Kiriş-kolon birleşim bölgelerinin ileri teknoloji malzemelerle güçlendirilmesi
Beam-column joints retrofitted with advanced technological materials
Yazar: SİNAN MURAT CANSUNAR
Danışman: PROF. DR. KADİR GÜLER
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / İnşaat Yapı Ana Bilim Dalı / Yapı Mühendisliği Bilim Dalı
Derece: Doktora, 2023
Konu: Deprem Mühendisliği = Earthquake Engineering ; İnşaat Mühendisliği = Civil Engineering

Özet:
Ülkemizin mevcut betonarme binalarında görülen en önemli sorunlarından bazıları, düşük beton basınç dayanımına sahip olması, donatıların yüksek karbonlu üretilmesi, kritik bölgelerde yeterli enine donatıların bulunmaması ve aderans koşullarının sağlanmamasıdır. Bu olumsuzluklardan dolayı binanın dayanım ve süneklilik kapasiteleri azalmakta, deprem etkisinde hasarlar oluşmakta ve binalarda can kaybına neden olan kısmi veya toptan göçmeler görülmektedir.
Tez çalışmasında, basınç dayanımı düşük ve basınç dayanımı normal betonarme çerçevelerin kiriş-kolon birleşimleri yerine, potansiyel plastik mafsalların meydana geleceği kiriş ve kolonların kiritik uç bölgeleri güçlendirilmiştir. Betonarme çerçeve sistemin kiriş ve kolon kritik uç bölgelerinin çimento esaslı harçla birlikte uygulanan karbon esaslı kompozitle sarılarak, yatay yük taşıma kapasitelerinin arttırılıp, daha sünek davranması, çerçevenin mukavemetinde önemli ölçüde azalma ve kararsız denge olmaksızın, deprem sırasında ortaya çıkan enerjinin büyük kısmını elastik sınırın ötesinde, elastik olmayan davranışla ve tersinir dönüşümlü büyük şekildeğiştirmelerle yutma yeteneği araştırılmıştır. Kiriş-kolon birleşimlerinin, günümüzde şiddetli depremler etkisinde doğrusal olmayan davranış için tasarlanan betonarme çerçevelerin kritik bölgeleri olduğu varsayılmaktadır. Birleşimin hemen üstündeki ve altındaki kolonlarda ve kirişlerde meydana gelen eğilme momentleri sonucu birleşimlerde, kiriş ve kolonlarda büyük yatay ve düşey kayma gerilmeleri meydana gelmektedir. Bu birleşim bölgeleri eğer doğru tasarlanmadıysa, büyük kesme hasarları kaçınılmaz olmaktadır. Bu çalışmada, yeni nesil karbon esaslı liflerin çimento esaslı harçla beraber, kiriş-kolon birleşim bölgelerinden olan kiriş ve kolonların kritik uç bölgelerinin sargı etkisinin katkısının deneysel çalışmalarla binanın kesit sünekliliğini, eleman sünekliliğini, sistem sünekliliğini, birbiri ile ilişkili ve etkileşimli olduğu araştırılmıştır. Bu güçlendirme tekniğinde, eleman bazlı uygulama ile, sistem bazlı sonuçlar elde edilmiştir. Bu tez çalışmasının verileri, deprem yönetmeliklerindeki "Kiriş-Kolon Birleşim Bölgelerinin Güçlendirilmesi" konularına katkıda bulunacağı öngörülmektedir.
 Beton dayanımı düşük ve enine donatı aralığı seyrek yalın (çıplak) çerçevelerde, kiriş ve kolon kritik uçlarının ankrajsız ve tam sargı ile sarılması, dayanımda, rijitlikte, enerji yutma kapasitesinde ve süneklilikte artışlar meydana getirmiştir. Kiriş ve kolon uçlarının güçlendirilmesiyle, birleşim panel bölgesinde etriye bulunmamasının olumsuzluğu belirli oranda ortadan kalkmaktadır. Bu güçlendirme tekniğinde sistemin yatay yük taşıma kapasitesi, rijitliği ve sünekliliği aynı anda artabilmiştir. Düşük beton basınç dayanımı (C15) olan numunelerin TRM ile güçlendirilmesi, davranış, süneklik ve enerji yutma artışı, beton basınç dayanımı normal numunelere göre (C26) daha etkindir. Düşük beton basınç dayanımlı C15 beton sınıfındaki numunelerde sargı kat sayısı arttıkça, numunenin dayanımı ve şekildeğiştirme kapasitesi(süneklilik) oldukça yükselmekte, sargı etkisi daha etkin çalışmaktadır. Gerilme-şekildeğiştirme grafiğinden (Şekil 2.52) de anlaşılacağı üzere, 3 kat ve 2 kat sargılı C15 li numunelerin referans numuneye göre, azalma kolunun eğiminin düştüğü ve basınç bölgesindeki betonun daha büyük basınç birim şekildeğiştirme düzeyine gitmesi sağlanmıştır. Gerilme-şekildeğiştirme grafiğinden (Şekil 2.53) de anlaşılacağı üzere, 3 kat, 2 kat ve 1 kat sargılı sargılı C26 lı numunelerin referans numuneye göre, azalma kolunun eğiminin aynı düzeyde gittiği görülmüştür. Bu sonuçları çerçeve deneyleri ile karşılaştırdığımızda, 3 kat sargıyla sarılan düşük beton basınç dayanımlı çerçeve, 3 kat sargılı normal beton basınç dayanımlı çerçeveye göre %15 daha fazla şekildeğiştirme kapasitesi(süneklilik) artışı göstermiştir. Sargı etkisinin düşük beton basınç dayanımlı çerçevelerde daha etkili çalıştığı sonucuna ulaşılmıştır.

Abstract:
The research investigates experimentally the effect of confinement on structural behavior at the ends of beam-column in reinforced concrete (RC) frames. In this dissertation, instead of beam-column joints of reinforced concrete frames with low concrete compressive strength and normal concrete compressive strength, the critical ends of beams and columns where potential plastic hinges will occur were strengthened. In the experimental study, nine specimens consisting of 1/3-scaled RC frames having single-bay, representing the traditional deficiencies of existing buildings constructed without receiving proper engineering service is investigated. The RC frame specimens were produced to represent most of the existing buildings in Turkey that have damage potential. To decrease the probable damage to the existing buildings exposed to earthquakes, the carbon Textile Reinforced Mortar (TRM) strengthening technique (fully wrapping with non-anchored) was used on the ends of the RC frame elements to increase the energy dissipation and deformation capacity. The specimens were tested with deformation controlled under reversed cyclic lateral loading with constant axial loads. They were constructed satisfying the weak column-strong beam condition and consisting of low-strength concrete and normal- strength concrete, such as compressive strength of 15 MPa and 26 MPa. The test results were compared and evaluated considering stiffness, strength, energy dissipation capacity, structural damping, ductility, and damage propagation in detail. Comprehensive investigations of these experimental results reveal that the strengthening of a brittle frame with fully-TRM wrapping with non-anchored was effective in increasing the stiffness, ductility, and energy dissipation capacities of RC bare frames. It was also observed that the frame-only retrofitting with an infill wall is not enough to increase the ductility capacity. In this case, both the frame and infill wall must be retrofitted with TRM composite to increase the stiffness, lateral load carrying, ductility, and energy dissipation capacities of RC frames. The presented strengthening method can be an alternative strengthening technique to enhance the seismic performance of existing or moderately damaged RC buildings. It is expected that the results of this thesis will contribute to the issues of "Strengthening Beam-Column Joint" in earthquake international codes. RC frames, with or without masonry infill walls having low compressive concrete strength were prepared and tested under lateral cycling loading until failure (drift ratio up to 3~5%). The ends of RC frames elements are strengthened by applying carbon TRM by non-anchored fully-wrapped effectiveness of this strengthening is determined by comparing the properties of the original frames and those of the strengthening ones. Experimental and numerical results given in tables and figures were discussed comparatively. The results show that the presented strengthening method which is external jacketing and fully wrapping with a non-anchored of frames display a promising alternative strengthening technique to enhance the seismic performance of existing buildings having RC frame systems.
RC frame specimens were produced at 1/3-scaled due to the laboratory capacity having brick infill walls. The particle diameter of concrete aggregate, the brick size of the infill wall, and the geometry of the frame members of all specimens were scaled, accordingly. The scaling is accomplished to obtain the actual structural behavior of the frame as much as possible, as pointed out by Gajanan et al. (1999), Noor et al. (1992). Nine different RC frame specimens (Specimen1-Specimen9) were produced and tested under cyclic lateral loading with constant axial loading. Five of the specimens Specimen1(1C15), Specimen2(2C15), Specimen3(5C26), Specimen4(6C26) and Specimen8(9C15) were used as the reference specimens for comparison with the other four retrofitted frames, Specimen5(8C26), Specimen6(3C15), Specimen7(4C15) and Specimen9(10C26). Since frame Specimen1 and Specimen3 have stirrups at the beam-column joints and the spacing at confined zones of the beam and column ends are small (50 mm), a ductile behavior is expected inherently. Details of the specimens are shown in Fig. 4.3, Fig. 4.4, Fig. 4.5 and Fig. 4.6. 
All RC frames have the same geometry including the dimensions as shown in Fig. 4.7 and Fig. 4.8. The height of the frame is 1000 mm, and the height and the span of the foundation are 400 mm and 1533 mm, respectively. The column and the beam cross sections are both 100 mm×200 mm, whereas the cross-section of the foundation beam is 400 mm×700 mm. A longitudinal reinforcement of 4Ø8 is used for the columns and the beam whereas, the reinforcement of the foundation is 12Ø12. For the brittle specimens, the spacing of the transverse reinforcement of the columns and the beam is 130 mm and 150 mm, respectively, and the concrete cover is considered 15 mm similar to the literature (Özkaynak 2010), whereas, the specimen, Specimen1 and Specimen3 have a stirrup spacing 50 mm at the confinement zones of frame elements to comply with the Turkish Building Earthquake Code (TBEC 2018). The dimensions and the reinforcement details of the retrofitted specimens Specimen5(8C26) Specimen6(3C15), Specimen7(4C15), and Specimen9(10C26) are shown in Fig. 4.5 and Fig. 4.6. The end sections of the beam and columns were fully wrapped with non-anchored 3-layer TRM composite. Overlapping of the last layer of 200 mm is provided. Strong adhesion was provided by using cement-based mortar to prevent the debonding. 
Before wrapping with TRM composite, the corners of rectangular sections of the beam and the columns were rounded to avoid stress concentrations and to ensure the efficiency of confinement (Azam and Soudki 2014, Dang et al. 2020). Otherwise, premature failure can occur. The length of the wrapping zone at the beam is 300 mm, whereas the length of the wrapping zone at the columns is 200 mm. A special mortar having a thickness of 5 mm was applied between the layers of the wrapping to ensure the integration between the layers and the frame. Thus, the total thickness of the wrapping reached 15 mm on each face of the column and beam ends.
The stages of the construction of the specimens are shown in Fig. 2.1, Fig. 2.2, Fig. 2.23, Fig. 2.24, Fig. 2.25 and Fig. 2.27. First of all, the frames of all specimens were produced by placing reinforcements and pouring concrete. Then, TRM composite wrapping was applied to the Specimen5(8C26), Specimen6(3C15), Specimen7(4C15) and Specimen9(10C26) to generate the retrofitted specimens.
RC frames were tested in the ITU Civil Engineering Faculty Structural and Earthquake Engineering Laboratory. During the tests, the loading protocol accepted by the international codes (ACI 549.4R-13, ACI 440.2R-17, and ASTM. 2016 a) for the RC frames was applied to achieve the desired critical limits of targets as shown in Fig. 4.30. 
The test setup is shown in Fig. 4.1 which consists of (i) a strong floor to provide fixed support where the test setup is connected with high tensile bolts and (ii) a reaction wall to apply the cyclic loads on the beam-column joint with the help of (iii) a servo-controlled, 280 kN-capacity hydraulic actuator. In the experimental setup two rollers are used to prevent out-of-plane displacements (Fig. 3.4 ). The specimens were tested according to the loading protocol given in Fig. 4.30 to simulate the earthquake effect. Loading was applied up to the failure of the specimens. The load-deformation diagram was recorded with the help of a load cell and the actuator. Two high-strength anchor rods with a diameter of Ø18 mm were passed through the holes in the head of the hydraulic transmitter and attached to the end of the specimen with a thick steel plate in order to perform repetitive loading (push and pull loadings). Pre-stressed 24 bolts with a diameter of 18 mm were used to provide the fixed anchorage conditions of the foundation under the lateral loads. The general crack patterns, crack mechanisms, and crack widths at each loading step were observed and the envelopes of force-displacement hysteresis obtained from the quasi-static (QS) tests under deformation control are noted with the numerical readings (Mosalam et al. 1998). The critical events, such as the first diagonal cracking in the infill wall, the first cracking in RC members, and the rebar buckling were observed and noted together with the loading displacements. 
CDP-5, CDP-10, CDP-25, and SDP-100 type linear variation displacement transducers (LVDT, Model TML) were used to measure the displacements at different sections of the test specimens. LVDTs were located to receive and record data in both push and pull loading cycles. The general instrumentation layout of the specimens is given in Fig. 3.2 and Fig. 3.3. A total number of 20 LVDTs were installed on each specimen to obtain the displacements and to measure the general lift, slip, and out-of-plane movements for control purposes. Additionally, some of the LVDTs (channels no: 4, 27, 29, and no 30 in Fig. 3.2 and Fig. 3.3) were installed for recording rotational or translational displacements of the foundation due to the rocking motion during the tests. They were mounted on ∅5 anchor rods placed on the specimen during the production stage. In addition, strains were also measured by strain gauges fixed on the longitudinal reinforcement at the ends of the beam and columns (Fig. 3.2 and Fig. 3.3).
The results of the experiments are discussed for each specimen in terms of stiffness, strength, energy dissipation capacity, and structural damping. Hysteretic behavior of specimens is obtained under reversible repetitive loads, acting throughout the 
experiment. Displacements and strains were measured by assuming the load cycles are positive in the push and negative in the pull loop.
A comparison of these figures reveals that the strength and the energy dissipation of Specimen1(1C15), Specimen3(5C26), Specimen6(3C15) and Specimen9(10C26) are higher than Specimen2(2C15) and Specimen4(6C26), because Specimen2(2C15) and Specimen4(6C26) displays brittle behavior due to the large spacing of the transverse reinforcement. On the other hand, Specimen1(1C15) and Specimen3(5C26) behaves ductile due to the adequate stirrup spacing, and Specimen6(3C15) and Specimen9(10C26) behaves ductile due to the retrofitting by TRM at the ends of the beam and columns. Hysteretic behavior of Specimen7(4C15) and Specimen8(9C15) can be seen in Fig. 4.163. Due to the strengthening of column and beam ends of the frame with infill wall, its rigidity slightly increased. During the test of Specimen7(4C15), the infill wall was crushed at the corner of the frame and the rigidity of the specimen decreased. The infill walls were crushed at the corners of the frame and rigidity of Specimen8(9C15) decreased while in Specimen7(4C15) no crush was observed at the infill wall during the tests, so the behavior of Specimen7(4C15) seems to be less ductile, because of limited damage development on the infill wall (Fig. 4.163). As it is anticipated, the frame with infill wall behaves brittle when compared to bare frames. Strengthening of bare frame specimens at the beam and column ends by wrapping enables more energy dissipation capacity than strengthened frames with infill wall. Experimental results reveals that, the infill walls should also be strengthened with those of frames. 
Since the inelastic behavior arises, the stiffness and the strength of the RC frame reduces and the frame system respond to these effects with hysteretic behavior under reversible loads. An envelope of the load-displacement curves of the frame system is obtained by combining the maximum load values occurring in each hysteretic cycles. This curve can be accepted as a unidirectional load-drift ratio curve which shows the damage extent and the overall behavior of the frame system (Fig. 4.167).
Ductility is the ability of a structural system to absorb more energy transmitted from the earthquake motion. It increases as deformations get larger. It is expected that a significant reduction in strength and unstable equilibrium should not appear as the system goes beyond the elastic limit. In the beam-column critical ends where the potential plastic hinges are expected to occur, the ability to consume energy is weakened due to insufficient transverse reinforcement. 
During the experiments, the strain gauges are used to measure the strains of the longitudinal reinforcement of the frame elements to observe their behavior. Stiffness is required to limit the lateral displacement in structures and it has a significant impact on structural performance. As seen in Fig. 4.25, Fig. 4.40, Fig. 4.114 for C15 and Fig. 4.58, Fig. 4.75, Fig. 4.178 for C26, the slope of the hysteretic curves decreases as the number of cycles increases, which clearly shows stiffness degradation in both directions for each cycle. The peaks in both directions are connected with a straight line to display the overall behavior of the frame in two directions and the slope of the line was defined as the stiffness of the loop as (Eq 4.1), where P_i^+and P_i^-correspond to the maximum positive and negative lateral load, and  and  denote the maximum positive and negative displacement of each cycle. Energy dissipation ability is one of the most essential properties of structures in point of the seismic-resistant design. In literature, it is of prime importance whether structural members have enough energy dissipation capacity to reach the desired performance target while incorporating the hysteretic behavior of the structural elements. RC frame members have critical regions close to their joints where the internal effects are high and plastic deformations are expected to develop, i.e., the controlled damages are expected due to bending moments beyond the yielding. As the ductility of the systems increases, more energy can be absorbed and consumed within the sections of the structure, where inelastic deformations take place. Energy dissipation is proportional to the enclosed area of each loop in the hysteretic curves at each drift ratio. Damping is also another property for dissipating their energy. As the damping ratio in structures increases, seismic demands decrease due to the energy dissipation and nonlinear deformations.
 As a result, experimental results show that the low and normal strength concrete frames which are inherently brittle can be easily and significantly strengthened to increase the ductility capabilities of the bare frame systems by wrapping the critical beam-column ends with TRM composite. Therefore, the application of TRM wrapping to RC bare frames mainly enhances the ductile behavior and energy dissipation of the existing buildings with low and normal concrete strength and poor reinforcement details (no transverse reinforcement at beam-column joints and no confined zones at the beam-column ends) and workmanship. Experimental results show that confinement of low and normal-strength RC frame member ends by wrapping TRM is a suitable strengthening technique to enhance the shear strength and ductility capacity. Wrapping with TRM composite seems to be a promising strengthening method to increase its ductility capacity. Furthermore, the study shows that the brittle behavior of undamaged existing bare frame systems can be improved by using this wrapping application. The results displayed that the proposed strengthening technique increases the stiffness and the strength of the frame system structures. It was concluded that strengthening should be applied not only to frames but also to infill walls to enhance ductility capacity. For the existing buildings, it is hard to access and apply strengthening to the beam-column joints due to the insufficient transverse reinforcement. This work showed that easy-to-apply strengthening at beam-column ends yields good system behavior. In addition, this confinement technique could be applied in moderately damaged buildings after an earthquake. The results presented in this dissertation are based on a limited number of test specimens. Therefore, further studies should be carried out to determine the effectiveness of TRM composite on the strengthening of RC buildings in seismic zones.