ارزيابي رفتار لرزه‌اي قاب‌هاي بتن مسلح در اثر برخورد با ساختمان‌هاي مجاور

Modeling and Studying the Seismic Performance of Adjacent Reinforced Concrete Frames with Applying the Effect of Pounding

در هنگام زلزله، ارتعاش غيرهم فاز ساختمان‌هاي مجاور باعث برخورد و ضربه بين آن‌ها مي‌شود كه شرايط بارگذاري بحراني‌تري از حالت ارتعاش بدون ضربه ايجاد مي‌كند. فراواني نمونه‌هايي از عدم اجراي درز انقطاع در ساختمان‌هاي موجود، اهميت پرداختن به اين موضوع را افزايش مي‌دهد. در پژوهش حاضر پس از توسعه و پياده‌سازي مدل ويسكو الاستيك غيرخطي براي شبيه سازي برخورد سازه‌ها در نرم افزار مورد استفاده، با مدلسازي قاب‌هاي بتن مسلح مجاور هم در شرايط مختلف و بررسي رفتار آن‌ها تحت اثر مجموعه‌اي از ركوردهاي واقعي زلزله و تحت شتاب افزاينده، اثر برخورد سازه ها مطالعه مي‌شود. اين قاب‌ها نخست به روش مستقيم طراحي بر اساس تغيير مكان براي اطمينان به دستيابي به يك تغييرمكان از پيش مشخص در سطح خطر مورد نظر، طراحي شده سپس با قرار دادن مدل‌هاي سازه‌اي با ارتفاع متفاوت در كنار يكديگر و فاصله درز انقطاع مختلف، تحت مجموعه ركوردهاي نمونه انتخابي مورد تحليل ديناميكي افزاينده قرار گرفته و اثر برخورد سازه‌ها بر تغييرمكان نسبي طبقات و ظرفيت فروريزش بررسي مي‌شوند. نتايج به دست آمده گوياي اين مطلب است كه با در نظر گرفتن شكست برشي سازه‌هاي مجاور هم با تراز طبقات متفاوت، ظرفيت فروريزش سازه‌هاي مجاور پس از برخورد به شدت كاهش مي‌يابد. بنابراين آثار موضعي برخورد در محل ضربه اهميت بالايي دارد و لازم است در ارزيابي لرزه‌اي سازه‌ها مورد بررسي دقيق قرار گيرد

It is well known that the interaction between adjacent buildings with limited separation distance or in the other ‎words earthquake-induced structural pounding has considerable effect on seismic performance of buildings. Earthquake induced vibrations may cause impact between two adjacent buildings with inadequate separation distance. Pounding hazard is considerable particularly in populated residential regions because of the limited separation distance due to limitation of lands. This ‎phenomenon may result in substantial damage or even contributes to total structural collapse of structures. Major seismic events during the past decade such as those that have occurred in Northridge, Imperial Valley (May 18,1940), California (1994), Kobe, Japan (1995), Turkey (1999), Taiwan (1999) and Bhuj, Central Western India (2001) have continued to demonstrate the destructive power of earthquakes, with destruction of engineered buildings, bridges, industrial and port facilities as well as giving rise to great economic losses. Among the possible structural damages, seismic induced pounding has been commonly observed in several earthquakes. As the cost of land in cities increases, the need to build multistory buildings in close proximity to each other also increases. Sometimes, construction materials, other objects and any projections from a building may also decrease the spacing provided between the buildings. This leads to the problem of pounding of these closely placed buildings when responding to earthquake ground motion. The main purpose of this paper is the modeling of adjacent structures in order to study the effects of pounding. Among the existing models, nonlinear visco-elastic model has been selected for numerical simulation of pounding. To study the effect of pounding on seismic performance of reinforced concrete structures, two RC frames with 4 and 6 stories are selected and their seismic performance under pounding effects are numerically studied. The effect of storey and total height of structures, size of separation distance and mass of buildings in series on the impact has been investigated in adjacent RC frames. The selected frames have been designed according to direct displacement based design method in order to investigate the effect of impact on ductility demand and obtain a desired maximum induced displacement at a considered hazard level. Then by putting the structural models with different dynamic characteristics close together, the effect of altitude, size of gap, and story height on performance of two adjacent structures has been studied. The nonlinear time history analysis and incremental dynamic analysis (IDA) has been done to predict the seismic collapse capacity of systems. The results of analysis show that the effect of pounding severely depends on the phase difference of vibrations of adjacent structures. The phase difference itself, depends on mass, stiffness and seismic capacity of adjacent systems and the value of imposed plastic deformation as well. The effect of pounding in structural systems with the same height with little difference in initial periods is negligible, whereas the local effect of pounding especially in the case of floor to column impact is considerable