Search

Controlled rocking steel braced frames (CRSBFs) are being developed as a seismic force resisting system that can be constructed economically to avoid structural damage and residual deformations following an earthquake. In a CRSBF, selected columns are permitted to uplift from the foundation in response to severe seismic loading, and posttensioning and energy dissipation are selected to control the magnitude of the rocking response. Despite extensive experimental testing to demonstrate that this behavior is stable and repeatable, there has been a lack of comprehensive guidance for potential designers of CRSBFs. This paper proposes a performance-based design methodology for CRSBFs, which consists of defining the performance objectives, designing the base rocking joint based on a single-degree-of-freedom (SDOF) model, and capacity protecting the rest of the structure for the maximum forces expected during the rocking response. This paper focuses on the design of the base rocking joint, while Part II proposes a method for capacity designing the rest of the structure with due consideration of higher-mode effects. This paper shows how to design the base rocking joint to achieve a targeted response based on the results of 30,492 analyses of SDOF systems with flag-shaped hystereses. The SDOF results suggest that the rotation demands on the rocking joint depend on the initial stiffness of the system. For structures with initial periods less than about 0.4 s, the base rocking joint requires smaller force reduction factors than are codified for ductile seismic force-resisting systems, but much larger force reduction factors could be used for structures with longer initial periods, provided that the frame members are capacity designed and that there is sufficient deformation capacity in the posttensioning and energy dissipation elements. These conclusions are validated by considering the peak and residual displacement response of three example structures at two seismic hazard levels. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study of the seismic response of a 0.6-scale three-story building with nonlinear viscous dampers under the design basis earthquake (DBE) and the maximum considered earthquake (MCE). The test structure consists of a moment-resisting frame (MRF) with reduced beam sections (RBS), a frame with nonlinear viscous dampers and associated bracing (called the DBF), and a gravity load system. The paper focuses on quantifying and assessing the seismic response of the test structure under the DBE and MCE. Three MRF designs were studied for 100, 75, and 60%, respectively, of the required base shear design strength according to ASCE 7-10. The DBF with nonlinear viscous dampers was designed to control the lateral drift demands. Earthquake simulations using ensembles of ground motions were conducted using the real-time hybrid simulation method. Experimental results show that a high level of seismic performance can be achieved under DBE and MCE ground motions, even for a building structure designed for as little as 60% of the design base shear strength required by ASCE 7-10 for a building structure without dampers. [ABSTRACT FROM AUTHOR]

Steel moment frame (MF) is one of the most prevalent and well researched steel seismic force resisting systems worldwide. However, efficiency and feasibility of using MF diminishes for large spans. In this paper, a recently developed steel seismic force resisting system, named buckling-restrained knee-braced truss moment frame (BRKBTMF), is suggested as a better alternative system. The BRKBTMF uses buckling-restrained knee-braces with open web trusses. In this paper, a four-story prototype office building was designed using both MF and BRKBTMF systems. Performance of the prototype building with different span lengths was systematically studied. The results show that BRKBTMF uses less structural steel, yet it performs better with lower structural damage, repair costs, and probability of collapse as compared with the MF for all span lengths and under all earthquake intensities considered. Hence, it is concluded that BRKBTMF is a viable and effective alternative seismic force resisting system for long-span applications. [ABSTRACT FROM AUTHOR]

Buckling-restrained braced frames (BRBFs) are often used as primary seismic force resisting systems to achieve the desired seismic performance of building frames. However, significant residual displacements of these systems may cause concerns on their postearthquake performance, especially under long duration earthquakes and strong aftershocks. Although backup moment frames (MFs) along with the BRBFs, referred to as dual systems, can be used to mitigate their residual interstory drifts, there is no design criterion on how to determine the section sizes in the MF for this purpose. In the research reported in this paper, a simple mitigation technique was developed, which relies on a stiffness-based design in which the residual interstory drifts can be effectively controlled by increasing the elastic story stiffness using column sections with a higher moment of inertia. Nonlinear time-history analyses were carried out on low-to-high rise BRBFs for a set of 20 design basis earthquake (DBE) level ground motions to evaluate their residual interstory drift response for both pinned and rigid beam-to-column connections. Based on these analytical results, a simple expression is also proposed for predicting the residual drift ratios of low-to-high rise BRBFs. [ABSTRACT FROM AUTHOR]

Controlled rocking steel braced frames (CRSBFs) are intended to have a self-centering response that avoids damage to main structural elements. To ensure that all nonlinearity is confined to the intended elements at the rocking joint, the frame must be adequately capacity designed. This requires accurate predictions of the peak forces that are likely to develop in all members of the frame while the rocking mechanism reaches its peak rotation. Previous studies have shown that the peak forces in CRSBF members are likely to be strongly influenced by higher mode effects, but these effects can be mitigated by designing multiple nonlinear mechanisms. This paper proposes methods for estimating the peak forces in frame elements, designing an additional mechanism if it is desired to mitigate higher mode effects, and predicting the reduction in response that will be achieved by adding this mechanism. The methods are validated by designing buildings with two, six, and 12 stories, including three alternative designs that use multiple mechanisms to mitigate the higher mode effects. The six frames are modeled using OpenSees and are subjected to 44 ground motions at the maximum considered earthquake level. The peak forces in the taller frames without additional mechanisms are dominated by higher mode effects, but these effects can be estimated using the proposed method. These forces can also be reduced by designing multiple mechanisms, and the proposed method provides a reasonable design-level prediction of this force reduction. [ABSTRACT FROM AUTHOR]

Conventional buckling braces are the most commonly used steel bracing systems but are characterized by intense local midlength buckling that leads to unstable energy dissipation and finally fracture. Moreover, they are unable to dissipate energy in the event of low-level earthquakes and provide limited post-yielding stiffness. The present paper proposes a prototype design of conventional buckling braces with improved seismic performance by introducing eccentricity along the brace length. Steel braces with intentional eccentricity (BIE) are deformed by small story drift. The inherent moment caused by eccentricity affects their response, and the braces display trilinear behavior under tension and transition smoothly to post-buckling behavior under compression. Owing to the moment contribution, stresses and strains are uniformly distributed along the brace length, delaying local buckling concentration, and, thus, the member's life is appreciably extended. The present experimental work on BIEs reveals that the braces offer a reduced stiffness up to 56% on the initial stiffness maintaining the same strength with the corresponding conventional brace, provide large tensile post-yielding stiffness of 14-18% of the initial stiffness, stably dissipate energy during cyclic loading, and delay local buckling and fracture until large story drift levels (up to 2 and 4%, respectively). [ABSTRACT FROM AUTHOR]

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]

This paper discusses the structural characterization of a novel hybrid shear-wall system formed by coupling standard platformframe panels with an external reinforced concrete shelter formed of precast slabs screwed to the wooden frames. The external RC skin is intended as a supplementary bracing system, increasing strength and dissipative capacity of the bare timber frame. The structural performance of such hybrid shear wall under monotonic and cyclic loading was first theorized analytically on the basis of code provisions and then confirmed via experimental tests. The novel shear walls demonstrated to fulfill the requirements prescribed by Eurocode 8. In particular, the analyzed system belongs to high ductility class (HDC). Finally the seismic response of a reference building realized with the innovative hybrid shear walls was simulated by means of a numerical model validated on experimental tests; the suitable behavior factor for the building was estimated [ABSTRACT FROM AUTHOR]

This paper presents the seismic performance of buildings with a hybrid bracing system in which buckling-restrained braces (BRBs) are used at the lower stories with the higher level of ductility demands, and conventional buckling-type braces consisting of steel hollow structural sections (HSS) are used in the upper stories with relatively smaller ductility demands to minimize the probability of their fracture under reversed cyclic displacements. This type of hybrid braced frame (HBF) could prove economical, especially for retrofitted buildings in seismically active regions. A series of nonlinear time-history analyses was conducted to investigate the seismic performance of 3- and 6-story buildings with the hybrid bracing system. The main parameters studied are the seismic design parameters, maximum interstory drift ratios, fracture response of the HSS, and the optimal deployment of the BRBs. The seismic performance of the HBFs was compared with conventional concentrically braced frames and buckling-restrained braced frames (BRBFs) designed according to the current code provisions. Results showed that using BRBs at only a few lower levels of the HBFs gives similar performance to that of BRBFs. The incident of brace fractures in conventional HSS can be delayed or completely eliminated by using the proposed hybrid bracing systems. [ABSTRACT FROM AUTHOR]

In a previous study by the authors, the cyclic behavior of a novel type of pin-connected angle steel buckling-restrained brace (ABRB) was examined, and the failure mechanism in the core projection of the ABRB induced by an excessive bending effect caused by end rotation was discussed. In this paper, the occurrence mechanism of end rotation modes and the bending effect in the core projection of an ABRB are first investigated based on the previous test results, which shows that end rotation demands would be significantly increased with the presence of a gap and an additional bending effect could be observed if the end rotation demands were large enough to cause two-point contact at the core ends. Then, a new method to predict the flexural demand on pin-connected BRBs is proposed by considering the effect of the end rotation modes, clearance, initial eccentricity, and initial deflection of casing. The design criteria to prevent yielding of the core projection are presented and further verified by the previous test results. Furthermore, the effects of key influential parameters on the flexural demand on core projection are discussed based on the analytical results. It is found that such a bending effect can be significantly reduced by decreasing the gap or increasing the constrained length of the core stiffening segment. The C-mode end rotation with single curvature bending configuration is found to be generally the most unfavorable case for core projection design. Finally, several design recommendations are provided for pin-connected BRBs. [ABSTRACT FROM AUTHOR]

Part I of this work was concerned with the experimental and numerical analysis of the lateral force, or thrust, exerted by the inner core of a buckling-restrained brace (BRB) when, after buckling in compression, it arrives into contact with the external retaining case. As Part II of that study, this paper considers the case of a steel-bolted BRB, with an empty gap between core and containment struts, to attempt the derivation of engineering analytical expressions for such a thrust. It is found that the relationship between applied axial force and buckled wavelength does not obey Euler's formula. [ABSTRACT FROM AUTHOR]

This paper describes structural optimization formulations for trusses and applies them to size and geometrically optimize lateral bracing systems. The paper provides analytical solutions for simple and limit cases and uses numerical optimization for cases where analytical solutions are not available. The analysis is based on small displacements, constant or no connection costs, static loads, and elastic behavior. This work provides guidance to improve the common engineering practice of locating the bracing point at the middle or at the top of the bay/story panel. [ABSTRACT FROM AUTHOR]

Circular arches subjected to in-plane loading are restrained with discrete lateral bracings to improve their out-of-plane stabilities in many cases. The arch segment between two adjacent bracings may buckle out of plane if the effective length of the arch segment is sufficiently large. During out-of-plane buckling, adjacent arch segments may provide elastic restraints which increase the out-of-plane stability of the arch segment significantly. This paper is concerned with out-of-plane elastic buckling of circular arch segments with elastic end restraints. The analytical method for deriving the out-of-plane buckling loads of circular arches with equal rotational restraints or equal warping restraints at both ends is formulated. For the convenience of structural designers, simple approximate formulas for predicting the out-of-plane buckling loads of these arches are developed by the curve-fitting method. As for circular arches with unequal rotational restraints, the approximate formula for the buckling load of the arch with equal rotational restraints is modified with a reduction factor. Comparisons with the finite-element results and the analytical solutions show that the predictions by approximate formulas are sufficiently accurate. [ABSTRACT FROM AUTHOR]

Light-gauge metal sheeting is often used in steel building and bridge industries as concrete deck formwork. Besides providing support to the fresh concrete, the sheeting acts as a shear diaphragm and provides continuous lateral bracing to the top flanges of the beams to which they are attached. An adequate stability-bracing system must possess sufficient stiffness and strength to control deformations and brace forces. Strength requirements for shear diaphragms are currently not well established. A computational study was conducted to develop strength requirements for shear diaphragms bracing simply supported steel beams. Both end-fastener and sidelap-fastener connections were modeled in the study. To the best of the authors' knowledge, this is the first study to quantify the stability-induced forces in sidelap fasteners. The effects of deck width and number of end and sidelap fasteners on brace forces were investigated. Expressions were developed to estimate the stability-induced brace forces in end-fastener and sidelap-fastener connections. [ABSTRACT FROM AUTHOR]

Steel concentrically braced frames designed prior to the implementation of capacity design principles in seismic design provisions may exhibit poor inelastic response under seismic excitation. These older, nonductile concentrically braced frames (NCBFs) used several configurations, with the chevron configuration being one of the most common. The response of chevron-configured NCBFs is unknown, as relatively large axial and flexural demands are imposed on the beam after brace buckling. Current code requirements for special concentrically braced frames (SCBFs) promote full yielding of the braces while the beam remains elastic, but NCBFs develop a mechanism in which the beam yields, deforms plastically, and limits tensile elongation of the brace. However, if ductile, this plastic mechanism may meet current performance limits and not require retrofitting. To examine this issue, four tests of two-story NCBFs were conducted at the National Center for Research on Earthquake Engineering in Taipei, Taiwan. The tests were designed to form a yielding-beam plastic mechanism as opposed to a yielding brace-elastic beam plastic mechanism. The results show the yielding-beam mechanism provided adequate lateral strength and deformation capacity and beams in a chevron configuration are a low priority with regard to retrofit, whereas retrofitting to mitigate brace and connection deficiencies is a high priority. The tests also demonstrate that modern CBFs can sustain lateral force and deformation demands if the yielding-beam, as opposed to yielding-brace, plastic mechanism is formed. The results support the modification of the stringent design requirements for chevron SCBFs. [ABSTRACT FROM AUTHOR]

Gusset plate connections with single-angle steel members are commonly used in power transmission towers. However, little research has been carried out with regard to the compressive behavior and design of gusset plates connected with single-angle members. Due to the complexity of the connections and load eccentricity, it is difficult to predict the compressive strength of the gusset plates. In this study, two full-scale experimental tests and a numerical parametric study were conducted to investigate the compressive behavior of such gusset plate connections. In the tests, the buckling failure mode and lateral-torsional deformation were observed. Finite-element (FE) models were then established and validated through comparison against the test results. In the parametric study, the effects of gusset plate thickness, unbraced length, distance to bending line, and load on the adjacent bracing member were examined. On the basis of the findings of this study and currently available design methods, two design methods were proposed for predicting the compressive strength of gusset plates connected with single-angle members. Good agreements were observed between the FE and design results in terms of the compressive strength and it was found that one method based on plate buckling gave a better prediction of the strength of gusset plates connected with single-angle members than the other based on effective column. [ABSTRACT FROM AUTHOR]

A correction to the article "Performance-Based Seismic Design of Controlled Rocking Steel Braced Frames. II: Design of Capacity-Protected Elements" that was published in a previous issue of the journal is presented.

Large-scale shake table tests were performed at E-Defense to examine the out-of-plane stability of buckling-restrained braces (BRBs). Two specimens were subjected repeatedly to a near-fault ground motion with increasing amplification. The test specimens comprised a single-bay, single-story steel frame and a pair of BRBs placed in a chevron arrangement. The specimens were not braced at the brace-to-beam intersection in order to produce a condition where the BRBs were susceptible to out-of-plane instability. Standard BRBs were used in the first specimen, while BRBs with a flexible segment at each end of the steel core were used in the second specimen. A simple stability model predicted the BRBs in the second specimen to fail because of out-of-plane buckling. The first specimen exhibited excellent ductility during the shake table tests, while the second specimen developed severe out-of-plane deformation that compromised the ductility of the BRBs. Based on the experimental observations and the stability model, a methodology is proposed to evaluate bracing requirements at the brace-to-beam intersection. [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]

This work presents an experimental and numerical analysis of the lateral forces exerted by the inner core of a buckling-restrained brace (BRB) when, after buckling in compression, it arrives into contact with the external restraining case, from which it is separated by a small empty gap. Experiments are carried out on reduced-scale specimens to measure the thrust acting on the containment struts. Refined, nonlinear two-dimensional (2D) FEM models are adopted to obtain numerical estimates of the thrust. The results show (1) the capability of 2D, plane stress FEM models to correctly reproduce the cyclic behavior of the studied BRBs, and (2) the difficulty in accurately measuring/computing the thrust associated to a given axial load/displacement. [ABSTRACT FROM AUTHOR]

The Yielding Brace System is a highly ductile bracing system in which seismic energy is dissipated by the yielding fingers of a specially engineered cast steel connector. When the brace is severely loaded in tension and compression, the fingers yield in flexure, thus providing a full, symmetric hysteresis. Second-order geometric effects result in an increase in postyield stiffness at large displacements. The mechanics of the system are first presented, including several first principle equations used to predict a connector's response. These equations are then used to design a prototype connector. The geometry of this prototype is evaluated using nonlinear finite element analysis. Following this analysis, the results of full-scale axial component testing of the prototype are discussed. These results include tensile coupon tests from material taken directly from unyielded portions of the test specimens. The prototype design and testing program presented demonstrate that the Yielding Brace System is a ductile connector that can enhance the energy dissipation and displacement capacity of braced frames. Good agreement between predicted and experimental response showed that the mechanics of the connector were well understood and that the method used to design the prototype was effective in meeting the targeted performance. [ABSTRACT FROM AUTHOR]