Mevcut Bir Betonarme Binanın Tdy’07’de Belirtilen Doğrusal Elastik Olmayan Analiz Yöntemlerine Göre Performans Değerlendirmesi

Abstract

Bilimin ve bilgisayar teknolojilerinin gelişmesine paralel olarak yapı mühendisliğinde de birtakım ilerlemeler kat edilmiştir. Eskiden beri teoride mevcut olan adım-adım itme (statik itme) analizi ve deprem ivme kayıtlarının kullanılarak yapı davranışının gözlemlenebilmesi gibi kavramların bu gelişmelere bağlı olarak uygulanabilmesi olanaklı hâle gelmiştir. Özellikle son yirmi yılda ortaya çıkan şekil değiştirme esaslı yapısal tasarım ve değerlendirme yöntemi, geleneksel olan kuvvet esaslı yöntem karşısında popülerliğini her geçen gün daha da fazla artırmakta ve gelişme göstermektedir. Bu tez kapsamında, ülkemizde deprem konusunda artan farkındalığın sonucu olarak uygulanmaya başlanan “Kentsel Dönüşüm” projelerinde en sık kullanılan yapı taşıyıcı sistemlerinden birinin incelenmesi amaçlanmıştır. Yapılan gözlemlere paralel olarak, düşük kiriş yüksekliğine sahip dişli (asmolen) döşeme sisteminin, özellikle çok katlı olmayan yapılarda sıklıkla kullanıldığı tespit edilmiş ve kiriş yüksekliğinin sınırlı tutulduğu bu sistemlerin deprem performansının incelenmesinin bu tezin amacı doğrultusunda uygun olduğu düşünülmüştür. Bu tür binalarda genellikle, asansör ve merdiven bölgelerini veya bu bölgelerin bir bölümünü çevreleyen betonarme perdeler kullanılmaktadır. Kolonların ve perdelerin birbirine bağlantısı düşük yükseklikli ve yassı kirişlerden oluşan dişli döşeme sistemi ile sağlanmaktadır. Özellikle kirişlerde düşük kesit yüksekliğinin sonucu olarak moment taşıma kapasitesinin sınırlı olması bu bağlantı bölgelerinin güvenilirliği konusunda kuşku uyandırmaktadır. Bu durumun neticesinde, deprem yüklerinin çerçeve davranışı oluşturularak karşılanması mümkün olamamakta ve tüm bu yükler özellikle perde elemanlar tarafından karşılanmaktadır. Bu çalışmada, ele alınan bir adet binanın bu özel durumun sonucu olarak, tüm kiriş, kolon ve perde elemanlarında meydana gelen etkiler incelenmiştir. TDY’07’ye uygun olarak boyutlandırılmış 8 katlı, kiriş yüksekliği sabit, asmolen döşeme sistemindeki yapının TDY’07 kıstasları dikkate alınarak şekil değiştirme esaslı performans değerlendirilmesi yapılmıştır. Perde + çerçeve sistemli yapıda bir adet U şeklinde perde mevcuttur. Performans değerlendirmesi yapabilmek maksadıyla doğrusal olmayan artımsal itme analizi ve doğrusal olmayan zaman tanım alanında dinamik hesap yöntemleri kullanılmıştır. Perde davranışını daha iyi gözlemleyebilmek için ise perdeler; lifli plastik mafsala sahip çubuk eleman ve çok katmanlı kabuk modeli olmak üzere iki ayrı biçimde tarif edilmiştir. Farklı analiz yöntemlerine ve farklı perde tiplerine göre oluşturulan modeller sonlu elemanlar yöntemi kullanılarak değerlendirilmiş ve birbirleri arasında karşılaştırmalar yapılmıştır.

Advancements in science and computer technologies leads to some progress on structural engineering. These developments make implementation of pushover analysis and observation of the structural behaviour by using real or modified seismic datas possible which were known theoratically in the past. Researchers, engineers and some other technical disciplines used that scientific heritage for creating better solutions about structural design concepts. This started a new era in structural engineering, which is called “displacement based design” in technical literature. Although this new concept is only around two decades old, it seems that popularity of it increases day by day against traditional concepts such as “force based design”. The new concept becomes popular since it leads more trustable, economical and observable solutions compared to traditional techniques. On the other hand, evaluation of the multiple performance of a building for any desired level is possible anymore by using displacement based design what brings a totally new subject to the structural engineering literature. This new concept has started to appear in the earhtquake and building codes of some countries since then the codes has been upgraded several times to improve the results. The concept is mentioned the first time in Chapter 7 of Turkish Seismic Code (TSC) in 2007. TSC’07 allows designers to evaluate the performance of a building what may has different purposes of using. Both linear and nonlinear behaviour approaches may be used to gain the result of performance level. However, the nonlinear behaviour of structural members must be recognised in displacement based design concept since the method demands it. This study focuses on to investigate one of the most popular building design type what is sustained in the scope of “National Urban Transformation Projects”. Some recent major earthquakes occured in Turkey raised awareness on earthquake disasters and finding solutions to beware of their destructive effects. The main purpose of these projects is to check performances of existing buildings and retrofit or rebuild them if necessary. The other goal for the researchers is to explore new techniques to strengthen the buildings. This thesis studies flat-slab floor systems used with low-height sectioned beams in RC structures since it is one of the most common building type especially for the low-rise transformation projects. In general, one core shear wall which is located mostly around of stairs, fire escapes, shafts or elevators is used with slab flat systems to help resist lateral loads. As it is observed from the current projects despite the column and shear wall elements designed conservatively, beam members which has very limited dimensions through their heights can cause lack of the structural earthquake resisting performance. This may create the redistribution of moments resulted from lateral loads through the vertical load carrying members columns and shear walls. Therefore, extraordinary and unpredictable loads may be needed to carry by these vertical members. When it is considered designers usually ignore such cases, this raises questions about the transformation projects which aim to make stronger and more capable buildings against earthquake loads. In this study, an 8 storey RC flat-slab building with constant height of the beam members designed together with an U shaped core wall and columns to generate a framework system. To evaluate the performance level of the building nonlinear static and dynamic analyses are implemented on the structure. Before starting the analyses plastic hinges for every each column and beam frame elements are defined according to their specific sections and then the hinges assigned to related members thus nonlinear behaviour of the frame elements are represented by the plastic hinges in the both ends of all the frames. Unlike the columns and beams, shear walls designed in two different ways with specific nonlinearity options for every each of them to observe the behaviours better. One of this modelling technique what is called in this study is “equivalent frame shear wall” which use fiber hinge method in the bottom ends of all the shear walls in the building and the other is “multi-layer shell shear wall” which use spread nonlinearity through the shear walls from bottom to top of the height of building. The main reason for using the different modelling techniques for shear walls is their different geometrical shapes if compared with the other load carrying members of a structure. Shear walls are the major structural members to resist lateral forces especially in high-rise buildings. However, the nonlinear behaviours of them could not be solved enough, yet. Therefore, different apporaches to simulate nonlinearity of these complicated elements conducted in this study as mentioned above to capture more accurate results. To give a summary information about these two methods by one sentence as explanation, finite number of the fibers represent discretisation of the different materials along the cross section allow to capture accurate responses on the structural elements and the other approach for representing the behaviour of RC shear walls which use multi-layer shell element model is based on the composite material mechanics. TSC’07 includes three types of nonlinear analyses which are single-mode considered static pushover analysis, multi-mode considered static pushover analysis and dynamic effects considered time history analysis. Among all of these methods single-mode considered static pushover analysis and time history analysis are choosen to exhibit the nonlinear behaviour and performance level of the building. Structural system has symmetry just on a single direction in the plan thus different analyses runned to observe the effects of structural members in the building for both of the methods through related directions. In literature, dynamic analyses are accepted as the most extensive methods between all of the others. On the other hand, an undesirable side of that method is it’s too much time consuming and demanding to choose one between different assumptions of the theories of structural dynamics what makes decisions to be taken more carefully. It can be also said that this method requires a higher level of technical knowledge when compared with static pushover analysis. However, static pushover analysis method allows to run the analyses in a considerable less amount of the solving time if compared by the dynamic analysis methods. The main disadvantages of this method can be said as being less reliable due to considering only a single free vibration mode of a building and some restrictions to gain more trustable results what makes the method to not be useful for the most of cases in the majority of buildings on the earth. Consequently, these two different approaches to find out the nonlinear behaviour of the building in this study compared between each other. The results checked carefully and the performance levels are determined for every each different method and loading case. Performance of the building is tried to checked under two main titles which are “structural system based results” and “structural element based results”. The structural system based results are less trustable and they give less informations about buildings’ earthquake resistance features if compared with the element based results. Besides, they can not be used to determine the performance level of any building if nonlinear elastic methods are used according to TSC’07. However, they are very useful to observe general building behaviour thus some main problems or some inadequacies of a building can be easily seen. The structural system based results of the studied building in the thesis are investigated under four subtitles which are “top story displacement of the building”, “base shear force of the building”, “story drift ratios of the building” and “story shear forces of the building”. The results show that pushover analyses estimate bigger quantities compared to time history analyses about the building’s top story displacement and story drift ratios. In contradiction to the previous case the time history results are higher than the pushover analyses results about the base shear forces of the building. Although not yet certain, it is observed that the two different analyses methods’ story shear forces results match on the top floors apparently. On the other hand, the pushover results are more dominant on the middle floors and the time history results are more dominant on the bottom floors. The element based results which are the main parameters to obtain a building’s earthquake performance according to TSC’07 are investigated for every each structural member carefully. The results are checked for the beam, column and shear wall members. Nonlinear behaviours of the beams and columns are checked on their both frame ends. The nonlinear behaviours for the beams are defined by bending plastic hinges. The definition of the columns’ nonlinear behaviours are made different than the beams since the columns are under axial normal force effects. Due to their different cases PMM plastic hinges which can simulate the mutual effects of the axial force and the moments from different directions are assigned to every each type of the column sections. Differently from the beams and the columns, behaviours of the core shear walls which are the main lateral load carriers are observed under both the shear forces and the bending moments effects. After all, these results show that main parameter for the performance level is the beam members. With a few exceptions the columns are in the “Minimum Damage” or “Apparent Damage” levels according to TSC’07. The shear walls estimate even better results as “Minimum Damage” except two shear wall members located at the first two storey levels under one of the loading direction of pushover analysis which shows for them “Apparent Damage” level. On the other side, the most of the beams demand too high rotations which make the behaviour of them to pass beyond elastic borders of their capacities. This issue can be seen in the both analysis methods besides that behaviours of the beams drag the building into undesired and dangerous performance levels as appeared for static pushover analyses “Life Safety” and for dynamic time history analyses “Collapse” in their worst cases respectively. All in all, much more studies should be done to investigate the building performance levels on similar structures and compared with each other to capture matching results if they exist thus healthier and more reliable comments can be given.