Sismik Yalıtım Kavramı Ve Taban Yalıtımı Uygulanmış Betonarme Bir Yapının Zaman Tanım Alanında Analizi

Abstract

Ülkemizin karşı karşıya bulunduğu deprem tehlikesi, bu konuyla ilgili herkesin bilgisi dâhilindedir ve alınması gereken önlemler gerek akademik çalışmalarda gerekse görsel ve işitsel medyada sıklıkla gündeme getirilmektedir. Geçtiğimiz yüzyıllar boyunca depreme dayanıklı tasarım İnşaat Mühendisliğinin en önemli uğraş alanlarından biri olarak ortaya çıkmış, bilim insanları depreme dayanıklı tasarım için gelenekselleşmiş birçok kural ve yöntem ortaya koymuştur. Bu yöntemler arasında nispeten daha yenilikçi ve teknolojik bir yaklaşım olan sismik yalıtım kavramı alışılanın aksine, depreme karşı daha büyük, dolayısıyla daha dirençli kesitler önermek yerine, yapıya etki edecek deprem yüklerini azaltma yoluna giderek, mevcut riski minimalize etmeyi hedeflemektedir. Yer hareketlerine karşı oldukça basit bir yaklaşımla, yapıyı yerden ayırma fikrine dayalı olarak ortaya çıkan sismik yalıtım kavramının günümüz modern sistemlerine uyarlanması noktasına gelindiğinde, konu karmaşık bir hâl almış, farklı davranış hedeflerine uygun çeşitli sistemler geliştirilmiş, uygulama prosedürleri yönetmeliklere konu olmuştur. Bu çalışmada öncelikle sismik yalıtımın amacı ve ana ilkeleri özetlenmiştir. Sismik yalıtımın binalara kazandırdığı başlıca özellik olan periyot artırımı yoluyla deprem kuvvetlerinin azaltılması ve sönümün artırılmasıyla depremin istenmeyen etkilerinden kaçınılmasına değinilmiştir. Taban yalıtımı kavramının ortaya çıktığı 1800’lü yılların ikinci yarısından günümüze yapılan çalışmalar kısaca özetlenmiş, sismik yalıtım uygulamalarının günümüzdeki popülerliğine ulaşırken izlediği yol ortaya koyulmuş ve konuya emek vermiş değerli araştırmacıların çalışmalarına atıfta bulunulmuştur. İlerleyen bölümlerde, günümüzde kullanılan modern yalıtıcı sistemler avantaj ve dezavantajlarıyla tanıtılmış, uygulama ilkelerine değinilmiştir. Yapı sistemlerinin depreme dayanıklı tasarımı, uygulayıcıların karşısına bir takım zorlayıcı koşullarla çıkmaktadır. Bu sebeple uygulamaların yaygınlaşmasında, denetiminde ve kontrol edilebilirliğinde yönetmeliğe bağlılık esastır. Deprem tehlikesiyle yüz yüze olan birçok gelişmiş ülkede sismik yalıtım yönetmeliklere bağlı olarak uygulanmaktadır. Tez çalışması kapsamında bu yönetmeliklerden bahsedilmiş, Türkiye’de taslak olarak hazırlanmış, ancak halen resmiyet kazanamamış Deprem Yalıtımı Yönetmeliğinin ana ilkeleri özetlenmiştir. Dünyada ise yaygın olarak kullanılan Uniform Building Code yönetmeliğinin sismik yalıtıcı sistem tasarımı aşamaları açıklanarak, betonarme bir yapı için yönetmelik ilkelerine bağlı kalarak, kurşun çekirdekli kauçuk bir yalıtıcı mesnetin detaylandırılması gerçekleştirilmiş, tezi inceleyen ve konuyla ilgilenen araştırmacıların çalışmayı yalıtıcı mesnet tasarımıyla ilgili bir örnek olarak arşivlerinde bulundurmaları temenni edilmiştir.Sismik yalıtım uygulamalarının, yapı performansına olumlu katkısını savunan tez çalışmasında, bu iddiayı savunmak adına, örnek bir dört katlı betonarme yapı bilgisayar ortamında, Computers and Structures firmasının ürettiği ETABS yazılımı kullanılarak, hem deprem etkilerinden yalıtılmış haliyle hem de ankastre mesnetli haliyle modellenmiş ve yapı 1999 Marmara Depreminde İzmit Meteoroloji İstasyonu tarafından kaydedilen ivme kayıtlarının uygulanması ile zaman tanım alanında analiz edilmiştir. Sonuçlar incelendiğinde, sismik yalıtımın ana ilke ve hedeflerine ulaşıldığı, gerek sayısal verilerin sunulduğu çizelgelerle gerekse grafikler yardımıyla sunulmuştur.

Seismic risk of Turkey is known by everyone who is related to this subject. In this sense civil engineers have to take preventions about attenuation of seismic risks on buildings. Civil engineers developed different techniques and products over the years on this subject. This master of science thesis, explains base isolation technique which is an innovator method for prevent buildings from earthquake motions. In first chapter, seismic base isolation concept is explained from the view of purpose, method , durability and main principles of isolation procedure. Isolation word in the term of base isolation, corresponds that the meaning of the state of being seperated, this seperation represents the seperation between structural system. In another sense, the term seismic isolation is more accurate in that the structure seperated from the effects of the earthquake. Conventional seismic design procedures holds on the capacity demand relationship concept. Earthquakes are uncontrollable, so in that sense planner has to accept demand and make sure that the capacity exceeds it. Studies of seismic base isolation seeking for answers to how engineers can create proper links between structures and foundations to control seismic efects of buildings. Inertia forces due to the earthquake proportional to building mass and the earthquake ground motions. Capacity of structure must be increase proportional to increase of ground accelerations but it is not pratical to increase strenght of building indefinitely. Capacity based design philosopy reveals two handicapped statements. First approximation is increase of elastic strength but it needs very large and expensive sections and creates higher floor accelerations, that kind of strengthening may cause unlooked structural damages, another approximation is using ductility and limiting the elasting strenght, most of building codes allow using ductility to achieve capacity but ductility means that afford proper damages on structural elements.Base isolation is a different technique which attempts to reduce the demand rather than increase the capacity. Earthquakes are uncontrallable but demand can be modified by application of seismic base isolation techniques. The most important principle to control demand is period shifting effect of seismically isolated buildings. The effectiveness of base isolation in reducing structural forces is tied to the lenghtening of the natural period of the structure and fort this purpose the period ratio should be as large as practical. Its known that in higher period levels buildings exposed to lower earthquake accelerations, decreasing in accelereation also decreases the earthquake forces adopted the buildings. In second chapter, historical background of seismic isolation explained briefly, primer ideas about isolating interpreted with respect to researchers studies about topic and after that seismically isolated structures presented from near history. Some of the examples on seismic base isolation applications introduced briefly. These are,Seismic risk of Turkey is known by everyone who is related to this subject. In this sense civil engineers have to take preventions about attenuation of seismic risks on buildings. Civil engineers developed different techniques and products over the years on this subject. This master of science thesis, explains base isolation technique which is an innovator method for prevent buildings from earthquake motions. In first chapter, seismic base isolation concept is explained from the view of purpose, method , durability and main principles of isolation procedure. Isolation word in the term of base isolation, corresponds that the meaning of the state of being seperated, this seperation represents the seperation between structural system. In another sense, the term seismic isolation is more accurate in that the structure seperated from the effects of the earthquake. Conventional seismic design procedures holds on the capacity demand relationship concept. Earthquakes are uncontrollable, so in that sense planner has to accept demand and make sure that the capacity exceeds it. Studies of seismic base isolation seeking for answers to how engineers can create proper links between structures and foundations to control seismic efects of buildings. Inertia forces due to the earthquake proportional to building mass and the earthquake ground motions. Capacity of structure must be increase proportional to increase of ground accelerations but it is not pratical to increase strenght of building indefinitely. Capacity based design philosopy reveals two handicapped statements. First approximation is increase of elastic strength but it needs very large and expensive sections and creates higher floor accelerations, that kind of strengthening may cause unlooked structural damages, another approximation is using ductility and limiting the elasting strenght, most of building codes allow using ductility to achieve capacity but ductility means that afford proper damages on structural elements.Base isolation is a different technique which attempts to reduce the demand rather than increase the capacity. Earthquakes are uncontrallable but demand can be modified by application of seismic base isolation techniques. The most important principle to control demand is period shifting effect of seismically isolated buildings. The effectiveness of base isolation in reducing structural forces is tied to the lenghtening of the natural period of the structure and fort this purpose the period ratio should be as large as practical. Its known that in higher period levels buildings exposed to lower earthquake accelerations, decreasing in accelereation also decreases the earthquake forces adopted the buildings. In second chapter, historical background of seismic isolation explained briefly, primer ideas about isolating interpreted with respect to researchers studies about topic and after that seismically isolated structures presented from near history. Some of the examples on seismic base isolation applications introduced briefly. These are,Wiilliam Clayton Building in New Zealand, Foothill Community Justice and Law Center in United States of America, Tohoku Electric Center Building in Japan, South California University Hospital, Los Angeles City Hall and West Japan Postal Center also in Turkey there are many projects which are constructed by seismic isolation techniques. Turkey came across with base isolation concept after that 1999 Earthquake, first base isolation projects in Turkey represented with facts and photos about applications. Atatürk Airport, Kocaeli University Hospital, Tarabya Otel, Erzurum Hospital projects and etc. are reffered briefly in this study. Third chapter probes that the classification of seismic base isolation systems.. Researches are produced many kinds of base isolator elements over the years, isolator types categorized and introduced to the readers in the text, this classification is prepared under the elastomeric isolation systems and sliding isolation systems main titles. Despite wide variation in production, base isolation techniques have two fundamental approaches with usual features. In the first approach base isolation system introduces a layer of low lateral stiffness between the structure and the foundation. With this isolation layer the structure has a natural period that is much longer its fixed base natural period. This lengthening of period can reduce the pseudo- acceleration and hence the earthquake-induced force in the structure but deformation is increased, this is the deformation across the isolation system. The second type of isolation systems use rollers or sliders between the foundation and the base of structure. The shear force passed on to the structure accross the isolation interface is limited by the coefficient of friction as low as practical. The Dynamics of structures on roller or slider type of isolation systems is complicated because the slip process is nonlinear. Lead rubber bearings are a member of elastomeric isolaton systems and engineerig properties and design procedures were explained in this study. Lead rubber bearings produce a hysteresis curve under the lateral displacements, this curve is a combination of linear elastic force displacement relationship. Hysteresis curves of two different rubber isolators were drawn in the thesis and explained the meanings of the charecteristic parameters on the curve to the readers. These characteristic values will be used proggresive aspects in the modeling of structure system. In developed countries, seismic base isolation procedures are ruled with codes, these codes are explainig requirements for planning and implemantation of base isolation elements, fourth chapter of this thesis introduces building codes for seismic isolation from different countries and summarizes Uniform Building Code 97 which is a common and famous design code for base isolation. Other than UBC 97, Federal Emergency Management Agency , Structural Engineers Association of Northern California published diffferent codes but similar contexts with UBC 97 In Turkey, a scientific organisation named as Earthquake Isolation Association published a national code for base isolation but it is waiting for become official and useable in Turkey, this study also explains that Turkish Code Requirements. This study, takes an example of a four story reinforcement concrete building for present positive effects of seismic base isolation. Chapter 5 explains that design procedure of two different, elastomeric base isolators with respect to the UBC 97 requirements. Isolator type A is implemented under the corner columns of the building which bear small loads with respect to interior columns. For this reason modulus of shear is 0,5 MPA for isolator Type A and 0,7 MPa for Isolator Type B. Design of the isolator dimensions determined with respect to Uniform Building Code requirements. Displacement Strenght curves also known as hysteresis diagrams created for Isolator Type A and Type B, this curve contains that the parameters of Stiffness, Effective Stiffness, Yields Strenght and etc. That values are used for the identify isolators as link elements in ETABS software. In Chapter six, two different elastomeric bearings implented between structure and foundation on computer model with using ETABS structural analysis software. Both isolated and fixed models of structure analysed with nonlinear time history analysis method with using time-acceleration datas which were recorded from İzmit Meteorology Station by during the big Duzce Earthquake in 17 August 1999 . This record contains 5348 acceleration datas in both North-South direction and West-East direction. Total duration of record is 27,190 sec. Maximum acceleration value in North-South direction is 3,149m/sec2 at the time of 8,725 sec and in West-East direction 3,787 m/sec2 at the time of 9,135 sec. Ritz vectors method used for the dynamic anaylsis and forty mode shapes obtained after analysis to view dynamic behaviour of model. Acceleration datas applied to the building from East-West and North-South directions and structural results compared for fixed and isolated statements. Positive effects of period shifting was observed easily in the values of shear force and moment values on the structural elements such as columns and beams. In last chapter, conclusions and proposals about effectivity of seismic base isolation application discussed and determinations of study presented to the readers as a list. First and second period values, displacement of center of mass for each stories, relative displacements between stories, maximum value of column moments, story shears and modal participating mass ratios compared on tables for fixed and seismically isolated model of building. In addition to tables of structural result, researches can look over some results on graphical represents such as variation of total and relative story displacements or story shears with respect to time. This graphics are presented in the prefixes chapter. Graphical represent of relative displacements on stories are easily explain that the aim of seismic base isolation. On the fixed base model, stories are moving discretely but on the seismicaly isolated model graphic of relative displacement of stories continues almost overlapped. Consequently, this master of science thesis aimed that the represent positive dynamic effects of seismic base isolation technique on buildings for attenuating seismic risks on buildings. Results showed that internal forces on structural elements decreased dramatically and displacement of stories controlled as expectedly. Writer of this thesis hopes that this study will be a first step for his future researches.