Sürtünmeli Sarkaç Mesnetli Düşey Silindirik Sıvı Depolarının Deprem Yükleri Altındaki Davranışı

Abstract

Sıvı depoları; başlıca yakıt, endüstriyel kimyasallar, kullanma ve yangın söndürme suyu depolamakta kullanılan önemli mühendislik yapılarıdır. Sıvı depolarını depremin zarar verici etkilerinden korumayı hedefleyen yeni tekniklerden biri de sismik yalıtımdır. Sismik yalıtım sistemleri yardımıyla sıvı depolarının sönüm kapasitelerinin arttırılması ve periyot uzaması etkisiyle depo içerisinde dabesel bileşenden kaynaklanan hidrodinamik etkilerin azaltılması amaçlanmaktadır. Ancak, sistem bileşenlerinin doğrusal olmayan davranışı, mekanik özelliklerinin çevre koşulları, yaşlanma, yükleme koşulları vb. nedeniyle zaman içerisinde değişimi ve depo içerisindeki çalkalanma hareketi, depo ve sismik yalıtım sisteminin tasarımını zorlaştırmaktadır. Bu çalışmada sürtünmeli sarkaç sistemine mesnetlenen düşey silindirik sıvı depolarının yalıtım sistemi parametrelerinin seçimi ve boyutlandırmada kullanılacak kuvvetlerinin hesaplanması için bir yöntem önerilmiştir. Bu kapsamda, deponun hizmet süresince sismik yalıtım sistemi elemanlarının mekanik özelliklerinde oluşabilecek değişim ve bunun depo tasarımına yansıyacak özellikleri parametrik olarak değerlendirilmektedir. Sürtünmeli sarkaç mesnetleri Bouc-Wen modeli ile modellenmiştir. Düşey silindirik sıvı depolarında hidrodinamik etkilerin hesaplanmasında Veletsos tarafından geliştirilen model kullanılmıştır. Sismik yalıtım sistemi bileşenlerinin mekanik özelliklerinin, zaman içerisinde çevresel etkiler nedeniyle değişiminin, depo tasarım parametreleri üzerindeki etkilerini değerlendirmek amacıyla MATLAB üzerinde çalışan bir yazılım geliştirilmiştir. Söz konusu yazılım tarafından üretilen grafikler kullanılarak tasarımda izlenen yöntem, bir örnek üzerinde açıklanmıştır. Sürtünmeli sarkaç sisteminin depo tasarım kuvvetlerini önemli ölçüde azalttığı, çalkalanma yüksekliğinde ise bir miktar artışa neden olduğu görülmüştür.

Liquid storage tanks are considered as critical elements of infrastructure systems. These structures are mainly used to store fuel, industrial chemicals and water. Failure of fuel or industrial storage tanks following earthquakes may result in substantial environmental and financial damages (Jaiswal et al., 2004), (Koller and Malhotra, 2004). Studies on the seismic response of tanks show that the bottom portion of the contained liquid moves in unison with the tank while the portion near the free surface oscillates with a long period sloshing motion. Dynamic models built on this principle make the assumption that the continuous liquid media can be represented with two components: a short period impulsive component and a long period convective component responsible for the sloshing motion. Impulsive component is predominantly responsible for the hydrodynamic pressures acting on the tank wall and foundation in ground supported liquid storage tanks. Field studies conducted by various researchers to investigate the seismic damage mechanisms of liquid tanks reveal that these structures generally perform poorly during earthquakes due to a lack of a substantial ductility mechanism that can dissipate large amounts of energy and that new methods should be developed to increase their performance. Seismic isolation is an example of these new methods that aim to protect the liquid storage tanks against earthquakes by increasing their energy dissipation capacity and by lengthening their vibration periods to decrease the hydrodynamic pressures generated by the short period impulsive component. Recently, the application of seismic isolation and energy dissipation systems has been extended to critical fuel, chemical and fire-fighting water storage tanks. Although there are only a few seismically isolated liquid storage tanks, the number is steadily increasing. However, information on the observed performance of isolated tanks subjected to strong earthquakes is very limited and currently there are no provisions for these structures in the current tank design codes. Sloshing phenomena and the nonlinear behavior of seismic isolation components which have mechanical characteristics that are subject to change due to aging, environmental and loading conditions etc. complicate the design of both the tank and the seismic isolation system. Therefore, there is a growing need to develop new methods and tools to design and evaluate seismically isolated tanks. This paper begins by outlining the mechanical analogue system to be used for calculating the overturning moment and the base shear in tank wall as well as the free surface displacements for an upright cylindrical tank with rigid walls subjected to a horizontal base excitation. Force-displacement characteristics of the friction pendulum bearings were modeled with the Bouc-Wen hysteresis model. The effect of variation of friction on the response of the upright cylindrical liquid storage tanks has been investigated through a parametric study, which was conducted using a script that utilizes the Matlab state-space solvers. Aforementioned Matlab script also has the capability to generate normalized plots of the results of the parametric analysis in order to facilitate the design of the seismic isolation system and the estimation of the forces and moments to be used in tank design. Parametric approach to selection of seismic isolation system parameters and the calculation of tank design forces was explained through a case study. The liquid tank used in the case study has a radius of 42 m and is filled with LNG up to 36 m. Friction pendulum bearings used in this study had curvatures of 1m, 1.55 m and 2.23 m. The range of coefficient of friction at the slider interface used in the analysis varied between 0.03 and 0.08. Although, the use of a single convective mode is usually sufficient for most design applications, five convective modes were used in this study to capture the interaction between the long period sloshing component and the isolation system. Damping provided by the liquid storage tanks is usually very limited. Even though a single acceleration record (Scaled Erzincan 1992 EW (0.6g)) was used in this example, this approach can easily be adapted to handle multiple earthquake acceleration records. Tank design forces were decreased in the order of 50% with a slight increase in the freeboard height. In general, seismic isolation of liquid storage tanks with friction pendulum bearings was found to be an effective method for decreasing hydrodynamic effects in liquid storage tanks.