Seismic Response and Design of Steel Multi-Tiered Concentrically Braced Frames

RÉSUMÉ Les contreventements concentriques en treillis à multiples segments (CCMS) sont des contreventements constitués de diagonales formant deux ou plusieurs panneaux qui sont superposés sur la hauteur du contreventement. Les CCMS sont couramment utilisés en Amérique du Nord afin d’offrir une résistance latérale pour les bâtiments d’un seul étage de grande hauteur,comme les bâtiments industriels, les installations sportives, les centres de congrès ou les hangars d'avion. Dans ces structures, la configuration CCMS est préférable puisque l'utilisation de diagonales de contreventement simples partant des fondations jusqu’au niveau du toit n’est plus pratique. Dans les CCMS, la longueur et la taille des diagonales sont réduites de manière significative, ce qui est favorable pour rencontrer les limites d'élancement prescrites dans les normes parasismiques. De plus, les colonnes peuvent être considérées comme contreventées latéralement dans le plan du cadre à niveau intermédiaire entre deux panneaux, ce qui contribue également à réduire la taille des poteaux et la quantité requise d'acier. Les colonnes de gravité adjacentes qui sont situées dans le même plan qu’un CCMS peuvent aussi être considérées comme contreventées latéralement en ajoutant des membrures horizontales aux niveaux intermédiaires entre les panneaux. Dans les CCMS, les colonnes sont en général des sections en W orientées de telle sorte que la flexion hors-plan se produise selon l’axe fort de la colonne, permettant ainsi à la colonne de résister au flambement hors plan sur toute la hauteur du cadre.----------ABSTRACT Steel multi-tiered braced frames (MT-BFs) are commonly used in North America to provide lateral resistance for tall single storey buildings such as industrial buildings, sport facilities, convention centers, or airplane hangars. In these structures, MT-BF configuration is preferable, as the use of single bracing members extending from the foundation to the roof level is no longer practical. MT-BFs consist of tall steel braced frames built with multiple bracing panels stacked over the height of the frame. In MT-BFs, brace lengths and sizes are reduced significantly, which is favourable to satisfy the slenderness limits specified in the seismic provisions. Additionally, the columns can be considered as laterally braced in the plane of the frame at every tier point, which also contributes to reducing the steel tonnage. Adjacent gravity columns located along MT-BF lines can similarly be laterally braced by adding horizontal struts at tier levels. In MTBFs, columns are typically I-shaped members oriented such that strong axis bending develops outofplane, so the column can resist out-of-plane buckling over the full building height

Seismic Behaviour and Design of Two-Bay Steel Multi-Tiered Braced Frames and Other Special Steel Concentrically Braced Frames in Single-Storey Buildings

«RÉSUMÉ:Pour les structures de bâtiments en acier de grande hauteur comme les centres sportifs, les hangars d’avions, les bâtiments industriels ou les entrepôts, il est courant d’utiliser des contreventements concentriques en treillis à segments multiples (CCSM). Ces CCSM sont constitués de plusieurs panneaux contreventés superposés sur la hauteur de l’étage. Chacun de ces panneaux est appelé un segment. Cette configuration de contreventements résulte en l’utilisation de diagonales plus courtes, ce qui permet généralement de minimiser l’aire des sections choisies et de respecter plus facilement les critères d’élancement et de rapport largeur sur épaisseur. Des bielles intermédiaires sont placées entre chaque segment contreventé, afin de résister aux efforts axiaux débalancés des diagonales qui surviennent lorsque les diagonales flambent en compression. La redistribution de ces efforts axiaux débalancés au moyen des bielles permet d’éviter un comportement de cadre contreventé en « K », pour lequel les colonnes doivent résister d’importantes sollicitations flexionnelles dans le plan du cadre. Dans des CCSM, les colonnes sont généralement constituées de profilés en I orientés de tel manière à ce que le flambement hors plan survienne autour de l’axe fort du profilé, sur toute la hauteur de l’étage.» et «----------ABSTRACT: In tall single-storey steel structures such as sports facilities, airplane hangars, industrial buildings or warehouses, multi-tiered concentrically braced frames (MT-CBFs) are commonly used. MT-CBFs consist of a bracing system where multiple braced panels are stacked over each other along the storey height. Each panel is referred to as a tier. This bracing configuration results in shorter braces, which typically leads to minimized brace sections and easier compliance with slenderness and width-to-thickness requirements. Intermediate struts are provided at tier levels, to resist unbalanced brace axial loads that develop after the compression braces have buckled. This redistribution of unbalanced axial loads through struts prevents undesirable K-braced frame behaviour, where the columns must resist large in-plane flexural demands. The columns typically consist of I-shaped members oriented such that strong-axis bending is utilized to resist out-of-plane bending moments over the frame height. Struts allow the columns to be laterally braced at tier levels for weak-axis buckling. Results from previous studies showed that under seismic loading inelastic deformations in MT-CBFs tend to concentrate in one critical tier. Two major concerns arise from this phenomenon. On one hand, significant inelastic deformations can occur in the bracing members of the critical tier, which may result in brace fracture due to low cycle fatigue. On the other hand, concentration of inelastic drifts in one tier leads to in-plane bending demands in the columns, which may cause instability. Both Canadian and American seismic design provisions require MT-CBFs to be designed for seismic loading assuming a concentration of inelastic deformations in one tier.

Assessment of seismic design provisions for multi-tiered ordinary concentrically braced frames

Multi-tiered braced frames (MT-BFs) are commonly used as lateral force resisting systems in tall single-story buildings such as performing arts and sports centers, industrial warehouses, and airplane hangars. Horizontal members (struts) are used to divide the tall single-story into several bracing panels or tiers, without intermediate floors or out-of-plane supports. Special conditions in MT-BFs during nonlinear seismic response lead to concentration of drifts in the tiers and impose additional flexural demands on the columns. These flexural demands, in combination with axial demands, can cause column instability and compromise the seismic performance of the frames. The seismic design provisions for multi-tiered ordinary concentrically braced frames (MT-OCBFs) are assessed in this study. MT-OCBFs are intended to achieve modest levels of ductility, and a relatively simple design procedure is used for them. The current design approach, contained in the 2010 AISC Seismic Provisions, requires an axial force amplification for the columns. In contrast, the newest design approach, contained in the 2016 AISC Seismic Provisions, requires an additional axial force amplification to approximately account for imposed flexural demands on the columns. This new requirement leads to larger column sizes, which in turn can dramatically modify seismic response. A set of eighteen frames, with varying total frame height, brace configurations (X, chevron, and split-X), and tier heights, are designed as per both provisions. Their seismic performance is assessed by employing nonlinear static and time history analyses on a three-dimensional, numerical model developed using the OpenSees simulation platform. The results show that the 2010 AISC Seismic Provisions severely underestimate column demands in MT-OCBFs, leading to significant inelastic drift concentration in one tier and column buckling. The 2016 AISC Seismic Provisions lead to larger columns, which improve redistribution of inelastic drift over the frame height and reduce story drifts, but do not necessarily reduce inelastic drift concentration. Potential for brace loss due to low-cycle fatigue fracture is apparent in both designs. These new provisions reduce the propensity for column buckling, but it is not necessarily prevented. Brace configuration also influences demands on the column. In general, the split-X configuration leads to larger in-plane flexural demands in taller frames and onset of column buckling at small story drift values. In contrast, the shortest frames in this study exhibited relatively better performance with the chevron bracing configuration

Seismic performance of Chilean concentrically braced frame industrial structures: effects of recent code modifications

Few seismic design codes for industrial structures exist worldwide. Among them, the Chilean design code was put to the test by the Maule earthquake of 2010, one of the largest seismic events in recent years. Although the seismic performance of industrial steel structures designed under these provisions was satisfactory, the standard was revised based on the accumulated evidence on the performance after the event and the advances in seismic design since the code was released in 2003. The revision process led to a number of modifications to the provisions, including those for structures based on concentrically braced frames (CBFs), a structural typology widely used in the industry. The modifications, mainly aimed at improving seismic performance in severe events, ranged from the seismic demand to the provisions for sizing structural elements and connections. This work evaluates the effect of these modifications on the design and seismic performance of CBFs. For this purpose, six industrial steel structures were designed using the current standard and the proposed version. The performance was evaluated through static non-linear analyses in 3D models according to the methodology prescribed by the FEMA P695 standard. The models included the non-linearity of braces, columns, beams or struts, and anchor bolts. The results showed similar performance between the structures designed using the proposed and the current version of the standard, in terms of overstrength and response modification factors. However, the performance improved when comparing the maximum drift that the structures can reach and the energy levels they are able to accumulate at these drifts. In terms of the cost–performance ratio, the improvement in performance is associated with moderate increases in cost

Seismic Evaluation and Retrofit of Existing Concentrically Braced Steel Frames in Canada

RÉSUMÉ-Les nouveaux bâtiments en acier conçus selon les dispositions sismiques du Code national du bâtiment du Canada (CNB) (CRNC 2010) et les règles de calcul des charpentes en acier CSA S16 (CSA 2009) doivent résister en toute sécurité aux charges sismiques et développer une ductilité suffisante tout en maintenant une résistance et une rigidité adéquates. La conception parasismique avec les détails de conception spécifiques aux structures en acier a été introduite dans l'édition 1989 de la norme CSA. Ainsi, les structures conçues avant les années 1990 pourraient ne pas développer la réponse sismique ductile souhaitée. À ce jour, les recherches consacrées à l'évaluation sismique des cadres à contreventements concentriques existants conçus conformément aux codes des années 1980 sont très limitées au Canada.----------ABSTRACT-New steel buildings designed according to the seismic provisions of the National building Code of Canada (NBCC) (NRCC 2010) and the steel structures design standard CSA S16 (CSA 2009) are conceived to safely resist seismic loads and develop sufficient ductility while maintaining adequate strength and stiffness. The special seismic design and detailing requirements for steel structures were introduced in the 1989 edition of the CSA standard. Thus, the structures designed prior to 1990’s may not develop the ductile seismic response. To date, very limited research in Canada has been devoted into the seismic evaluation of existing concentrically braced frames designed in accordance with the 1980’s codes

Capacity Design Optimization of Steel Building Frameworks Using Nonlinear Time-History Analysis

This study proposes a seismic design optimization method for steel building frameworks following the capacity design principle. Currently, when a structural design employs an elastic analysis to evaluate structural demands, the analysis results can be used only for the design of fuse members, and the inelastic demands on non-fuse members have to be obtained by hand calculations. Also, the elastic-analysis-based design method is unable to warrant a fully valid seismic design since the evaluation tool cannot always capture the true inelastic behaviour of a structure. The proposed method is to overcome these shortcomings by adopting the most sophisticated nonlinear dynamic procedure, i.e., Nonlinear Time- (or Response-) History Analysis as the evaluation tool for seismic demands. The proposed optimal design formulation includes three objectives: the minimum weight or cost of the seismic force resisting system, the minimum seismic input energy or potential earthquake damage and the maximum hysteretic energy ratio of fuse members. The explicit design constraints include the plastic rotation limits on individual frame members and the inter-story drift limits on the overall performance of the structure. Strength designs of each member are treated as implicit constraints through considering both geometric and material nonlinearities of the structure in the nonlinear dynamic analysis procedure. A multi-objective Genetic Algorithm is employed to search for the Pareto-optimal solutions. The study provides design examples for moment resisting frames and eccentrically braced frames. In the examples some numerical strategies, such as integrating load and resistance factors in analysis, grouping design variables of a link and the beams outside the link, rounding-off the objective function values, are introduced. The design examples confirm that the proposed optimization formulation is able to conduct automated capacity design of steel frames. In particular, the third objective, to maximize the hysteretic energy ratio of fuse members, drives the optimization algorithm to search for design solutions with favorable plastic mechanisms, which is the essence of the capacity design principle. For the proposed inelastic-analysis-based design method, the seismic performance factors (i.e., ductility- and overstrength-related force reduction factors) are no longer needed. Furthermore, problem-dependent capacity design requirements, such as strong-column-weak-beam for moment resisting frames, are not included in the design formulation. Thus, the proposed design method is general and applicable to various types of building frames

Seismic Design and Qualification of All-Steel Buckling-Restrained Braced Frames for Canadian Applications

RÉSUMÉ Les contreventements à diagonales ductiles confinées (DDC) sont devenus le système de contreventement standard pour les bâtiments et pont pour résister aux charges sismiques. Ce système intègre un concept spécifique de diagonales, appelé, DDC, qui ne flambent pas en compression fournissant ainsi un mécanisme de dissipation d'énergie stable. La capacité de dissipation de l'énergie sismique des DDC est fortement reliée à la performance du mécanisme de retenue du flambement qui prévient le noyau ductile interne de subir une déformation latérale excessive. La force transmise au système de confinement dépend de plusieurs facteurs qui doivent évalués avec précision et intégré lors de la conception.----------ABSTRACT Buckling-Restrained Braced Frames (BRBF) have become one of the standard ductile lateral systems for building structures and bridges to resist the earthquake loads. This system incorporates special diagonal elements, called BRB, that do not buckle in compression hence providing a stable and nearly symmetrical hysteretic response in tension and compression with high seismic energy dissipation capacity. The energy dissipation capability of BRBs is strongly linked to the performance of buckling restraining mechanism that constrains the internal ductile core from undergoing excessive lateral deformation. The force demand on the restraining system depends on several factors that should be precisely estimated and implemented in design

Inelastic behavior and seismic collapse prevention performance of low-ductility steel braced frames

Seismic-force-resisting systems (SFRSs) in areas of low and moderate seismicity such as the east coast of the United States have minimal ductility. Unlike high-ductility systems, whose design encompasses extensive seismic detailing and proportioning requirements to provide said ductility, the analogous requirements for low-ductility steel SFRSs are minimal, and in some instances nonexistent. Ordinary concentrically-braced frames (OCBFs) have requirements that are intended to provide a small level of ductility, and steel SFRSs designed with a response modification factor R = 3 have no specific seismic requirements. Rather than relying on ductility for post-elastic performance, these systems—that have received almost no research attention—must prevent collapse through other means that are not explicitly defined or considered in the design process. Reserve capacity, or additional lateral force resisting capacity following incipience of inelastic and often brittle behavior within the SFRS, has been shown through post-earthquake reconnaissance to be the probable mechanism by which low-ductility concentrically braced frames (CBFs) provide collapse prevention. Aside from these anecdotal cases, however, the inelastic behavior and seismic collapse prevention performance of low-ductility CBFs remain untested and unsubstantiated, and these topics are not yet understood at a fundamental level to the extent of high-ductility systems. In this dissertation, the inelastic behavior and seismic collapse prevention performance of low-ductility CBFs are examined through a combination of numerical simulations and full-scale experimental testing. Sources of reserve capacity are systematically identified and evaluated through numerical simulations. The impact of common design choices, such as system type and system configuration, on reserve capacity is evaluated through full-scale tests of an R = 3 CBF in the chevron configuration and an OCBF (R = 3.25) in the split-x configuration. Test results and numerical simulations indicate that the chevron configuration is more suitable for providing reserve capacity, and that while some of the minor seismic detailing requirements of the OCBF provision improve performance, others are detrimental and lead to uneconomical designs. The knowledge gained from the initial numerical simulations and full-scale experimental testing inspired the development of a new low-ductility system for use in areas of low and moderate seismicity which provides collapse prevention through primary system ductility and intentional reserve capacity mechanisms: the R = 4 OCBF. In a reliability-based performance assessment this new system shows an improvement in behavior over the two primary low-ductility CBF SFRSs—the R = 3 CBF and the OCBF (R = 3.25)—while also minimizing design costs and complexity. This R = 4 OCBF concept is proposed for consideration in the upcoming 2022 AISC Seismic Provisions

Elastic And Inelastic Stability Of Two-Panel Tiered Concentrically Braced Frames

ABSTRACT ELASTIC AND INELASTIC STABILITY OF TWO-PANEL TIERED CONCENTRICALLY BRACED FRAMES Michael H. Bloom, B.S. Marquette University, 2013 Multi-panel, tiered concentrically braced frames are commonly used in the lateral resisting systems of industrial facilities for loads resulting from wind and earthquake. To date, minimal investigation has been performed on the effect of gravity and lateral loads on the local and global (system) stability of these framing systems. Recent research has evaluated the effects of in-plane and out-of-plane bending moments induced by inelastic brace deformation and transverse notional loads on the stability of columns in a two-panel concentrically braced frame with an x-bracing arrangement. Other recent research efforts have studied the effect that differential tier drifts resulting in weak-axis flexural yielding have on the strong-axis buckling strength of columns in a four-tier concentrically braced frame. A three-dimensional finite element analysis was used to impart varying levels of weak-axis flexural yielding onto various wide flange sections and the strong-axis buckling strength was analyzed. That study, however, consisted of analyzing columns isolated from the rest of the frame. This research effort utilizes the structural analysis program MASTAN2 to conduct multiple elastic and inelastic critical load analyses and nonlinear inelastic analyses on a two-panel, tiered concentrically braced frame. Multiple lateral loading conditions, frame height, frame slenderness, and column orientation scenarios are considered to determine the effects of these variables on the stability behavior of the frame. The results of this research effort indicate that the ratio of applied lateral load to applied gravity load and the frame aspect ratio have a profound effect on whether frame stability behavior is controlled by local member behavior or global (system) behavior