Çok katlı çelik yapılarda stabilite tasarım yaklaşımının farlı yönetmeliklere göre incelenmesi = Stability analysis design approach in steel structures according to tbdy-2018 and çythye-2016 and comparison of design methods on a 10-storey structure

Abstract

Çelik yapı tasarımı, yapılanın kullanım amacına uygun olarak, farklı yük kombinasyonları göz önüne alınarak gerçekleştirilmektedir. Tasarlanan yapılar, kullanım ömürleri boyunca beklenen stabilite seviyesini koruması gerekmektedir. Çelik yapıların stabilitesi, yapısal bütünlüğün ve dayanıklılığın sağlanmasındaki en önemli kriterlerin başında gelmektedir. Yapısal elemanlardaki dayanım ve rijitlikteki belirsizlikler, geometrik kusurlar, düşey taşıyıcı elemanlardaki deplasmanlar, ikinci mertebe etkilerinin hakim olmasına ve sonucunda yapının göçme mekanizması durumuna gelmesine neden olmaktadır. Stabilite kayıpları, kullanılan analiz yöntemine bağlı olarak, yapı elemanlarının kritik yük ve kapasite değerleri üzerinden hesaplanabileceği gibi doğrudan yöntemlerle de incelenmektedir. İncelenen 10 katlı yapı, 2018 yılında yürürlüğe giren Türkiye Bina Deprem Yönetmeliği (TBDY-2018) kapsamında yapı düzensizlikleri hesaplanmış olup, tez konusuyla doğrudan ilişkili önem arz eden bir düzensizliği bulunmamıştır. Stabilite tasarımı yöntemlerinde yaygın şekilde kabul gören beş farklı yöntem bulunmaktadır. Tezde analizi yapılan bina tipi için bu analiz yöntemlerinin dördü kullanılabilirlik şartlarını sağlamıştır. Bu sebeple çalışmada Çelik Yapıların Tasarım, Hesap ve Yapım Esasları (ÇYTHYE-2016) uyarınca sınırları çizilmiş "Genel Analiz Yöntemi" ve "Burkulma Boyu Yöntemi" ile tasarım yöntemlerinin her ikisi için birinci ve ikinci mertebe analizler yapılarak, 4 analiz yöntemi arasındaki fark incelenmiştir. İncelenen 10 katlı yapının farklı stabilite hesabı yöntemleriyle elde edilen analizlerinde, (hakim davranış olan) en fazla eksenel kuvvet etkisine maruz kalan 1. ve 2. katlardaki kolonların kapasiteleri incelenmiştir. Ayrıca aynı aks üzerine yerleştirilmiş kolon elemanları arasında, yerleşim kaynaklı farklı yüklemeler sonucunda oluşan, değişen kapasite değerleri göz önüne alınmıştır. Çalışmada Türkiye Bina Deprem Yönetmeliği 2018'in istemleri dikkate alınarak tasarım yapılmış yapıların, deprem yönetmeliğinin etkisinin hangi seviyede olduğu tartışılmıştır. Çalışmada farklı analiz yöntemlerinin kullanımı, yapıların maruz kaldığı yükler altındaki gerçekçi davranışlarını daha iyi anlamamıza ve bu sayede istenilen yapısal güvenlik ve performans seviyelerini daha doğru bir şekilde belirlememize yardımcı olacaktır. Böylece belirlenen yöntemlerden hangisinin görece daha gerçekçi sonuçlar verebileceği hakkında bilgi sahibi olmamız sağlancaktır. Ek olarak stabilite tasarımındaki Burkulma Boyu Yöntemi ile Tasarım için hayati önem arz eden (K) tutulma boyu katsayısının, bazı hallerdeki hesaplamaları için, Amerikan Yönetmeliğinin ek bir önerisi bulunmaktadır. Tezde tasarlanan 10 katlı yapının karakteristiği, bu bahsi geçen özel halleri içeren davranışta olacak şekilde dizayn edilerek, AISC360-10/16 yönetmeliğinin bu önerisiyle bahsi geçen dört farklı yöntemle tekrar analiz yapılmış ve sonuçlar bu açıdanda irdelenmiştir.

Steel is known for its high strenght as a construction material. Considering its adequacy in terms of strength, this strength can be achieved with smaller element sections compared to the reinforced concrete building. Although section strength is a very important parameter for the resistance of the structure under external loads, it is not sufficient on its own. In structural engineering, buildings are expected to resist external loads, especially earthquakes, with skills such as durability, rigidity, ductility and stability, especially in our country. However, the most critical of these skills is that the stability of the structure is at the desired level. The importance of structural stability is due to the fact that its deficiency or inadequacy results in total collapse. When the components of the behavior of a structure are examined, induction can be made under the headings of material, section (strength states), element and system behavior. Stability design inference determines the criteria for section and element behavior in the lower set, while examining the system behavior in the upper set. As mentioned before, steel is a material with high strength and therefore the element cross-sections satisfy this strength requirement with small cross-section areas, which creates another issue that needs to be taken into consideration. Small cross-sectional areas cause to local and global buckling or wrinkles. At this point, the problem in the structure becomes a stability problem rather than a strength problem. Stability analysis approach ensures that these problems are provide solutions and sets boundary regarding regulations. It is always desired to model and calculate the behavior of structures that is closest to their actual behavior. Especially in recent studies, when this actual behavior is examined, it has been observed that the effect of operating loads on the deformed system of the structure continues and as a result, the structure loses its stability and collapses occur. This effect, which we call second-order effects, has therefore begun to be examined and it has been observed that there is a stability problem as well as a strength problem in the above-mentioned elements and even sections. Analysis methods and criteria that could take this stability situation into consideration have been developed. In the thesis study, a 10-storey building was designed. While the design was being made, it was not considered symmetrically, aiming for the structure to exhibit relative torsional behavior, provided that it remained within the limits of torsional irregularity. For this reason, protrusions were made in the plan of the building and placed in a way to create a diagonal placement at the eccentric. The reason for choosing a 10-storey building is that it is desired to strain the building elements in terms of axial force. At the same time, since the drifts will be greater in tall xxviii structures, it was desired to increase the effects of second-order effects under both axial force and horizontal loads. Second order effects is tried to be enforced both on an element basis and on a system basis. The structure was designed by forming steel frames that transmit moments with a high level of ductility in the x direction. In the y direction, the main system was established with centrally braced steel frames with high ductility. According to the geography of Turkey, it is desired to be above average in terms of earthquake forcing and the central settlement is accepted as Istanbul. Since one of the basic elements of the analysis will be the drift differences in the floor diaphragms and the different displacements within the slabs will create complexity in reading the analysis results, reinforced concrete composite slabs were considered and a rigid diaphragm was assigned. At the same time, it was intended to benefit from the axial force created by the load created by the reinforced concrete slab. TBDY-2018 requirements have been fulfilled, thus the changes in the structure have been observed and what consequences they will cause in terms of stability design have been observed. From the perspective of earthquake regulations, it has been seen that the structure in the x direction is the most dominant element in terms of its main system and this is caused by the relative storey drift regulation. Displacement limits were the effective factor in determining column sections. As a result of situation, the second order effects of the system whose horizontal translation was restricted were limited. Since drift is not an design criterion for the diagonal direction, the determining factor for the diagonal direction has been the strength and so capacity criterion. Different main systems for x and y directions resulted in different column cross-sections. While HI1080-30-300-40 made of S355 material was used for moment-resisting frames, HI 1000-30-400-50 profile made of the same material was used for the braced frames. Centrally braced steel frames desing under axial force due to their structure. Another benefit of TBDY-2018 is that it requires the axial forces to be enlarged with the over strength factor, causing the sections to be enlarged. This causes small changes in the magnification coefficients for stability design to create significant differences in column capacities. In AISC 360-10/16, different calculation methods of the buckling length K coefficient are offered as suggestions for some cases. For the 4 analysis models created, 4 more models were created with this buckling length coefficient in AISC, and thus the differences in the K coefficient offered by AISC were also examined. AISC recommends this approach for cases where the K coefficient is high. AISC recommends this approach for cases where the K coefficient is high. These case are situations where the elements that contributed in the translation of the columns and their support conditions are very low compared to the column stiffness, or the other columns have little effect on the floor stiffness. It is a known in the literature that the more suitable and realistic between the methods is II.MGAYT, since the finite element method can dynamically take into account second order effects on a structure and element basis. Reserch in the thesis is to what extent other analysis methods can approach the design with II.MGAYT. For this reason, the design of the structure was calculated with II.MGAYT, and the capacity comparison was made with this structure in other methods. K coefficient is calculated at high values in the system in the edge columns and on the 2nd and upper floors. The reason is that it is holded by the beam from only one direction and while it is supported on the foundation on the 1st floor, it is been able to drifted more on the other floors. As a result of the calculations, it was seen that the columns where the K coefficient between II.MGAYT and I.MBBYT was lower than 2 gave similar results with a deviation of 4%. It is seen that in columns with K coefficient above 2, the higher it is, the more deviation it causes.. This difference in the edge and upper floor columns is 9% in the moment frame column and 41% in the centrally braced frame column. The reason for the difference between these two columns is that, as mentioned before, the braced column has a much higher K value than the other column. In Addition, braced frames in terms of design combinations, it is under the influence of high axial force as a result of multiplication with the over strength factor causes this. It has been revealed that we examine at the calculation made with the K coefficient recommended in AISC, it is observed that AISC obtains more realistic results, with a difference between the II.MGAYT and I.MBBYT design not exceeding 4% for all columns. Between II.MGAYT and II.MBBYT, the same situation as between I.MBBYT and II.MGAYT above applies. Compared to the I.MBBYT method, II.MBBYT column capacities decreased by a maximum of 4%, excluding the second floor edge columns, and showed results closer to II.MGAYT. Although the difference in the moment frame columns, which is 9% in I.BBYT, decreased to 6%, the result for the centric braced frame columns is significantly different with 29%. Similarly at the results of AISC, the same results is calculated due to the K coefficient and give results similar to II.MGAYT. Although approximate results appear between II.MBBYT and I.MBBYT, it is observed that II.MBBYT is closer to the analysis results of II.MGAYT. The reason for this is that the amplification coefficients (B1 and B2) in I.MBBYT are used instead of second-order geometric nonlinear analysis with finite elements when considering second-order effects on a system basis. It has been concluded that the second order analysis results with finite elements give more realistic results compared to the calculation made with approximate coefficients. When I.MGAYT and II.MGAYT were examined, it was observed that they gave approximate results. There are capacity differences between 6% and 9% between II.MGAYT and 2nd Floor. When we look at the reason for the difference in the ratios, it is observed that although the cross-sectional internal forces found as a result of the first and second order analysis are very close to each other, the design force remains higher than the second order due to the B1 and B2 coefficients during the design. Considering all the results, it was seen that it would be more appropriate to design the steel structure with the second order General Analysis Method due to the different ratios. It has been observed that similar or different results can be obtained between design methods due to a number of parameters. Especially, the fact that buckling strength in columns is a stability problem rather than a capacity problem has revealed the importance of the designer interpreting and using the Buckling Length Coefficient correctly if stability designs are used for methods other than II.MGAYT. It was concluded that in such a case, it is more realistic to use the buckling coefficient recommended by AISC360/10-16.