the frames is evaluated using nonlinear response history analysis. The results show that the seismic performance of the braced frames is improved as nonlinear seismic demand on the bracing members is reduced when mobilizing the gravity columns for lateral resistance. Furthermore, gravity columns bending moment demands are distributed between braced frame and gravity columns in proportion of their relative flexural stiffness. Adequate seismic performance and cost-effective design can be achieved when columns of both types are designed to resist their respective share of the flexural demand. Alternatively, satisfactory response was achieved when the gravity columns are verified for the seismic induced bending moments acting together with concomitant axial loads. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a real-time dynamic substructuring (RTDS) test program carried out on bridge structures equipped with two innovative viscous seismic protective devices: a seismic damping unit and a shock transmission unit. In the RTDS tests, the seismic protective units were physically tested in the laboratory using a high performance dynamic actuator imposing, in real time, the displacement time histories obtained from numerical simulations being run in parallel. The integration scheme used in the test program was the Rosenbrock-W variant, and the integration was performed using The MathWorks’ Simulink and XPC target computer environment. The numerical counterpart included the bridge columns and the additional energy dissipation properties. The nonlinear response of these components was accounted for in the numerical models. The tests were run under various ground motions, and the influence of modeling assumptions such as damping and initial stiffness was investigated. Finally, the test results are compared to the predictions from nonlinear dynamic time history analyses performed using commercially available computer programs. The results indicate that simple numerical modeling techniques can lead to accurate prediction of the displacement response of bridge structures equipped with the seismic protective systems studied. [Copyright &y& Elsevier]