شکست برشی قاب بتنی در اندرکنش با دیوار پرکننده آجری

وجود دیوارهای پرکننده آجری و چگونگی اتصال آن به قاب بتنی بحث بسیار مهم و تعیین‌کننده در عملکرد سازه در مقابل زلزله است. این بحث در سال های اخیر با توجه به زلزله های پیش آمده و خرابی های حاصل از این زلزله ها اهمیت زیادی پیدا کرده و مورد بررسی و تحقیق قرارگرفته است. دیوار پر کننده علاوه بر اثرات مثبت در سازه دارای اثرات منفی نیز می باشد. در این مقاله به بررسی اندرکنش بین قاب بتنی و دیوار پرکننده آجری در یک قاب 3 دهنه 5 طبقه پرداخته خواهد شد. دیوار پرکننده آجری به‌صورت دستک معادل قطری ستون به ستون بر طبق روابط دستورالعمل بهسازی لرزه ای تعریف شده و برای مدل‌سازی و تحلیل استاتیکی غیرخطی از نرم‌افزار sap 2000 استفاده شده است. نتایج حاصل بیانگر این است که خاموت برشی موجود در ستون های بتنی در طراحی معمولی جواب گوی برش ایجاد شده در بالا و پایین ستون های متصل به دیوار پرکننده آجری نیست و باعث تشکیل مفصل برشی و شکست برشی در این نواحی از ستون می شود. شکست برشی در ستون های یک سازه باعث از بین رفتن ظرفیت محوری ستون و در نتیجه فروریزش سازه خواهد گردید. برای جلوگیری از این شکست باید خاموت ها برای بیشینه نیروها طراحی شود.

Shear Failure of Concrete Infilled Frame

Infill walls are one of the most important and problems in behavior of concrete structures during earthquake. This matter has become so important over the past few years as a result of the great damages caused by recent earthquakes. Masonry infill walls have negative and positive effects in building response.In this paper interaction of masonry infill with concrete frame with 3 bays and 5 stories is surveyed. Masonry infill walls are modeled by node to node equivalent struts and SAP2000 is used for analysis. The results show that shear reinforcement of concrete columns in ordinary design is not sufficient and shear failure is occurred. Shear failure in columns cause loosing axial capacity of columns and results in collapsing of buildings. Stirrups should be designed for maximum forces in order to avoid this brittle failure.An ideal form of structure is considered normally in order to analyze the structure, which undoubtedly has differences with its actual model. The actual model has also some differences with the computational model such as defects in the existence of infill walls, which will be neglected from their effects on the structure analysis and design. Distribution of these elements and their effects on stiffness and lateral strength of the constructions have generally overlooked during the design process. Seismic retrofitting of the structures needs detailed evaluation of these elements in reaction of structures to the applied loading. Although existence of the infill walls basically provides higher stiffness and strength for the frames, their detrimental effects on the structure performance is ignored due to the lack of adequate information about the behavior of frames and infill walls.According to the failure mode of structures with masonry infill walls, it is seen that masonry infill walls have positive and negative effects on structure. Because of considerable stiffness and resistance of infill walls, existing of that should cause high torsion in stiffness and resistance of structure in plan and view. Thus an important question is: how the interaction between masonry infill walls and concrete frame should be considered in design of structures for preventing brittle failure of columns in future earthquakes. From a technical point of view, the modeling is categorized into:i) Detailed modeling (micro)ii) Simple modeling (macro)The first category is based on the definition of finite element model of infill wall and is solved by formal plasticity and elasticity methods. In the second category, a general behavior of infill wall is important, and in this case one or several elements are used to model the behavior of infill wall. A 5story frame with 3 bays has been investigated in this study. All frames have been filled by masonry walls with thickness of 0.23 m. Lateral force resisting system is intermediate concrete moment frame and the type II of soil according to Iranian seismic code of practice (Standard No. 2800). In this method of analysis, by assessing the target displacement of structures, lateral forces are increasingly applied to the structural model up to control point displacement achieve target displacement. Control point displacement is considered at roof level. An important point is adjacent the masonry walls to concrete frame cause intensive shear force at top and bottom of columns. Inadequacy of shear reinforcement in columns cause shear failure and also the results confirm it. Shear failure will eliminate the axial capacity of columns, and finally the building will collapse.