Beton dolgulu çelik kolonların bulonlu alın levha kiriş birleşimlerinin çevrimsel davranışına berkitme levhalarının etkisi
The influence of rib stiffeners on cyclic behavior of bolted end-plate connections of concrete-filled steel columns
Yazar: ARYAN AZIMI
Danışman: DOÇ. DR. KADİR ÖZAKGÜL
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Yapı Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2024
Konu: İnşaat Mühendisliği = Civil Engineering

Özet:
Bu çalışmada beton dolgulu çelik kolon (BDÇK) ve I-kesitli kirişlerin alın levhalı boyuna bulonlu (through-bolt) birleşimlerine berkitme levhalarının etkisi incelenmiştir. Son yıllarda birçok araştırmanın konusu olan BDÇK elamanlar, diğer muadilleri olan betonarme ve çelik profillere göre önemli üstünlüklere sahiptirler. Ancak BDÇK ve I-kesitli kirişlerin birleşimlerinin tasarımında onaylanmış bir yönetmeliğin eksikliğinden dolayı, beton dolgulu çelik elemanların kullanımı çok sınırlı kalmıştır ve bu alanda farklı çalışmalara ihtiyaç duyulmaktadır. 
Beton ve çeliğin bu kesitlerde beraber kullanılmasının sağladığı avantajlar ve beton dolgulu çelik kolon ve I-kesitli kiriş birleşimleri için araştırmacılar tarafından önerilen farklı birleşim tipleri ayrıntılı olarak ilk bölümde incelenmiştir. Ayrıca moment aktaran birleşimlerin tasarımı için yaygın olarak tercih edilen alın levhalı birleşimler bahsedilmiş ve bu birleşimlerde kullanılan berkitme levhalarının etkileri ve bunlarla ilgili çalışmalar irdelenmiştir.
İkinci Bölümde ise ABAQUS sonlu elemanlar yazılımını kullanılarak ve deneysel bir çalışma referans alınarak, beş adet farklı kiriş yüksekliklerine ve eksenel yüke sahip kare kesitli beton dolgulu boyuna bulonlu berkitmesiz alın levhalı birleşim bu sayısal çalışmanın gerçekleştirilmesi için modellenmiştir. Modellerin kesit özellikleri, malzeme özellikleri, elemanlar arasında yüzeylerin etkileşimi, yük ve sınır koşulları, sonlu elemanlar ağının oluşturulması ve eleman tipleri ile ilgili tüm bilgiler detaylı olarak bu bölümde verilmiştir. Ayrıca referans alınan çalışmanın deneysel ve sayısal sonuçları yorumlanmış ve bu çalışmada modellenen örneklerin analiz sonuçları ayrıntılı bir şekilde gösterilmiştir. 
AISC 358-22 yönetmeliğinin berkitme levhalarının tasarımı ile ilgili sınırlamalar ve berkitmeli birleşimlerin oluşturulması ise üçüncü bölümde ele alınmıştır. Farklı berkitme levha kalınlığı, açısı ve çelik kalitesine sahip berkitmeler doğrulanmış modeller üzerine ilave edilmiştir. Ayrıca oluşturulan berkitme levhalarında bitiş yüksekliğinin belirlenmesi ve yeni modellerin isimlendirmesi açıklanmıştır.   
Beton dolgulu çelik kolon ve I-kesitli kirişlerin alın levhalı boyuna bulonlu birleşimlerinde, berkitme levhalarının birleşim davranışına olan etkisinin analiz sonuçları ise dördüncü bölümde sunulmuştur. Çevrimsel yükleme altında birleşimlerin nihai dayanımları, süneklikleri, berkitme ve kirişlerinde plastikleşme ve burkulmalar ve bulon davranışı detaylı olarak incelenmiştir. 
En son bölümde ise berkitme özelliklerinin birleşim davranışına olan etkisi irdelenmiştir. Berkitme levhalarının kalınlığının ve açısının nihai dayanımın artışında en önemli parametre olarak görülürken, çelik kalitesinin etkisi sınırlı olmuştur. Berkitme levhalarının varlığı kirişlerde yerel burkulmaları önleyemediği için sünek davranışa etkisi görülmemiştir, ancak bulon öngerilme geriliminin korunmasında önemli etken olduğu tespit edilmiştir.

Abstract:
This study presents a numerical study on the influence of stiffened on bolted end-plate connections of concrete-filled steel tubular (CFST) column and I-section beams. Concrete-filled steel tubular as a composite section has been one of the popular topics for researcher in recent years due to their characteristic. Using concrete and steel tubes together gives different features to CFST sections. The steel tubes induce the preservation of the compression strength of concrete, while the buckling of steel tubes is also delayed. Concrete-filled steel sections have significant advantages, such as high load-carrying capacity over their counterparts, reinforced concrete and steel profiles. These superiorities make them appropriate construction materials, particularly for special structures. However, due to the construction difficulties of the connections of CFST and I-section beams and the lack of approved regulations and design guidelines, the use of concrete-filled steel elements has remained very limited and requires further studies in this field.
The connection of steel construction has an essential role in the reliability of structures. Many researchers have proposed various types of connections for concrete-filled steel tube column and I-section beams. The welding between beams and steel tube was one of them; however, tube wall distortion makes it more susceptible to the risk of brittle failure. Different types of diaphragms, including external, internal, and through diaphragms, have also been investigated by the researcher, and they depict good seismic performance. Through-bolts and blind bolts are another way to connect CFST column and I-section beams. Especially the through bolt, due to its ability to act as a connector between concrete and steel tube, seems a good selection for moment resistance frames.  
To conduct and verify the accuracy of this research, finite element (FE) analysis was utilized to simulate a previously reported study that involved both experimental and numerical results in the literature. The simulation in ABAQUS finite element software included five specimens of square concrete-filled steel. The FE analyses consider both material and geometrical nonlinearities. The average compressive strength of concrete and steel elements material properties reported in reference work used and confined concrete stress-strain behavior was considered for modeling of concrete. The interaction between the concrete core and steel tube was surface-to-surface, and normal behavior was assigned as a "hard contact," which prevents elements from penetrating each other but allows separation, while a "penalty" was assigned for tangential behavior. The load application process in ABAQUS started with a 50 kN bolt pre-tension, followed by constant axial load and then cyclic loading at the top of the column. Element types for bolts, concrete, end-plate, and steel tubes were an eight-node brick element C3D8R, and an eight-node brick element SC8R was used for beams element. The obtained load-displacement (P-Δ) relationships and failure mode from cyclic loading confirmed the accuracy of the simulated un-stiffened end-plate connection of the CFST column and I-section beams.
End-plate bolted connections are widely used in moment-resistance frames, which include stiffened and unstiffened plates. Many investigations showed that stiffened end-plate with rib stiffeners improves the connection performance without changing the end-plate thickness and bolt diameter. Further, a better load distribution among bolts was reported. Other researcher also examined the influnce of different rib stiffeners parameters on the behavior of connection and rib stiffeners for I-section Column and beams joints. Unlike previous investigations, the effect of rib stiffeners on the cyclic behavior of through bolted end-plate connections of CFST column and I-section beams was investigated in this study.
For this, two validated un-stiffened specimens (J-S13 and J-S33) with the same axial load but different beam dimensions were used to assemble rib stiffeners on the end-plate connection of CFST column and I-section beams in ABAQUS software. The rib stiffeners were chosen by considering the restrictions of the AISC 358-22 standard on rib stiffeners. The stiffener thicknesses of 2, 4 and 6 mm were used to examine the rib stiffener thickness thinner or thicker than the beam web thickness. S235 and S355 steel grades, which had lower and higher yield strength than beams, were considered. Furthermore, the effect of different rib stiffener slopes was investigated for 20°, 30°, and 40°.
Analysis of stiffened connections was achieved under the same cyclic loading protocol of un-stiffened connections, and load-displacement (P-Δ) skeleton curves were obtained to determine the effect of rib stiffener on the ultimate strength and ductility of connections. The influence of rib stiffeners on ultimate strength was significant. The increase in rib thickness induces higher ultimate strength, while the thinner ribs depict less ultimate strength. For the slope of the stiffener, it was vice versa; an increase in the slope causes less ultimate strength. Further, despite models made with S355 steel having higher ultimate strength than those made with S235 steel, the difference is not remarkable. Despite favorable results in the ultimate strength due to the inadequacy of rib stiffener to delay local buckling of beams, ductility of connection was not affected in a constructive way. For a great majority of models the ductility was less than main models. 
The effect of various parameters on the failure mode of rib stiffeners was also analyzed. The buckling of 2 mm rib stiffeners was grave, while for 4 and 6 mm stiffeners, respectively, it was at the end of the stiffener and out of the stiffener. Additionally, the study obtained the equivalent plastic strain (PEEQ) concentration of the connection, revealing that the plasticization center on the beam moves further with thicker rib stiffness.
Although there are different benefits of through-bolts, using the same through-bolt on both sides of a connection makes them fragile to failure; hence, comprehension of their behavior is essential. The pre-stress values obtained for the through-bolts after the cyclic loading showed that the J-S13 stiffened connections have the same behavior as the unstiffened main model, and the presence of rib stiffener did not have a significant impact. However, for the J-S33 stiffened connection, the through-bolt behavior was found to be positively affected by the addition of the rib stiffener.