411 research outputs found

    Influence of masonry infill on the seismic performance of concentrically braced frames

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    This paper presents an experimental and analytical study to investigate the effect of masonry infill on the seismic performance of special Concentrically Braced Frames (CBFs). Cyclic lateral load tests are conducted on three half-scale specimens including two special CBFs with and without masonry infill and a moment resisting steel frame with masonry infill for comparison purposes. Companion analyses are performed to study the influence of masonry infill on the potential rupture of gusset plates and top-seat angle connections by using detailed FE models validated with experimental results. It is shown that the presence of masonry infill could increase the lateral stiffness and load carrying capacity of the special CBF by 33% and 41%, respectively. However, the interaction between masonry infill and the frame significantly increased the strain demands and failure potential of the connections. The results of the experimental tests and analytical simulations indicate that ignoring the influence of masonry infill in the seismic design process of CBFs results in a premature fracture of the connection weld lines and a significant reduction in the deformation capacity and ductility of the frame. This can adversely influence the seismic performance of the structure under strong earthquakes. The results of this study compare well with the damage observations after the 2003 earthquake in Bam, Iran

    Net Section Fracture Assessment of Welded Rectangular Hollow Structural Sections

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    Rectangular Hollow Sections (RHS) because of their high resistance to tension, as well as compression, are commonly used as a bracing member with slotted gusset plate connections in steel structures. Since in this type of connection only part of the section contributes in transferring the tensile load to the gusset plate, shear lag failure may occur in the connection. The AISC specification decreases the effective section net area by a factor to consider the effect of shear lag for a limited connection configuration. This study investigates the effective parameters on the shear lag phenomenon for rectangular hollow section members connected at corners using a single concentric gusset plate. The results of the numerical analysis show that the connection length and connection eccentricity are the only effective parameters in the shear lag, and the effect of gusset plate thickness is negligible because of the symmetric connection. The ultimate tensile capacity of the suggested connection in this study were compared to the typical RHS connection presented in the AISC and the similar double angle sections connected at both legs. The comparison indicates that tensile performance of the suggested connection in this study because of its lower connection eccentricity is much higher than the typical slotted connection and double angle connections. Therefore, a new equation is suggested based on the finite element analyses to modify the AISC equation for these connections

    STR-863: DEVELOPMENT OF A NOVEL REPLACEABLE CONNECTION FOR SEISMICALLY DESIGNED STEEL CONCENTRICALLY BRACED FRAMES

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    There is increasing demand, from both engineers and their clients, for structures that can be rapidly returned to occupancy following an earthquake, while also maintaining or reducing initial costs. One possible way towards this goal is to ensure that seismic damage occurs only within elements that can be removed and replaced following a damaging earthquake. For concentrically braced frames that use hollow structural sections, the current design practice requires field welding of the brace to the gusset in a way that causes the brace to buckle out-of-plane. In the event of a damaging earthquake, the out-of-plane brace buckling may damage both the gusset plate and also any adjacent exterior cladding. The plate cannot be easily replaced, resulting in expensive and time-consuming repairs, and the damaged cladding could endanger the lives of people evacuating the building and of other pedestrians. This paper discusses the development of an alternative connection that can be bolted into place and that confines damage to replaceable components. The proposed connection is expected to result in reduced erection costs and be easier to repair following a major earthquake. Moreover, the new connection causes buckling to occur in plane, preventing dangerous damage to the cladding. Potential challenges in the design of such a connection are discussed and evaluated, and a finite element model that was created to confirm the performance of the proposed connection is also introduced. Finally, future areas of research and development of the connection are identified

    Effects of Connections Detailing and Friction Dissipation Devices on the Seismic Response of a Hospital Steel Braced Frame Building

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    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

    Lateral capacity and seismic characteristic of hybrid cold formed and hot rolled steel systems

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    This thesis addresses the application of hybrid cold-formed steel (CFS) - Hot-Rolled Steel (HRS) structures, as a new lateral force resisting system for light weight steel framed buildings in seismic regions. The study considers hysteretic behaviour, as well as maximum lateral load resisting capacity through comprehensive testing and advanced numerical analyses. The study identifies the advantages and disadvantages of the proposed hybrid system and provides in depth knowledge about performance characteristics of this innovative structural system, in order to facilitate the use of this system in earthquake-prone regions. The project is divided into three main parts: experimental, numerical and analytical studies. A comprehensive literature review is performed as a part of this study, in order to discover the existing gaps in the current knowledge regarding the structural performance of CFS structures and the methods for lateral performance enhancement. The literature review suggests that although CFS walls are not new, and have been used as non-structural components for many years, their application as main load-bearing structural frames is relatively new. That is, appropriate guidelines that address the seismic design of CFS structures have not yet been fully developed in the literature. In addition, the lateral design of these systems is not adequately detailed in the available standards of practice. There have been several attempts to improve the seismic performance of such structural system by different bracing or sheathing configurations. However, there is minimal background information available on hybrid systems such as hot rolled-cold formed structures. In this study, a series of CFS-HRS hybrid shear walls are constructed in order to investigate the lateral behaviour of the walls with different configurations to obtain the optimum combination of HRS and CFS. Different configurations are considered to provide the most efficient load transfer pattern from cold formed steel part of the wall to the Hot Rolled section, which is responsible for withstanding the lateral loads. The CFS part is aimed to transfer lateral loads to HRS part without any internal local failure. The ideal failure condition is the HRS yielding. Therefore, the optimum rigidity of the HRS part is of great importance to prevent any local failure happening prior to reaching the maximum lateral capacity of the HRS. For each experimental specimen, the hysteretic envelope curve is plotted, and different characteristics are evaluated. Since the failure mode of such systems is very complicated, the test results will provide the possible failure modes to be utilised for any further investigation or any optimisation analysis in numerical and analytical studies. In addition, Non-linear finite element (FE) analysis is employed using the ABAQUS software [1], in order to investigate the seismic performance of the proposed hybrid shear walls in multi-storey light steel frames. The nonlinear analysis accounts for different structural characteristics, including material non-linearity, geometric imperfection and residual stresses. The numerical models are verified based on experimental test results. The principal objective of this part of the study is aseismic optimisation of the proposed hybrid system and finding the corresponding dimensions and configurations to improve the strength and stiffness to achieve the objective. Using the hybrid wall panel system, a 4-storey building in an earthquake prone region is designed as per the relevant codes of practice. For the designed 4-storey building, the CFS part of the panel only bears the gravity loads, while a hot rolled steel collector transfers the lateral load to the HRS part acting as the main lateral load resisting system. Finally, the building is designed using different lateral load resisting systems and the results are compared with those from the proposed hybrid system in terms of cost. Furthermore, based on the real failure mode shapes obtained from test specimens, a Finite Strip Method program is developed to evaluate the elastic buckling mode shapes of a single stud with an arbitrary section detail. The code is helpful for design of CFS studs as explained in Chapters 3 and 5

    High-fidelity inelastic post-buckling response for balanced design and performance improvement of X-braced moment resisting frames

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    In this paper, the nonlinear post buckling response of X-Braced Moment Resisting Frame (X-BMRF) systems are studied. The X-BMRF comprises of X-bracing diagonals attached to the moment frame by corner gusset plates to form the structural system acting as a dual frame. In common practice today, one of the X-bracing diagonal members is discontinuous, and a middle gusset plate is used to connect the diagonals to each other at the intersection. In this study, the effect of mid-connection details and different types sizes of corner gusset plate connection are well measured to evaluate behavioral characteristics of the above systems. An accurate and robust three-dimensional finite element modeling of the above systems validatedverified against available test data and numerical simulation are demonstrated. Then, a number of X-BMRFs are designed and analyzed under monotonic (and cyclic) loading(s), and later ductility values and energy dissipation ratios of such systems are appraised. The results are used to evaluate the secondary yield mechanisms, probable failure modes, and to quantify the loading share of story shear when different rigidity ratios between the X-bracing and moment frame systems are deliberated. Finally, the results can provide a suitable ground to present a new set of balanced design criteria which can improve nonlinear performance and assure maximum system ductility of such system
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