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|ارزیابی کارایی دیواره های ساختمانی پر با عناصر فیوز ساختاری بر اساس تست بار چرخه ای صفحه ای
|عنوان انگلیسی مقاله:
|Performance Evaluation of Different Masonry Infill Walls with Structural Fuse Elements Based on In-Plane Cyclic Load Testing
|رشته های مرتبط:
|مهندسی عمران، سازه و مدیریت ساخت، ساختمان های بتنی
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This paper discusses the performance of a structural fuse concept developed for use as a seismic isolation system in the design and retrofit of masonry infill walls. An experimental program was developed and executed to study the behavior of the structural fuse system under cyclic loads, and to evaluate the performance of the system with various masonry materials. Cyclic tests were performed by applying displacement controlled loads at the first, second, and third stories of a two-bay, three-story steel test frame with brick infill walls; using a quasi-static loading protocol to create a first mode response in the structural system. A parametric study was also completed by replacing the brick infill panels with infill walls constructed of concrete masonry units and autoclaved aerated concrete blocks, and applying monotonically increasing, displacement controlled loads at the top story of the test frame.
Keywords: masonry infill walls; seismic isolation; cyclic lateral load testing; structural fuse
Masonry infill walls are a common building element found throughout the world. Infill walls constructed of various masonry materials are often used in both concrete and steel structures to infill the frame openings . This type of construction is particularly common in developing countries where masonry materials such as clay bricks, concrete masonry units, and hollow clay tiles are readily available . In many cases, infill walls are treated as architectural elements and their influence on the behavior of the structure is not considered. This design philosophy can lead to uneconomical design as well as unexpected behavior and even catastrophic collapse. It has been widely documented by many researchers that masonry infill walls significantly influence the in-plane behavior and response of structural frames [3–۱۸]. Masonry infill walls increase the stiffness of structural frames, and in general help to limit building deflection under lateral loads. Although this increase in stiffness is beneficial for limiting building drift during wind storms and minor to moderate earthquakes, it can have a negative impact on the performance of structures during major seismic events. A comprehensive literature review on these issues is presented by Aliaari . The structural properties of masonry infill walls are often overlooked by designers who do not consider the increase in stiffness and potential decrease in ductility introduced into structural frames by the addition of unreinforced masonry infill materials. Typically, concrete and steel building frames are designed to resist all of the gravity and lateral loads, including wind and seismic forces. Infill walls are often treated as nonstructural elements even though they have a significant influence on the in-plane behavior and seismic performance of the structure . Ignoring the contribution of masonry infill walls to the strength and stiffness of building frames can lead to damage in the masonry walls as well as resulting in an inefficient use of materials and uneconomical design . According to Colombo et al. , neglecting the effects of the infill panels, as suggested by various building codes, does not lead to a safe seismic design. The assumption that masonry infill walls in concrete and steel frames will only increase the lateral load capacity of these structures is a common misconception . This oversight can result in severe structural damage and collapse in buildings where the ability of the frame to safely dissipate seismic input energy has been significantly overestimated. Overstressing of the masonry walls and the formation of a collapse mechanism in the structural frame can occur if the composite interaction between masonry infill walls and bounding frames is not accounted for during design. Two common design philosophies have been developed which consider the influence of infill walls on the response of structural frames to lateral loads . One approach is to isolate the masonry infill walls from the bounding frame with a physical gap . This allows the frame to act independently of the masonry walls and to be designed without consideration of the interaction between the frame elements and the infill panels. Isolating the infill panels from the frame can prevent severe cracking and damage to the masonry materials. This is an important consideration since falling debris from damaged and collapsed infill walls is a major life safety issue for buildings in seismic areas. By isolating the infill walls from the concrete or steel frame, the structure can dissipate the seismic input energy in a predictable and safe manner. A second common design method for infilled structural frames is to recognize the contribution of the infill panels to the strength and stiffness of the building frame and design the masonry walls as structural elements . In many cases, the size of the structural members in a building frame is dictated by building drift limits or other serviceability criteria rather than strength considerations. By taking into account the structural properties of the masonry infill walls in the calculation of building drift, the framing members can be reduced in size, resulting in a more economical design. This approach requires careful consideration of the composite interaction between the structural frame and the infill panels so that both can be designed to safely resist lateral loads. A disadvantage of this method is that the structural frame cannot act independently of the infill walls when subjected to large seismic forces. The added stiffness of the infill panels decreases the natural period of the structure, which results in higher seismic loads . Severe cracking of the masonry infill walls and shedding of debris can take place if the infill walls are not properly designed and detailed. A new concept in the performance and design of masonry infill walls is the idea of a structural fuse system [19,25–۲۷]. The structural fuse concept combines the two common design approaches by allowing masonry infill walls to be engaged with the bounding frame up to a predetermined level of lateral load. Brittle failure of the infill walls or frame elements is prevented by the introduction of a fuse mechanism, which isolates the infill material from the frame under higher loads. For lower levels of load, the strength and stiffness of the masonry material work compositely with the structural frame to limit lateral deflections. Under higher lateral loads, the infill panels are disengaged from the structure using the fuse mechanism, which prevents damage to the masonry walls and the formation of a frame failure mechanism. With this system, the structural frame can be designed to resist high lateral forces without the influence of the masonry material. The fuse element is the key component of the structural fuse system. The purpose of the fuse is to serve as a link between the structural frame and the masonry infill walls and prevent damage to the infill material. An extensive analytical and experimental study has been carried out on fuse materials and systems [19,25,28]. Under typical loading conditions, the fuse mechanism transfers the story shear forces from the structural frame to the masonry infill panels, which help to resist the in-plane lateral loads and limit frame deflections. If the story shear forces are sufficiently high to cause inelastic behavior in the masonry panels, the fuse element is designed to “break” and disengage the infill wall from the frame before damage occurs to the masonry material. An initial experimental study [25,28] showed that the structural fuse concept works well as a seismic isolation system. In that study, monotonically increasing, displacement controlled loads were applied at the top story of a two-bay, three-story steel test frame with brick infill walls and lumber disk fuse elements. According to the test results, the brick infill walls made a significant contribution to the in-plane stiffness of the test frame, up to the point where the fuse elements failed. The fuse mechanism successfully isolated the infill panels from the test frame, preventing damage to the brick masonry material. Since the initial study [25,28] concentrated on concept development and proof-of-concepts based on monotonic tests, a follow-up experimental program was developed to study the response of the structural fuse system under a cyclic loading history, and to evaluate the performance of the system with various masonry infill materials.