Türkiye Çelik Yönetmeliği'ne (ÇYTHYE-2016) göre stabilite tasarım yöntemlerinin irdelenmesi = Investigation of stability design methods according to Turkish Steel Code (2016)

Abstract

Çelik yapıların stabilitesini(kararlılığını) etkileyebilecek birçok faktör bulunmaktadır. Elemanların ve yapı sisteminin şekil değiştirmeleri, şekil değiştirmelerden oluşan ikinci mertebe etkiler, geometrik ön kusurlar, dayanım ve rijitliklerdeki belirsizlikler yapı stabilitesini önemli derecede etkilemektedir. Bu nedenle yapı stabilitesinin tahkikinin yapılabilmesi için stabilite tasarım yöntemlerinin kullanılması gerekmektedir. Amerikan Şartnamesi AISC360-16(Specification for Structural Steel Buildings, Çelik Binalar İçin Tasarım ve İnşaat Yönetmeliği-2016), ÇYTHYE(Çelik Yapıların Tasarım, Hesap ve Yapım Esasları Yönetmeliği-2016)'da Burkulma Boyu(BB) ve Genel Analiz(GA) yöntemleri yer almaktadır. Genel Analiz yöntemi teknolojik gelişmeler neticesinde geliştirilen ve yapı stabilitesini irdelemek için sunulan bir yöntemdir. Burkulma boyu yöntemi yarım yüzyılı aşkın süredir bilinen ve kullanılan yöntem olmasından dolayı sınırlı koşullarda genel analiz yöntemine alternatif olarak kullanılabilir. Her iki yöntem için ikinci mertebe etkilerin belirlenmesinde doğrusal olmayan analiz kullanılır. Doğrusal olmayan analiz yöntemine, belirli sınırlar çerçevesinde yaklaşık ikinci mertebe analizi alternatif olarak kullanılabilmektedir. Yöntemler harmanlandığında dört farklı stabilite tasarım yöntemini oluşturmaktadır. Bu yöntemler, doğrusal olmayan analiz ile BB yöntemi, doğrusal olmayan analiz ile GA yöntemi, yaklaşık ikinci mertebe analizi ile BB yöntemi ve yaklaşık ikinci mertebe analizi ile GA yöntemleridir. Yapı stabilitesini etkileyen, elemanların ve sistemin şekil değiştirmeleri, ikinci mertebe etkiler, geometrik ön kusurlar, dayanım ve rijitliklerdeki belirsizlikler basit ifadelerle tez içerisinde açıklanmış, yöntemlerin içerisinde nasıl ele alındığı ifade edilmiştir. Stabilite tasarımı yöntemlerinin, çerçeve sistemi üzerinde çalışmasını irdelemek amacıyla çerçeve sistemi örneği, hesap detaylarıyla çözülmüştür. Stabilite tasarımı için sunulan dört yöntemin, uygulama sınır durumlarını sağlayan sekiz katlı çelik yapı sistemi sap2000 programı ile çözülmüştür. Tüm yapılan çalışmaların kapsamlı değerlendirmesi yapılmıştır. Değerlendirmeler neticesinde hesaplamalardaki netlik ve uygulanabilirlik bakımından GA yöntemi BB yönteminden daha güvenilir olduğu görülmüştür. Doğrusal olmayan analiz yöntemi, YİMA yöntemine göre şekil değiştirmeleri analizde dikkate alabilmesi sebebiyle gerçek davranışa daha yakın sonuçlar vermektedir. YİMA yöntemi, doğrusal olmayan analiz yönteminden daha küçük değerler elde etmeksizin daha güvenli bölgede kalmaktadır. Süperpozisyon prensibinin kullanılması gerekli durumlarda doğrusal olmayan analiz yöntemi uygulanamadığından YİMA yönteminin kullanılması avantaj sağlamaktadır.

Stability for all building systems; Although it is a problem that must be overcome in terms of life safety and building safety, it has special importance, especially for steel structures. The high strength of the steel provides easy access to sufficient bearing capacity with small cross-sections in the carrier system elements. However, while the buckling problem is encountered in the structural element formed with small sections and subjected to axial pressure load, the stability problem is also met when the holistic behavior of the same structural system is examined. For this reason, it is necessary to carry out the design by examining the stability situation based on elements and throughout the system during the design phase. It would be a correct method to consider the factors affecting the stability of the element and the structure with a comprehensive research in the stability design. In this context, international steel regulations and Turkish Steel Regulations (ÇYTYHE-2016) offer design opportunities with two different methods: General Analysis (GA) and Buckling Length (BB) method. There are two different analysis methods to consider the second-order effects of the design methods. These methods are nonlinear geometric analysis (nonlinear) and approximate second-order analysis methods (YIMA). When the presented methods are blended, they form four different design and analysis methods within certain boundary situations. Aim: This study is aimed to compare the four methods within the limits of applicability and to highlight their advantages and disadvantages. Method: Within the scope of the study, a single-story plane steel frame system and an eight-story steel structure model were designed with first-order analysis and stability controls with four different methods, and the results were analyzed. When we examine the structure of the frame sample with the results of first-order analysis, after all the wear stability design of the columns (A and B axle columns) that carry both horizontal and vertical loads are completed; It is seen that the first-order results for column A have greater internal forces than the stability design results. The negative consequences of this situation for this column are the stability design internal force values are greater than the first order values. It has been observed that vertical, horizontal and second order effects are effective on the internal forces of the column in both columns. With the effect of vertical load from the weights of the vertical and horizontal loads, axial compressive force and positive moment occur in the A column, while horizontal loads consist of axial tensile force and negative moment values. The propagation of the second-order effects, on the other hand, has an opposite sign to the internal forces formed by vertical loads. Therefore, the stability design results resulted in lower internal force values since the second order effects are the opposite direction (opposite sign) of the internal force values formed by vertical loads on the A column. For the B column, the internal force values are greater in the stability design as this is exactly the opposite of the situation. Considering the case of the opposite case of the direction of fictitious load application, it can be predicted that the results obtained will be exactly the opposite. The values that continue to be examined within the internal force values are the values where the moment values are affected more than the axial values. This shows that second order effects are more effective on instant values. When the calculation details of the GA and BB methods used in stability design are examined, the BB method aims to reduce the element design compressive force strength with the coefficient K. For this reason, the capacity ratio that will occur in the element is taken into account with the reduced axial force strength. In the GA method, the deformations of the element and the system are taken into account by reducing the element stiffnesses without changing the design strengths of the element. When the displacement values are examined, it is seen that the BB-YIMA method has the same displacement values as the first order analysis. It is understood that the buckling length coefficient K used in the BB method does not affect the system displacements. In GA methods, on the other hand, higher values were obtained than the first-order displacement values due to the stiffness reductions. More displacement values can cause more effective second-order effects and destabilizing factors. More displacement values in terms of stiffness reduction provide the opportunity to better examine the stability of the structure. It can be taken into consideration that nonlinear analysis (nonlinear) in terms of geometry change is the method recommended to be used by the regulation, giving results close to the real behavior. Comparing the internal force values in the non-linear analysis and approximately second order analysis results, the results were close to each other in the axial force values. In the moment values, it is seen that the second order analysis gives higher moment values than the nonlinear analysis values. Assuming that the nonlinear analysis results give results close to the real behavior, it can be said that the results show close values when the second order analysis for column elements and the nonlinear analysis element capacity ratio results are compared. When the results are analyzed in terms of internal forces, it is seen that the axial force values are close to each other in both methods. However, when the moment values are compared, it can be said that the results of the second order analysis reach 1% to 5% higher values than the nonlinear analysis results. This situation shows that the stability design made with approximate second order analysis is in the safer region. This situation, which seems to be a disadvantage, allows the use of approximately second-order analysis and the spectrum (mode combining) method, which is commonly used by design engineers, in stability design. The buckling length coefficient K used in the buckling length method does not affect the structural displacement. The general analysis method tries to force the limit values with the displacement of the designed structure and the second order effects that may occur due to the reduction in stiffnesses. Such a compulsion gives the designer a chance to better examine the stability of the structure. Column elements used in steel structures generally have a stronger inertia in one direction than the other. For this reason, they are usually designed to transmit torque in the strong direction and as articulated in the weak direction. The Buckling Length method, which is used for stability design, designs with the coefficient K. The buckling length coefficient K determines the axial slenderness ratios of the element and the direction that will form the boundary condition for the flexural buckling condition of the element. In this determination, it is important that the slenderness ratio in the strong side of the element exceeds the slenderness ratio in the weak side, for the method to take into account the uncertainties in strength and stiffness. Two negative situations can be encountered in the buckling length method. The first case is the case where the strong axis cannot exceed the weak axis slenderness ratio and the compressive strength of the element cannot be reduced. This is the situation encountered in three-dimensional building design. In the second case, large slenderness ratios are obtained with the buckling length coefficient K. The compressive force strength of the element can be reduced too much and it can create situations of staying in the extreme safe zone in the stability design. The biggest disadvantage of the method is that the calculation of the K coefficient for the buckling length method is a matter of debate and therefore a clear K coefficient calculation is not recommended by the standards. Since there is no such situation in the general analysis method, the GA method is more advantageous due to the clarity and applicability limit situations in its calculation. If the axial and moment values are examined as a result of the studies, the Buckling Length and General Analysis methods examine different internal force values. The axial force in the BB method focuses on the moment value in the GA method. However, the results of the combined effects equations under bending and pressure, showing the element capacity results, are close to each other.