Kısa alın levhalı kiriş-kolon birleşimlerinin çevrimsel tekrarlı yükler altında davranışlarının incelenmesi ve deprem etkileri altında yapısal davranışa katkılarının araştırılması
The study of the behavior of header end-plate beam-to-column connections under cyclic loads and the investigation of their contributions to structural behavior under seismic effects
Yazar: ADEM KARASU
Danışman: DOÇ. DR. CÜNEYT VATANSEVER
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Yapı Mühendisliği Bilim Dalı
Derece: Doktora, 2021
Konu: İnşaat Mühendisliği = Civil Engineering

Özet:
Bu çalışmanın ilk kısmı, eğilme momenti aktarmadığı varsayılan ve başlangıç rijitliği ihmal edilen kısa alın levhalı kiriş-kolon birleşimlerinin doğrusal olmayan davranışının araştırılmasına yönelik deneysel çalışma sonuçlarını içermektedir. Rijitlik ve dayanım bakımından bu tür birleşimlerin çevrimsel davranışlarının anlaşılabilmesi amacı ile 16 adet numune tekrarlı yükler altında test edilmiştir. Çalışma kapsamında test edilen numunelerin sonlu eleman modelleri hazırlanarak deneysel sonuçlar ile uyumluluğu gösterilmiştir. Alın levhası kalınlığı, alın levhası yüksekliği ve bulon sıra sayısının birleşimin doğrusal olmayan davranışına olan etkisi deneysel çalışma ve sonlu eleman analizleri yardımı ile araştırılmıştır. Deneysel çalışma ile moment-dönme ilişkileri elde edilen birleşimlerin, başlangıç rijitlikleri, eğilme momenti kapasiteleri ve dönme kapasiteleri belirlenmiştir. Deneysel ve sonlu eleman analiz sonuçlarına göre, alın levhası kalınlığı ve alın levhası yüksekliğinin arttırılmasının birleşimin eğilme momenti dayanımını arttırdığı gözlemlenirken, bulon sıra sayısının birleşimin doğrusal olmayan davranışına etkisinin önemli bir düzeyde olmadığı görülmüştür. Yapılan çalışmalar sonucunda bu tür birleşimlerin başlangıç rijitliğinin belirlenmesinde, kiriş gövdesinde oluşan çekme gerilmeleri belirleyici olmaktadır. Ayrıca birleşimlerin alın levhası yüksekliğinin arttırılması ile birlikte başlangıç rijitliklerinin arttığı gözlemlenmiştir. Alın levhası kalınlığının veya kullanılan bulon sıra sayısının arttırılmasının birleşimlerin başlangıç rijitliğine önemli bir etkisi olmamıştır. Yapılan çalışmaya göre, ince alın levhası kullanılması durumunda birleşimin eğilme momenti kapasitesinde alın levhasının eğilme momenti kapasitesi belirleyici olmaktadır. Bu nedenle bu tür birleşimler için akma çizgisi parametresi ve eğilme momenti kapasitesi, deney sonuçları ve sonlu eleman analizlerine göre elde edilen akma çizgileri üzerinden virtüel iş teoremine göre oluşturulan denklemler yardımı ile hesaplanmıştır. Ayrıca, bileşen yöntemi kullanılarak kısa alın levhalı birleşimlerin, başlangıç rijitliğinin hesabı gösterilmiştir. Deneysel çalışmalar ile birlikte bu tür birleşimlerin uygun bir şekilde tasarlanması durumunda, Avrupa Standartlarına göre yarı-rijit birleşim olarak sınıflandırılabileceği gösterilmiştir.
Çalışmanın ikinci kısmında, deneysel çalışma ile elde edilen birleşim davranış modeli yapı sistemine entegre edilerek, bu tür birleşimlerin yapı davranışına olan katkıları araştırılmıştır. Bu amaç doğrultusunda, Türkiye'de tasarımı planlanan, taşıyıcı sistemi sadece düşey yük taşımak üzere tasarlanan çerçeve sistemi ile bina çevresinde teşkil edilen süneklik düzeyi yüksek moment aktaran çerçevelerden oluşan örnek çelik binalarda, sadece düşey yük taşıyan çerçeve sistemlerinin yapının göçme kapasitesine olan etkisi araştırılmıştır. Sadece düşey yükleri taşıyan çerçeve sisteminin kiriş-kolon birleşimleri kısa alın levhalı bulonlu bağlantılar olarak tasarlanmıştır. Bu amaçla, yürürlükteki yönetmelik ve standartlar esas alınarak, 4- ve 8-katlı tipik çelik binaların tasarımı yapılmıştır. Kısa alın levhalı kiriş-kolon birleşimlerinin moment-dönme davranışı, deneysel sonuçlar esas alınarak kalibre edilen analitik model kullanılarak temsil edilmiştir. 4- ve 8-katlı bina taşıyıcı sistemlerinin göçme performansının değerlendirilmesi ve göçme anına kadar yapı davranışının izlenmesi amacı ile, sadece düşey yük taşıyan çerçevelerin dikkate alındığı ve alınmadığı iki boyutlu analitik modellerin, doğrusal olmayan statik itme analizleri ve zaman tanım alanında doğrusal olmayan dinamik analizleri gerçekleştirilmiştir. Sadece düşey yük taşıyan çerçeve sistemlerin yapıya sağladığı avantajlar ve modeller arasındaki yapısal davranış farklılıkları araştırılmıştır. Yapının zaman tanım alanında doğrusal olmayan dinamik analizlerinde, doğrusal olmayan statik itme analiz sonuçlarına göre, yanal yük taşıma kapasitesinde artış, birinci kat göreli kat ötelemesi değerlerinde azalma görülmüştür. Yapılan analizler sonucunda, kısa alın levhalı birleşimlerin kullanıldığı sadece düşey yük taşıyan çerçevelerin yapıya ilave rijitlik ve dayanım kazandırdığı ve şiddetli deprem etkileri altında yapının göreli kat öteleme değerlerini ve göçme olasılığını önemli ölçüde azalttığı sonucuna ulaşılmıştır.

Abstract:
After the 1994 Northridge earthquake, bolted end-plate connections have seen a rise in popularity as engineers seek alternatives to the welded connections. The use of bolted end-plate connections has become popular due to ease of fabrication, erection and proper seismic behavior. Header end-plate connection is a kind of bolted end-plate connections whose length is less than the depth of the beam. Moment-rotation characteristics of these type of steel connections are indicative of the connection's stiffness, strength and ductility. 
Most of the experimental studies were conducted on monotonic loading on header end-plate connections. The lack of cyclic test results of header end-plate connections motivated the study presented in the thesis. That's why, the hysteretic response of this type of connection must be investigated to better understand the behavior of structural system under seismic loads when these connections are used as a beam-to-column connections. Therefore, we experimentally investigated nonlinear behavior of bolted header end-plate connections under cyclic loading in this study.
The behavior of the bolted header end-plate steel connections is entirely dependent upon and highly sensitive to the connection's geometric variables. It is essential that the influence of these parameters on the performance of header end-plate connections must be investigated to make proper assessment. The first part of the study presents the results from experimental study on the actual behavior of header end-plate connections whose flexural moment capacity and initial rigidity are neglected during the desing procedures. To better understand the hysteretic behavior of these connections in terms of the stiffness and the strength, sixteen specimens were considered and subjected to cyclic loads. The moment-rotation relations of the connections governed by three parameters such as, initial stiffness, moment capacity and rotation capacity were obtained. Results revealed that the moment capacity increases with the increase in end-plate thickness and depth of the connection. However, for the equal connection depth, increasing the number of bolt rows has not influenced the connection behavior remarkably. Also results show that beam web strains due to tensile forces control the initial stiffness of the header end-plate connection. Increasing the end-plate thickness and number of bolt rows has not influenced the initial stiffness of the connection substantially.  
In order to identify the effect of different parameters on the behavior of connection and to understand important local effects which are difficult to measure with sufficient accuracy, 3-D FE models which account for both geometrical and material nonlinearities are developed. Furthermore, the flexural strength of the connections with thin end-plate was determined by using yield line analysis which estimates the yield moment of the end-plate. For this purpose, based on the experimental and the FE analyses results, yield line patterns were determined and in order to obtain the flexural strength, the equality of the work done by the internal forces to that done by the external forces is generated. In addition, initial stiffness of the end-plate connections was estimated based on the simulation of the connection by using a set of rigid or flexible components. Tests on the header end-plate connections revealed that the behavior of the specimens with the end-plate thickness of t=15 mm and t=20 mm has satisfied requirements stipulated by the european standards for partial strength and semi-rigid connections. However, the connection with 20 mm thick end-plate exhibited insufficient behavior in terms of ductility and rotation capacity. 
Based on the test and finite element analysis results, the behavior of header end-plate connections is controlled by two limit states such as end-plate tearing and rupture of the beam web. According to this study, thin plate behavior should be adopted to ensure that the connections have enough rotation capacity under cyclic loading when more energy dissipation is intended during the earthquakes. Therefore, a limit for header end-plate thickness, called critical thickness, was defined in the study to distinguish between thick and thin end-plate behavior.
Before the Northridge earthquake, welded moment resisting frames had been considered to be the most ideal structural system. Following the Northridge Earthquake a lot of steel moment resisting frames experienced beam-to-column fractures. The damage observed after the earthquake ranged from minor cracking to completely severed members. However, structural collapse was not observed after the Northridge earthquake. The most likely reason that the buildings remained standing is that the gravity framing acted as a "backup system" preventing structural collapse. The damage observed after the Northridge Earthquake exposed weakness of design procedures and led to the development of performance-based guidelines. 
Secondary systems such as the gravity framing system are typically ignored in seismic performance assessment of structures. The simple connections such as header end-plate connections that may be used in gravity frames are assumed to provide minimal lateral resistance to the structure. However, quasi-static experimental tests show that header end-plate connections have some inherent flexural capacity reaching up to 34% of the plastic flexural capacity Mp, of the gravity beam with increasing strength and stiffness at large displacements. Thus, it may be possible to provide a contribution to the energy dissipated during the earthquake, if gravity frames with these types of connections are used combining with the special moment frames. 
In most of the studies conducted, beam-to-column connections of the interior gravity frames are typically designed with shear tab connections. The motivation for the study presented here was the lack of studies where the header end-plate beam-to-column connections are used as the connections of gravity frames. 
The second part of the study presents the influence of gravity framing on the collapse risk of steel frame buildings with perimeter special moment frames (SMFs) designed in Turkey. For this, four- and eight-story buildings have been designed considering the related current codes and standards in Turkey. Header end-plate connections were used for the beam-to-column joints of the gravity frame system. A nonlinear analytical model that simulates the hysteretic behavior of header end-plate connections was calibrated with experimental test results obtained during the study. The lateral force resisting system of the buildings consists of special moment frames with reduced beam section in both principle directions.
For each building, two different models have been developed. The first model ignores the gravity framing by assuming that the beam-to-column connections in interior gravity frame are acting as pinned as assumed in practical engineering and all the gravity columns have zero rotational stiffness (named BF model). The second model considers the gravity framing (named GF model). All the models ignore the presence of composite slab. A concentrated plasticity approach was used herein to simulate the inelastic behavior of lateral force resisting system of the buildings. A fictitious single-bay frame is attached to the main SMF using axially rigid trusses to account for the gravity framing as part of 2D analytical model representations of the same steel building in its East-West loading direction. Geometric nonlinearities are captured using the simplified P-Delta formulation in the analytical models. Doubler plates were inserted at the panel zones of the interior SMF columns to satisfy the shear requirement. Steel SMFs were designed with strong column/weak beam ratios larger than 1.0. Rayleigh damping was incorporated in the analytical models. The stiffness-proportional term was assigned to elastic beam and column elements while the mass-proportional term was assigned to all the frame nodes. Two percent Rayleigh damping ratio was assigned at the first and third mode of all buildings.
The nonlinear static pushover analysis (NSPA) has much value in understanding significant characteristics that are not being explored in a nonlinear response history analysis (NRHA). In a pushover analysis, engineers generally focus on the demand/capacity ratio rather than visualization of the building behavior. Therefore, it is proposed to employ a combination of NSPA and NRHAs to understand the seismic performance of steel buildings with SMFs. Hence, nonlinear static pushover analysis (NSPA) and nonlinear response history analyses (NRHAs) were implemented for both four-story and eight-story steel buildings with and without the gravity framing to quantify their collapse performance and monitor the system-level seismic response of the building through collapse. The advantage of presence of the gravity framing is investigated and differences in structural responses between the models are also examined. 2D analytical model simulations were successfully conducted. The following conclusions were drawn from this study.
The existing rules in TSCB are found to be very strict for the control of story drift ratios during the design process, leading to overly rigid buildings. To mitigate the requirements of inter-story drift ratio, these rules should be updated. The period of vibration of the analysed structures decreases when the gravity frame is considered as a part of the structural system. However, since the steel buildings designed in accordance with TSCB are very rigid, the decrease in the vibration period is low.
When the models were excited by different ground motions, median responses of the detailed models showed an increase in lateral force carrying capacity and a decrease in first-story drift demand, compared to the nonlinear static pushover analyses results. In addition, steel frame buildings with SMFs achieve a static over-strength factor larger than code-specified over-strength factor (D=3) regardless of the considering gravity framing.
Dynamic shaking effects caused the engineering demands to change from the static case. As a result of the higher mode effects, the taller the buildings the larger dynamic amplification of story shear forces of the building. Therefore, it is clear that quantification of demand parameters from NSPA only is questionable for structures that have considerable higher mode effects.
Gravity frames can contribute to the mitigation of drift demands by adding lateral stiffness and strength. When considering gravity framing, as a result of the pushover (PO) analyses, the base shear strength was obtained 18.5% and 10.8% larger than that of the bare frame only for the four-story and eight-story buildings respectively. As a result of the incremental dynamic analyses, the median collapse intensity of the buildings was average increased about of 27% compared with bare BF models for both of buildings. 
Models that include the interior gravity frames had smaller drift demands and greater global drift capacity of the buildings than those without. Bottom story collapse mechanism that is not a desirable scenario for ductile design, is triggered when the gravity framing is not regarded as a part of the building during the nonlinear dynamic analyses. The gravity framing system appears to be effective at providing reserve strength and stiffness to the system which helps to prevent or delay collapse. The probability of collapse decreased significantly when the gravity framing is included in the analysis. Incorporation of the gravity framing improved the collapse capacity based on the FEMA P-695 methodology.