Download or read book Improving the Earthquake Resilience of Existing Multi Storey Concentrically Braced Frames Office Buildings in Moderate to High Seismic Zones written by Ovidiu Mircea Serban and published by . This book was released on 2015 with total page 216 pages. Available in PDF, EPUB and Kindle. Book excerpt: In the past decades, concentrically braced frames (CBF) have been frequently employed as earthquake resistant systems for low- and middle-rise buildings. Their configuration and straightforward design made them appealing for engineers. It is noted that building structures designed and built in Canada prior to 1970 were not proportioned to carry seismic loads, while those constructed between 1970 and 1985 were designed to withstand lower seismic forces than those required by the current code. As a consequence, these buildings are characterized by lack of seismic resilience and therefore are vulnerable to earthquakes. Herein, the building’s resilience is defined as the capability of a system to maintain a level of functionality in the aftermath of an earthquake event and is characterized by the performance metrics such as fragility, loss, and recovery functions. To quantify the seismic resilience of existing office buildings, a methodology was proposed and exemplified in a case study comprising of 3- and 6-storey fictitious CBF office buildings located in Quebec City and Vancouver. These buildings were designed in accordance with the requirements of the 1980 edition of the National Building Code and CSA/S16.1-M78 standard. It is noted that before 1985, Quebec City and Vancouver were located in the same seismic zone (the seismic demand was identical) and the Vancouver buildings were selected for comparison purpose. The proposed seismic resilience methodology consists in selecting the Rehabilitation Objective Class and the associated performance levels corresponding to earthquake hazard levels (e.g. 2%/50 yrs., 10%/50 yrs. and 50%/50 yrs.). To achieve this step, nonlinear dynamic time-history analyses are required and fragility curves computed for different hazard levels for both existing and retrofitted 3- and 6-storey fictitious buildings were generated from the Incremental Dynamic Analysis curves (IDA). Both aleatoric and epistemic uncertainties were considered. The loss estimation model is a function of system’s components deficiency determined by the use of performance limit thresholds for different damage states. In addition, functionality curves computed for different hazard levels using an exponential recovery model are also shown. The seismic assessment process was done according to performance based design principles and nonlinear time-history analysis by means of IDA using the OpenSees framework (Open System for Earthquake Engineering Simulation). Herein, all studied buildings were assessed against the current code demand. Based on the results, the buildings located in Quebec City and Vancouver show deficiencies at the level of structural members, especially the buildings located in Vancouver. Moreover, all brace-to-frame connections had insufficient strength and showed failure due to shearing of the welds. As reported from IDA curves, all existing buildings experienced collapse when subjected to ground motion intensities in agreement with the current code demand and a retrofit action was required. To respond to the Rehabilitation Objective Class defined as Basic Safety by the ASCE/SEI 41-13 provisions, the selected rehabilitation strategy consisted in local strengthening of system’s components (e.g. cover plating steel columns or beams and gusset plate replacement). According to the results, when the 3- and 6-storey retrofitted buildings located in Quebec City were subjected to ground motions scaled to the current code demand, their functionality was higher than 86.67% and 75.7%, respectively. Conversely, for the Vancouver buildings, besides the retrofit action, it is suggested to double the number of CBFs in order to pass the current code requirements. In conclusion, the proposed retrofit scheme for Quebec City buildings was able to improve the building performance and implicitly its earthquake resilience.

Download or read book Improving the Earthquake Resilience of Buildings written by Izuru Takewaki and published by Springer Science & Business Media. This book was released on 2012-07-26 with total page 332 pages. Available in PDF, EPUB and Kindle. Book excerpt: Engineers are always interested in the worst-case scenario. One of the most important and challenging missions of structural engineers may be to narrow the range of unexpected incidents in building structural design. Redundancy, robustness and resilience play an important role in such circumstances. Improving the Earthquake Resilience of Buildings: The worst case approach discusses the importance of worst-scenario approach for improved earthquake resilience of buildings and nuclear reactor facilities. Improving the Earthquake Resilience of Buildings: The worst case approach consists of two parts. The first part deals with the characterization and modeling of worst or critical ground motions on inelastic structures and the related worst-case scenario in the structural design of ordinary simple building structures. The second part of the book focuses on investigating the worst-case scenario for passively controlled and base-isolated buildings. This allows for detailed consideration of a range of topics including: A consideration of damage of building structures in the critical excitation method for improved building-earthquake resilience, A consideration of uncertainties of structural parameters in structural control and base-isolation for improved building-earthquake resilience, and New insights in structural design of super high-rise buildings under long-period ground motions. Improving the Earthquake Resilience of Buildings: The worst case approach is a valuable resource for researchers and engineers interested in learning and applying the worst-case scenario approach in the seismic-resistant design for more resilient structures.

Download or read book Seismic Fragility Assessment of Pre and Post Retrofit 1980s Concentrically Braced Frame Office Buildings in Moderate Seismic Zones written by Ming Zheng Wang and published by . This book was released on 2014 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Design Modelling and Seismic Performance of Outrigger Braced Frame Steel Buildings Subjected to Crustal and Subduction Earthquakes written by Aid Jnaid and published by . This book was released on 2018 with total page 247 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Seismic Evaluation Rehabilitation and Improved Design of Sub standard Steel Concentrically Braced Frame Buildings written by Derek Slovenec and published by . This book was released on 2015 with total page 593 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Techniques for the Seismic Rehabilitation of Existing Buildings written by and published by FEMA. This book was released on 2006 with total page 572 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Life Cycle of Structures and Infrastructure Systems written by Fabio Biondini and published by CRC Press. This book was released on 2023-06-28 with total page 6293 pages. Available in PDF, EPUB and Kindle. Book excerpt: Life-Cycle of Structures and Infrastructure Systems contains the lectures and papers presented at IALCCE 2023- The Eighth International Symposium on Life-Cycle Civil Engineering, held at Politecnico di Milano, Milan, Italy, 2-6 July, 2023. This book contains the full papers of 514 contributions presented at IALCCE 2023, including the Fazlur R. Khan Plenary Lecture, nine Keynote Lectures, and 504 technical papers from 45 countries. The papers cover recent advances and cutting-edge research in the field of life-cycle civil engineering, including emerging concepts and innovative applications related to life-cycle design, assessment, inspection, monitoring, repair, maintenance, rehabilitation, and management of structures and infrastructure systems under uncertainty. Major topics covered include life-cycle safety, reliability, risk, resilience and sustainability, life-cycle damaging processes, life-cycle design and assessment, life-cycle inspection and monitoring, life-cycle maintenance and management, life-cycle performance of special structures, life-cycle cost of structures and infrastructure systems, and life-cycle-oriented computational tools, among others. This Open Access Book provides both an up-to-date overview of the field of life-cycle civil engineering and significant contributions to the process of making more rational decisions to mitigate the life-cycle risk and improve the life-cycle reliability, resilience, and sustainability of structures and infrastructure systems exposed to multiple natural and human-made hazards in a changing climate. It will serve as a valuable reference to all concerned with life-cycle of civil engineering systems, including students, researchers, practicioners, consultants, contractors, decision makers, and representatives of managing bodies and public authorities from all branches of civil engineering.

Download or read book Risk Management Series Incremental Seismic Rehabilitation of Office Buildings Providing Protection to People and Buildings written by and published by FEMA. This book was released on with total page 90 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Innovative Methodologies for Resilient Buildings and Cities written by Izuru Takewaki and published by Frontiers Media SA. This book was released on 2019-09-19 with total page 135 pages. Available in PDF, EPUB and Kindle. Book excerpt: Resilient buildings and cities are in the center of common interests in modern academic communities for science and engineering related to built environment. Resilience of buildings and cities against multidisciplinary risks, e.g. earthquakes, strong winds, floods, etc., is strongly related to the sustainability of buildings and cities in which reduction of damage during a disaster and fast recovery from the damage are key issues. The reduction of damage is related to the level of resistance of buildings and the time of recovery is affected by the amount of supply of damaged members, assurance of restoration work, etc. Robustness, redundancy, resourcefulness, and rapidity are four key factors for supporting the full realization of design and construction of resilient buildings and cities. This research topic gathers cutting-edge and innovative research from various aspects, e.g. robustness of buildings and cities against earthquake risk, structural control and base-isolation for controlling damage risks, quantification of resilience measures, structural health monitoring, innovative structural engineering techniques for higher safety of buildings, resilience actions and tools at the urban scale, etc.

Download or read book Seismic Performance Evaluation and Economic Feasibility of Self centering Concentrically Braced Frames written by Mojtaba Dyanati Badabi and published by . This book was released on 2016 with total page 180 pages. Available in PDF, EPUB and Kindle. Book excerpt: Self-centering concentrically braced frame (SC-CBF) systems have been developed to increase the drift capacity of braced frame systems prior to damage to reduce post-earthquake damages in braced frames. However, due to special details required by the SC-CBF system, the construction cost of an SC-CBF is expected to be higher than that of a conventional CBF. While recent experimental research has shown better seismic performance of SC-CBF system subjected to design basis earthquakes, superior seismic performance of this system needs to be demonstrated for both structural and nonstructural components in all ground motion levels and more building configurations. Moreover, Stakeholders would be attracted to utilize SC-CBF if higher construction cost of this system can be paid back by lower earthquake induced losses during life time of the building. In this study, the seismic performance and economic effectiveness of SC-CBFs are assessed and compared with CBF system in three building configurations. First, probabilistic demand formulations are developed for engineering demand parameters (inter-story drift, residual drift and peak floor acceleration) using results of nonlinear time history analysis of the buildings under suites of ground motions. Then, Seismic fragility curves, engineering demand (inter-story drift, peak floor acceleration and residual drift) hazard curve and annual probabilities of exceeding damage states are used to evaluate and compare seismic performance of two systems. Finally, expected annual loss and life cycle cost of buildings are evaluated for prototype buildings considering both direct and indirect losses and prevailing uncertainties in all levels of loss analysis. These values are used evaluate economic benefit of using SC-CBF system instead of CBF system and pay-off time (time when the higher construction cost of SC-CBF system is paid back by the lower losses in earthquakes) for building configurations. Additionally, parametric study is performed to find acceptable increase in cost of SC-CBFs comparing to CBFs and impact of economic discount factor, ground motion suite and building occupancies on economic effectiveness of the SC-CBF system in three configurations. Results of this study indicates that, SC-CBF system generally shows better seismic performance due to damages to structural and non-structural drift sensitive components but worse performance due to damages to acceleration sensitive components. Therefore, loss mitigation in structural and non-structural damages are major source of economic benefit in SC-CBFs. SC-CBF system is not feasible for high rise buildings and low seismic active locations. If the cost of SC-CBFs are twice as CBF frames, the higher cost is paid back in a reasonable time during the life time of the buildings. SC-CBFs are more feasible for banks/financial institutions than general office buildings.

Download or read book Risk Management Series Incremental Seismic Rehabilitation of Office Buildings FEMA 397 December 2003 written by U. s. Department of Homeland Security and published by Createspace Independent Pub. This book was released on 2013-01-29 with total page 88 pages. Available in PDF, EPUB and Kindle. Book excerpt: This manual is intended to assist office building owners' personnel responsible for funding and operating existing office buildings across the United States. This publication and its companion documents are the products of a Federal Emergency Management Agency (FEMA) project to develop the concept of incremental seismic rehabilitation—that is, building modifications that reduce seismic risk by improving seismic performance and that are implemented over an extended period, often in conjunction with other repair, maintenance, or capital improvement activities. The manual was developed after the project team analyzed the management practices of office building owners of varying sizes located in various seismic zones in different parts of the United States. It focuses on the identified concerns and decision making practices of owners and managers of Class A, B, or C buildings, be they REITs, pension funds, partnerships, individuals, or other types of owners. Earthquakes are a serious threat to office safety and pose a significant potential liability to office building owners. Office buildings in 39 states are vulnerable to earthquake damage. Unsafe existing buildings expose office building owners and tenants to the following risks: Death and injury of tenants, occupants, and visitors; Damage to or collapse of buildings; Damage to and loss of furnishings, equipment, and other building contents; Disruption of office functions and building operations. The greatest earthquake risk is associated with existing office buildings that were designed and constructed before the use of modern building codes. For many parts of the United States, this includes buildings built as recently as the early 1990s. Although vulnerable office buildings need to be replaced with safe, new construction or rehabilitated to correct deficiencies, for many building owners new construction is limited, at times severely, by budgetary constraints, and seismic rehabilitation is expensive and disruptive. However, incremental seismic rehabilitation, an innovative approach that phases in a series of discrete rehabilitation actions over a period of several years, is an effective, affordable, and non-disruptive strategy for responsible mitigation action. It can be integrated efficiently into ongoing facility maintenance and capital improvement operations to minimize cost and disruption. The strategy of incremental seismic rehabilitation makes it possible to get started now on improving earthquake safety in your office building inventory. This manual provides owners of office buildings, be they Real Estate Investment Trusts (REITs), pension funds, partnerships, individuals, or other forms of ownership, with the information necessary to assess the seismic vulnerability of their buildings and to implement a program of incremental seismic rehabilitation for those buildings.

Download or read book Rapid Visual Screening of Buildings for Potential Seismic Hazards Supporting Documentation written by and published by Government Printing Office. This book was released on 2015 with total page 206 pages. Available in PDF, EPUB and Kindle. Book excerpt: The Rapid Visual Screening (RVS) handbook can be used by trained personnel to identify, inventory, and screen buildings that are potentially seismically vulnerable. The RVS procedure comprises a method and several forms that help users to quickly identify, inventory, and score buildings according to their risk of collapse if hit by major earthquakes. The RVS handbook describes how to identify the structural type and key weakness characteristics, how to complete the screening forms, and how to manage a successful RVS program.

Download or read book Seismic Performance of Steel Buildings with Braced Dual Configuration and Traditional Frame Systems Through Nonlinear Collapse Simulations written by Yudong Wang and published by . This book was released on 2018 with total page 203 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Effects of Connections Detailing and Friction Dissipation Devices on the Seismic Response of a Hospital Steel Braced Frame Building written by Masaaki Ohira and published by . This book was released on 2020 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Hospitals are post-disaster buildings designed to withstand seismic forces that are amplified with an importance factor of IE= 1.5. Their seismic force-resisting system (SFRS) should be designed with Rd> 2.0, while the interstorey drift at each floor is limited to 1.0%hs. Herein, Rd is the ductility-related force modification factor and hs is the storey height. Although the non-structural components and the hospital contents are not part of this research, they constitute a larger loss in the event of an earthquake. As such, both interstorey drifts and floor accelerations should be within the required limits. Concentrically braced frames (CBFs) are frequently employed as earthquake-resistant systems due to their high stiffness and moderate ductility. However, this system has shown several drawbacks such as the concentration of damage within a floor and high floor accelerations, which may be critical for acceleration-sensitive non-structural components. Recent experimental studies revealed that even moderately ductile concentrically braced frames (MD-CBF) may undergo unintended failure modes due to the limited deformation capacity of brace-to-frame connections. To overcome this drawback, it is proposed to provide an 8tg elliptical clearance band in the brace-to-frame gusset plate instead of a linear 2tg clearance, which is recommended by the code. Herein, tg is the thickness of the gusset plate. The results pointed out that gusset plates with 8tg elliptical clearance require less thickness than that with 2tg linear clearance and provide larger rotation capacity. In consequence, the ductility of MD-CBF with brace-to-frame gusset plates detailed with 8tg elliptical clearance is improved. Furthermore, in order to mitigate the floor acceleration, braces of CBFs can be replaced with sliding friction braces (SF), where each SF brace is made of a friction damper installed in-line with an HSS brace. The proposed sliding friction braced frame (SF-BF) system behaves elastically as a traditional CBF before friction devices are activated and experience nonlinear response after that. Thus, in the case of SF-BF system, the input energy is dissipated by friction devices and all adjacent members such as braces, connections, beams, and columns of the CBF system are designed to remain in the elastic range. It is noted that SF-BF systems are prone to residual interstorey drift, which can be mitigated by: (i) using braced frame's columns continuous over all floors or (ii) adding back-up moment-resisting frames designed to provide the elastic frame action. The main objective of this thesis was three folds: (i) to investigate the inelastic behaviour of MD-CBF systems with 8tg elliptical clearance gusset plate versus 2tg linear clearance band; (ii) to develop an accurate numerical model for braces equipped with friction dampers using the OpenSees software and (iii) to examine the seismic response of SF-BF systems. To carry out this research, a detailed model of a 4-storey hospital located in Victoria, BC on Site Class C was developed in OpenSees and subjected to 10 historical ground motions for nonlinear time-history analysis. In this manner, a model replicating the MD-CBF with 2tg linear clearance band gusset plates for brace-to-frame connections and a model replicating the MD-CBF with 8tg elliptical clearance band for brace-to-frame gusset plate detail were developed and the nonlinear time-history responses expressed in terms of interstorey drift, residual interstorey drift and floor acceleration were compared. A force-based design method was applied to design the SF-BF system. By optimizing the slip length and slip force in the damper, the slip-lock phase exhibited due to the bearing of the pretensioned bolts can be postponed while maintaining the drift below the code limits. Dynamic instability may become an issue when dampers with large slip lengths are installed. From this research it was found that small difference was observed in the response of MD-CBFs when brace-to-frame gusset plates with 8tg elliptical clearance was selected instead of 2tg linear band detail. When the SF-BF system was designed using the force based design method, the HSS brace was proportioned such that the compression resistance of brace to be equal or greater than 130% slip force. Then, capacity design was employed to design the beams and columns of braced frames. An OpenSees model was developed to simulate the behaviour of Pall friction damper and brace assembly. From nonlinear dynamic analysis, it was found that large residual interstorey drift was observed when columns of braced frame were continuous over two storeys, although the interstorey drift is within the code limit, which is 1.0%hs for a hospital building. To mitigate the residual drift, continuous columns over the building height were considered. However, it was concluded that SF-BFs are not recommended for hospitals located in high risk seismic zones unless back-up moment-resisting frames designed for 25% base shear are provided.

Download or read book New Methods for Seismic Performance Evaluation and Retrofit of Nonductile Concentrically Braced Frames written by Andrew D. Sen and published by . This book was released on 2018 with total page 290 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Performance Based Seismic Demand Assessment of Concentrically Braced Steel Frame Buildings written by Chui-Hsin Chen and published by . This book was released on 2010 with total page 342 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Performance of Seismically Deficient Existing Braced Steel Frame Structures With Flexible Diaphragms in Halifax written by Alicia Gallagher and published by . This book was released on 2012 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Concentrically braced frames (CBFs) have been one of the fundamental structural systems for lateral force resistance chosen by designers for low-rise steel construction since the early part of the twentieth century. CBFs designed using the building codes and standards of the 1960s were designed using the principle that they remained in the linearly elastic range. The current design philosophy of the 2010 National Building Code of Canada (NBCC) and CSA-S16-09 is based on the principles of capacity design and recognises the cyclic inelastic behaviour of CBFs. Since no detailing or design requirements for an inelastic seismic response were included in structures designed with past building codes, these structures are likely to exhibit seismic deficiences, including lack of lateral resistance and insufficient ductility. Guidelines for evaluating the performance of CBFs are required in order to provide recommendations for seismic evaluation and rehabilitation for such existing buildings for future building codes. The behaviour of one-storey steel structures built with the 1965 National Building Code of Canada (NRCC 1965) and CSA-S16-65 (CSA 1965) under current building code standards for seismic design was studied in order to aid in establishing such guidelines. The response of a series of sixteen one-storey buildings with varying aspect ratios and heights was studied, subjected to ten artificial and ten historical earthquake ground motions. The nonlinear seismic behaviour of the CBFs was determined using an analytic Open Sees, Open System for Earthquake Engineering Simulatuion (OpenSees 201), model for nonlinear time history dynamic analysis, including drift and ductility demands on the braces. The intended performance level in the design earthquakes, as well as the acceptance criteria used in the braced frame analysis were established using FEMA P695 (FEMA 2009) criteria. In general, although acceptable performance was not acheived in all cases, the one-storey stee structures built with the 1965 National Building Code of Canada, on average, performed well, for the seven failure criteria outlined in this study.