Sürtünmeli Sarkaç Mesnetli Düşey Silindirik Sıvı Depolarının Deprem Yükleri Altındaki Davranışı
Seismic response of isolated upright cylindrical liquid storage tanks with the Friction Pendulum System
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.
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