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Traditional concentrically braced frames, CBF, are stiff and provide limited to moderate ductility, while moment resisting frames, MRF, are able to dissipate seismic energy when undergoing large lateral displacements. However, these traditional earthquake resistant systems do not show uniformly distributed damage along the building height. Changes in structural proprieties during nonlinear hysteresis behaviour may lead to drift concentration and weak-storey response. Moreover, both traditional systems are susceptible to long-duration subduction earthquakes. The pursuit of these issues led to the concept of utilizing multiple-resisting structural systems that act progressively so that the overall seismic resistance is not significantly reduced during long-duration earthquakes. The structural system consisting of a rigid braced frame that provides primary stable cyclic behavior and a moment frame acting as a backup system with good flexural behavior is the steel Braced Dual System studied herein. The objectives of this study are: a) to investigate the seismic response of steel Braced Dual building from yielding to failure, as well as, to identify the types of failure mechanism; b) to assess the seismic response of Braced Dual System against the traditional MRFs and CBFs with moderate ductility through incremental dynamic analysis; c) to evaluate the effect of long duration subduction earthquakes versus crustal type earthquakes on these building systems through collapse safety criteria using FEMA P695 procedure and to assess the probability of exceeding defined performance levels using fragility analysis. To carry out these objectives, detail numerical models were developed using the OpenSees framework. The prototype 8-storey office building is located on firm soil in Vancouver, B.C. and is subjected to two sets of crustal and subduction ground motions. Two traditional earthquake resistant systems (MD-CBF, MD-MRF) and the Braced Dual System are considered. Design is conducted according to NBCC2015 and CSA/S16-14. From nonlinear time history analysis, the following results are reported: for the Braced Dual System, two types of failure mechanism involving either one floor or two adjacent floors (in general the bottom floors) were identified which also involve flexural yielding of MRF beam of critical floors; the Braced Dual System provides larger ductility than the MD-CBF, shows significant increase of seismic resistant capacity for similar seismic demands, provides the largest collapse margin ratio and collapse safety capacity under both earthquake types. In addition, the building with Braced Dual System shows a progressive seismic behavior and a more uniform damage distribution along the building height. From fragility analysis resulted that at Collapse Prevention (CP) limit state, the Braced Dual System experiences 100% probability of exceedance after it was subjected to two times larger seismic demand than the MD-CBF or MD-MRF systems. All studied structural systems are sensitive to long duration subduction earthquake.
Concentrically braced frames (CBFs) are widely used in North America. The CBFs possess high stiffness and moderate ductility, while braces are designed to buckle in compression and yield in tension. However, after a brace experiences buckling, its compression strength diminishes and the system undergoes asymmetrical response, while the distribution of internal forces and deformations is influenced by the frequency content of ground motions. Despite the system's stiffness, CBFs are prone to concentrate damage within a floor which leads to the formation of storey mechanism. To preserve the stability of the system during the nonlinear seismic response, the National Building Code of Canada (NBCC) imposes limits on a building's height which depends on the selected ductility-related force modification factor, Rd. Thus, the height limit for buildings with moderately ductile concentrically braced frames, MD-CBFs, is 40 m and for limited ductility concentrically braced frames, LD-CBFs, is 60 m. To safely increase the height limit of ductile braced frame buildings, a system labelled Outrigger Braced Frame, OBF, is proposed and developed in this study. According to the Council on Tall Buildings and Urban Habitat (CTBUH), a building with more than 14 stories or more than 50 meters in height may be considered a high-rise building. The aim of this research is to develop, design, model, and study the seismic performance of mid-rise (e.g. tweleve-storey) and high-rise (e.g., sixteen-storey) OBF buildings subjected to dynamic loads. It is noted that the outrigger system functions by tying together a core system and a perimeter system. Herein, the core system is made of MD-CBFs and the perimeter system is made of gravity columns. Furthermore, only the core braces are designed to dissipate energy, while the outrigger's diagonals are designed to respond in the elastic range. The performance of OBF system is controlled by the amount of added stiffness and optimum location of outriggers across the building's height, the number of levels with outriggers and the intensity of seismic zone. All multi-storey buildings are located in high-risk seismic zone of Victoria, B.C. Canada, on Site Class C. The selection of ground motions was made to capture the seismic characteristics at buildings location. Herein, two sets of crustal and subduction ground motions were considered such as California records and the mega-thrust magnitude 9 Tohoku records, respectively. The nonlinear time-history dynamic analyses were conducted using the OpenSees software. The main objectives of this thesis are three-fold: i) to identify the effect of subduction versus crustal ground motions on the seismic response of low-rise, mid-rise and high-rise MD-CBF buildings and to study their seismic performance from yielding to failure, ii) to provide design method and optimum location for outriggers of OBF steel buildings, iii) to assess the collapse safety of the proposed mid-rise and high-rise OBF steel buildings using FEMA P695 procedure and to compare their seismic performance against that resulted for MD-CBF buildings. It is concluded that the OBF buildings are slightly stiffer than the corresponding MD-CBF buildings, and they experienced lower interstorey drift and residual interstorey drift than the MD-CBF buildings. In all case studies considered here, the collapse margin ratio (CMR) is greater for buildings subjected to crustal ground motions than subduction ground motions. Evaluation of seismic performance of sample 12-storey and 16-storey OBF buildings shows that these buildings are able to pass the collapse safety acceptance criteria, ACMR ≥ ACMR10%, when subjected to both sets of ground motions. On the other hand, the corresponding MD-CBF buildings are not able to pass the collapse safety acceptance criteria when subjected to subduction records set. Hence, special attention should be given when designing buildings in seismic regions which are prone to both types of earthquakes.
The special concentrically steel braced frame (SCBF) system is one of the most effective struc-tural systems to resist lateral forces. Because of its effectiveness and straightforward design, many SCBFs are incorporated in structures throughout the world. However, the highly nonlin-ear behavior associated with buckling and non-ductile fracture of braces reduces the ability of the system to dissipate energy resulting in undesirable modes of behavior. While many studies have investigated the cyclic behavior of individual braces or the behavior of subassemblies, the dynamic demands on the structural system under various seismic hazard levels needs additional study for performance-based earthquake engineering. Archetype buildings of SCBFs and buckling restrained braced frames (BRBFs) were analyzed using the computer program OpenSees (the Open System for Earthquake Engineering Simulation) to improve the understanding of the seismic behavior of braced frame systems, and to assess seismic demands for performance-based design. Numerical models were calibrated using test data determined from testing of conventional buckling braces, buckling restrained braces, and the braced frame specimens. In addition, fiber-based OpenSees models were constructed and compared with results of a sophisticated finite-element model that realistically captured local buckling and local fracture of structural elements. Because the OpenSees models are reasona-bly accurate and efficient, they were chosen to perform set of parametric computer simulations. The seismic demands of the system and structural elements were computed and interpreted for 3-, 6-, and 16-story SCBFs and BRBFs under various hazard levels. The analysis results show large seismic demands for the 3-story SCBF, which may result in unexpected damage of struc-tural and non-structural elements. The median expected probability of a brace buckling at one or more levels in a 3-story SCBF is more than 50% for an earthquake having a 50% probability of exceedance in 50 years (the service-level event). The possible need to replace braces fol-lowing such frequent events due to brace buckling should be considered in performance-based earthquake engineering assessments. In addition, brace fracture in SCBFs is likely for an earthquake having a 2% probability of exceedance in 50 years (the MCE-level event). Analy-ses show that in general, BRBF models had larger drift demands and residual drifts compared to SCBF systems, because of the BRBF's longer fundamental period. However, the tendency to form a weak story in BRBFs is less than that in SCBFs. Evaluation of seismic demand parameters were performed for 2-, 3-, 6-, 12-, and 16-story SCBFs and BRBFs, which demonstrated that short-period braced frame systems, especially SCBFs, had higher probability of collapse than longer-period braced frame systems. Substantially improved response was observed by lowering the response reduction factor of the 2-story SCBF building; this reduced the collapse risk at the hazard level of 2% probability of exceedance in 50 years. For long-period (taller) structures, although the collapse probability was lower compared to the short-period structures, weak story behavior was commonly observed in conventionally designed SCBF. A design parameter related to the ratios of story shear demand and capacity under a pushover analysis is proposed to modify member sizes to reduce weak story behavior efficiently. This is demonstrated for a 16-story SCBF building. Regarding local deformation and force demands, simple methods to estimate out-of-plane buck-ling deformation of braces and column axial force demands are proposed. The investigation of system performance and member behavior provides seismic demands to more accurately assess the socio-economic losses of SCBFs and BRBFs for performance-based earthquake engineering.
A state-of-the-art summary of recent developments in the behaviour, analysis and design of seismic resistant steel frames. Much more than a simple background volume, it gives the most recent results which can be used in the near future to improve the codified recommendations for steel structures in seismic zones. It contains new material which cann
"Performance-Based Earthquake Engineering necessitates the development of simulation models that can predict the nonlinear behavior of structural components as part of a building subjected to seismic loading. For reliable seismic assessment of buildings, these models need to be calibrated with large sets of experimental data. This thesis advances the state-of-knowledge on the collapse assessment of concentrically braced frames (CBFs) designed in seismic regions. The thesis discusses the development of a database that includes extensive information from more than 300 tests of steel braces that have been conducted worldwide over the past 40 years. Statistical information of various properties of steel braces that can be used for quantification of modeling uncertainties is summarized and implications regarding the expected yield properties of various steel types as part of current design provisions are discussed. The steel brace database is utilized to develop drift-based and dual-parameter fragility curves for different damage states of steel braces. These curves can be used as tools for rapid estimation of earthquake damage towards the next generation of performance-based evaluation methods for new and existing buildings. Through extensive calibrations of an inelastic fiber-based steel brace cyclic model, modeling recommendations for the post-buckling behaviour and fracture of steel braces due to low-cycle fatigue are developed for three different brace shapes. The effectiveness of these recommendations is demonstrated through two case studies including concentrically braced frames (CBFs) subjected to earthquake loading. The emphasis is on the accurate assessment of the collapse capacity of concentrically braced frames with the explicit consideration of strength and stiffness deterioration of various structural components that are part of local story mechanisms that develop in CBFs after the steel braces fracture. The influence of modeling classical damping on the collapse capacity of CBFs is also discussed." --
Concentrically braced frames (CBFs) have been used in steel construction as seismic-force-resisting systems for many decades and constitute a substantial proportion of existing building infrastructure. Until about 1990, CBFs were designed without the codified capacity-based and other ductile design provisions that ensure safety in today's special CBFs (SCBFs) used in regions with high seismic risk. Thousands of these older and potentially nonductile CBFs (NCBFs) remain in service in the high-seismicity areas of the west coast of the US and other more moderately seismically vulnerable regions. These NCBFs utilize a wide variety of connections, components, and frame configurations with deficiencies expected to lead to significant damage and potential collapse in earthquakes. Seismic retrofit of NCBFs may be necessary to ensure occupant safety and building functionality, but current engineering guidance for determining retrofit need and type is limited. The state of practice evaluates the seismic vulnerability of CBFs using simplistic models for braces, beams, and columns, and the nonlinear behavior of connections is typically not considered; it is clear that the vulnerability depends on more complex component behavior. To develop more comprehensive engineering methods that can accurately estimate the vulnerability of NCBFs and the improved performance of retrofitted NCBFs, integrated experimental and computational research programs were conducted. First, two series of large-scale experiments of existing and retrofitted NCBF subassemblages were performed to investigate brace, connection, and beam deficiencies common to NCBFs. The experiments identified critical deficiencies but also beneficial yielding mechanisms (e.g., bolt-hole elongation, beam yielding in the chevron configuration, etc.) which could be retained in retrofit. Experimentally validated, nonlinear modeling approaches capable of simulating brace fracture, connection fracture, weak frame elements, and post-fracture response of components with secondary yielding mechanisms were then developed to advance numerical simulation capabilities. These models were used to enable system-level response-history analysis for seismic performance evaluation. Specifically, the seismic performance (including collapse) of three- and nine-story buildings were investigated at multiple (5) hazard levels. The models were also used to evaluate retrofit strategies; these results combined with the experimental work were used to develop a cost-effective seismic retrofit methodology based on balancing yielding mechanisms and suppressing severe failure modes.
This thesis examines collapse risk of tall steel braced frame buildings using rupture-to-rafters simulations due to suite of San Andreas earthquakes. Two key advancements in this work are the development of (i) a rational methodology for assigning scenario earthquake probabilities and (ii) an artificial correction-free approach to broadband ground motion simulation. The work can be divided into the following sections: earthquake source modeling, earthquake probability calculations, ground motion simulations, building response, and performance analysis. As a first step the kinematic source inversions of past earthquakes in the magnitude range of 6-8 are used to simulate 60 scenario earthquakes on the San Andreas fault. For each scenario earthquake a 30-year occurrence probability is calculated and we present a rational method to redistribute the forecast earthquake probabilities from UCERF to the simulated scenario earthquake. We illustrate the inner workings of the method through an example involving earthquakes on the San Andreas fault in southern California. Next, three-component broadband ground motion histories are computed at 636 sites in the greater Los Angeles metropolitan area by superposing short-period (0.2~s-2.0~s) empirical Green's function synthetics on top of long-period (> 2.0~s) spectral element synthetics. We superimpose these seismograms on low-frequency seismograms, computed from kinematic source models using the spectral element method, to produce broadband seismograms. Using the ground motions at 636 sites for the 60 scenario earthquakes, 3-D nonlinear analysis of several variants of an 18-story steel braced frame building, designed for three soil types using the 1994 and 1997 Uniform Building Code provisions and subjected to these ground motions, are conducted. Model performance is classified into one of five performance levels: Immediate Occupancy, Life Safety, Collapse Prevention, Red-Tagged, and Model Collapse. The results are combined with the 30-year probability of occurrence of the San Andreas scenario earthquakes using the PEER performance based earthquake engineering framework to determine the probability of exceedance of these limit states over the next 30 years.
"Capacity design principles have reduced the earthquake-induced collapse risk in steel frame buildings designed in seismic regions. Experiments suggest that the steel column behaviour may be significantly compromised due to member and local geometric instabilities, thereby increasing the associated collapse risk and likelihood of building demolition due to residual deformations. The High Yield Point (HYP400) steel is a steel material that has a higher yield stress and notch toughness but less strain hardening than conventional mild steels. HYP400 steel could enhance capacity design principles, such as the strong-column-weak-beam (SCWB) ratio when they are utilized in steel columns and potentially increase the collapse capacity of steel moment resisting frames (MRFs) under earthquake shaking. This thesis advances the state-of-knowledge through a multi-scale (from material to system) level study to assess the potential use of high-performance steel materials in minimizing earthquake-induced collapse of steel MRFs. The primary focus is on the characterization of the collapse behaviour of HYP400 and conventional steel hollow square section (HSS) columns by means of experimental testing and corroborating numerical simulations. Dual-parameter collapse-consistent loading histories (i.e., axial load and lateral drift demands) are developed to better quantify the flexural and axial demands in both interior and end columns in steel MRFs. These protocols reflect the asymmetric drifting of a building in one primary loading direction prior to dynamic instability ("ratcheting"). They also reflect the seismic demands imposed into steel columns within a steel MRF subjected to near-fault and long-duration ground motions. A landmark experimental program is conducted that characterizes the collapse behaviour of wide-flange and HSS steel columns under cyclic loading. The experimental program highlights the differences in the seismic demands and failure modes observed in steel columns depending on the imposed lateral and axial loading history, expected ground motion characteristics and building topology. It is shown that column axial shortening dominates the steel column stability. The hysteretic behaviour of HSS steel columns is further evaluated through corroborating finite element (FE) simulations. The steel column pre- and post-buckling behaviour is fully characterized depending on the type of steel material including the HYP400 steel. The FE results provide insight on the main differences of the lateral and axial damage progression between interior and end columns within the same steel MRF bay. The experimental data and corroborating finite element studies provide the basis for the development of a versatile steel column deterioration model that can explicitly simulate the axial-bending interaction, the column axial shortening due to local buckling induced softening and the cyclic deterioration in the column's strength and stiffness. Local buckling-induced softening is modeled through the development of an equivalent stress-strain formulation that includes a softening branch and can be fully characterized through conventional stub column tests. System level dynamic collapse simulation studies are conducted with over 80 archetype buildings with steel MRF systems ranging from 2 to 12-stories. Emphasis is placed on the importance of column axial shortening on the seismic performance of steel MRFs. It is shown that depending on the ground motion type, column axial shortening may result into slab tilting and catenary action prior to collapse. It is also shown that the use of the HYP400 steel columns can potentially enhance the collapse capacity of steel MRFs and reduce the expected residual lateral and vertical deformations in the aftermath of earthquakes." --
Seismic design of multi-story buildings requires capacity design principles that allow for distributed damage (plastic member deformations) to occur over the building height while preventing soft-story failure mechanisms that may lead to collapse. Seismic evaluation of steel concentrically braced frame (CBF) buildings has revealed that they exhibit soft-story behavior due to non-uniform brace degradation and non-ductile failure modes. This research proposes a rehabilitative design procedure for existing buildings that uses a stiff rocking core to redistribute plastic deformations along the structure’s height. Additionally, an improved design procedure for braced frame columns is proposed for new frame design. Several representative frames were designed and evaluated using nonlinear transient seismic finite element analysis and large-scale hybrid experimental testing. Predicted, analytical, and experimental response results show reasonable agreement, and the proposed techniques are believed to be reliable for achieving desirable seismic performance in low- to mid-rise steel braced frame structures.