Burkulması önlenmiş çapraz sistemli çerçevelerin doğrusal olmayan davranışlarının incelenmesi
Investigation of nonlinear behaviors of steel frame systems with buckling – restrainted braces
Yazar: RAMAZAN KARAGÖZ
Danışman: DOÇ. DR. KADİR ÖZAKGÜL
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Ana Bilim Dalı / Yapı Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: İnşaat Mühendisliği = Civil Engineering

Özet:
Bu tez çalışmasında, yaklaşık 40 yıl önce literatürlere girmeye başlayan, dünyada görece olarak yeni sayılabilecek; ülkemize de geçen yıl yönetmeliğimize giren burkulması önlenmiş çaprazlı (BÖÇ) çelik çerçeveli yapıların tasarımı ve doğrusal olmayan analizi yapılarak yapı davranışına etkileri incelenmiştir. Ayrıca Amerikan yönetmelikleri ile yönetmeliklerimizde bu çapraz tipinin tasarımı için oluşan farklılıklar ortaya koyulmuştur. Bu çalışma anlatımı için 6 bölüm oluşturulmuştur.
İlk bölümde, çalışmanın amacından bahsedildikten sonra burkulması önlenmiş çelik çaprazlı çerçevelerin kullanım ihtiyacının oluşumu; keşfi, gelişimi ve yönetmeliklere girişi ile ilgili önceki çalışmalardan genel olarak bahsedilmiştir. Çalışmanın kapsamı da yazılarak bu bölüm tamamlanmıştır.
İkinci bölümde, burkulması önlenmiş çaprazların üretimi ve bileşenleri ile ilgili bilgilere ilave olarak diğer çaprazlı sistemlere göre avantajlarına da değinilmiştir. Burkulması önlenmiş çaprazlı çelik çerçeveli yapı olarak tasarlanılacak sistem için yönetmeliğimiz TÜRKİYE BİNA DEPREM YÖNETMELİĞİ (TBDY) 2018'deki tasarım kriterleri ve hesap esaslarından bahsedilerek modellemeye dair bilgiler verilmiştir.
Üçüncü bölümde, TBDY 2018'de anlatılan iki farklı tasarım yöntemi, Dayanıma Göre Tasarım (DGT) ve Şekildeğiştirmeye Göre Tasarım (ŞDGT)'in hesap esasları anlatılmıştır. Bu esasların referans aldığı analiz yöntemleri, doğrusal ve doğrusal olmayan analizle ilgili bilgiler verilmiştir. Ayrıca Amerikan yönetmelikleri IBC 2000 ve FEMA 450'deki tasarım aşamalarından da bahsedilmiştir. Son olarak da doğrusal olmayan analiz yönteminin bir aşaması olan statik itme analizi sonucuyla elde edilebilecek parametreler gösterilmiştir.
Dördüncü bölümde ise Lehigh Üniversitesi ATLSS labaratuvarında deneysel ve nümerik olarak çalışılmış olan 4 katlı burkulması önlenmiş çapraz çerçeveli çelik yapının SAP 2000 ile hem Amerikan hem de kendi yönetmeliklerimiz göz önünde tutularak ayrı ayrı modellemeleri yapılmıştır.
Beşinci bölümde, doğrulaması yapılan 4 katlı yapının 8 ve 12 katlı olarak tasarımı yapılarak doğrusal olmayan davranışları incelendikten sonra tüm yapıların burkulması önlenmiş çaprazları, burkulması önlenmemiş yani tipik merkezi çapraz olarak değiştirilmesi sonucu çıkan farklar ortaya koyulmuştur.
Son bölümde ise tüm bu incelemeler ışığında yapı ile ilgili davranış belirtileri olan yapı taşıyıcı davranış katsayıları, dayanım fazlalığı katsayıları, göreli kat ötelemeleri ve tepe deplasman değerinin taban kesme kuvveti ile ilişkileri değerlendirilmiştir. Bu değerlendirmeler baz alınarak önerilerde bulunulmuştur.

Abstract:
During vigorous earthquake, steel moment – resisting frames are susceptible to large lateral displacements. Intrinsically, real attention is needed to limit interstory displacements throughout design process so that possible troubles stem from brittle or ductile fracture of beam – to – column connections and geometric nonlinearities can be reduced hereby heavy damage to nonstructural elements is abstained. In response to associated with many practical and financial subjects, as a lateral load resisting system for structures, engineers are increasingly turning to the use of concentrically braced steel frames. However, a lot of damages on concentrically brace frames in past earthquakes has increased anxieties about the ultimate deformation capability of this type of structure.
A few reasons for inadequate performance in braced frames have been put forward. For instance, unique braces frequently have just limited ductility or energy dissipation capacity during cycling loading and many typical connection details are intend to brittle behavior.
Because concentrically braced frame (CBF) systems have restricted ductility and energy dissipation capacity, many research efforts have gone towards improvement new CBF systems with steady hysteric behavior, remarkable ductility and huge energy dissipation capacity. Eventually, A CBF system with developed sismic behavior called buckling - restrainted braced frames has invented.
Buckling - restrainted braces have two basic parts: a steel core element which bear the whole brace axial load, and a restraining exterior part that forestalls the core from buckling in compression and allows it to yield in both tension and compression.
Typical buckling – restrainted brace configuration is confined within a concrete filled steel tube (CFT). The steel core is debonded from the confining concrete so that the core is able to carry the axial load independent of the concrete filled tube. The core is got thin at the middle to compose a particular zone of contained yielding.
Japanese people has discovered the BRB consept over 40 years ago and in the first 15 years, BRBs have been utilized in approximately 200 Japanese structures. However, BRB concept has not mentioned in the Turkish codes until last year. The new Turkish sismic code TBDY 2018 has rules about the buckling – restrained braces like design criterias, rotation limits etc.
In this thesis, for the investigation of nonlinear behaviors of steel frame systems with buckling – restrainted braces, an analytical study was carried out. The entire procees was explained below.
In the first part, after mentioning the purpose of the study, the need to use steel framed structures with buckling restrainted braces (BRB) were examined; previous studies on invention and development of buckling restrainted - braces, introduction of codes. Finally, scope of research is given.
In the second chapter, in addition to the information about the production and components of buckling restrainted braces, their advantages compared to other with braced systems were also mentioned. On the other hand, a few disadvantages of BRBs were explained. Some structures with BRBs were illustrated. Then, section types of BRBs was displayed. After displaying of connection details of buckling restrainted brace with beam and column, for modeling and analysis informations of buckling restrainted braces were indicated and for the design criterias of buckling – restrainted braced frames related codes were mentioned.
In the third chapter, two different design methods described in TBDY 2018, Design depend on Resistance and Design depend on to Strain were explained. The general informations were given about methods; linear and nonlinear analysis. Moreover, the design stages of American Codes, IBC 2000 and FEMA 450 were mentioned. Finally, the parameters that can be obtained with the static pushover analysis results, which is a step of the nonlinear analysis method, were shown.
In the fourth chapter, firstly, a study performed in Lehigh University on a four story building with buckling restrainted brace was modeled with SAP2000 educational version that is computer software was used for both steel structure's design and nonlinear analysis. SAP 2000 is a structural analysis program that has developed analytical techniques. After a four story building modeling and designing according to study of Lehigh University, this building was designed depend on Turkish Sismic Code (TBDY) 2018. Then, IBC 2000 and TBDY 2018 which means sismic codes are compared related to some parameters as story weights, story shears, load combinations and elements' sections. Finally, whole buildings with 4, 8 and 12 storey respectively, was modelled and designed based on TBDY 2018 and sections of elements of these buildings are indicated with tables. Respect to these sections, different views of entire buildings are represented.
In the fifth chapter, primarily, nonlinear static pushover analysis of 2D frame model with 4 storey was carried out and verified with results of the study performed in Lehigh University. Differences of these curves was ıgnorable. After non-linear modeling accuracy was comfirmed, nonlinear static pushover analysis of the 2D frames with 4, 8 and 12 storey was performed in order to understand non-linear behavior of higher frames according to Turkish Sismic Code. These buildings' 2D frames pushover curves was matched in order to evaluate local ductility. Besides, story displacements at a roof drift of 0.03 rad for all 2D frames were displayed, respectively. Moreover, story drifts at a roof drift 0.03 rad are monitorized in order to obtain brace maximum ductility demand. Then, buckling – restrainted brace forces versus their normalized axial deformations in order to maximum ductility demands were plotted. Also, based on static pushover analysis results of each 2D frame R system modification factor value was obtained. As the system overstrength factor, Ω for equivalent lateral force method used in seismic design is an important design parameter, this parameter was also calculated for each 2D frame. Eventually, buckling – restrainted and buckling - unrestrainted braced buildings with 12 storey were compared for elastic analysis.
In the last section, the obtained analysis results and evaluations were briefly summarized and suggestions based on these results were proposed.