Seismic response of liquid storage steel tanks with variable frequency pendulum isolator

The seismic response of liquid storage steel tanks (slender and broad) isolated with Variable Frequency Pendulum Isolators (VFPIs) is investigated under normal component of six near-fault ground motions. The continuous liquid mass is lumped as convective mass, impulsive mass and rigid mass. The corresponding stiffness associated with these lumped masses is worked out depending upon the properties of the tank wall and liquid mass. The frictional forces mobilized at the interface of the VFPI are assumed to be velocity independent. The governing equations of motion of liquid storage steel tanks isolated with the VFPIs are derived and solved in the incremental form using Newmark’s step-by-step method assuming linear variation of acceleration over small time interval. For comparative study, the seismic response of liquid storage steel tanks with the VFPIs is compared with that of the same liquid storage steel tanks isolated using the Friction Pendulum Systems (FPSs). In order to measure the effectiveness of isolation system, the seismic response of isolated steel tanks is compared with that of the non-isolated steel tanks. Further, a parametric study has been carried out to critically examine the behaviour of liquid storage steel tanks isolated with VFPIs. The important parameters considered are the friction coefficient of the VFPI, the Frequency Variation Factor (FVF) of the VFPI and the tank aspect ratio. The difference between the liquid storage tanks isolated with the VFPI and the FPS isolators subjected to the harmonic and far-field ground motions was also investigated in this study. Effect of vertical component of ground motions on the behaviour of the VFPI-isolated liquid storage tanks is investigated under triaxial ground excitations by considering the interaction of forces in two orthogonal directions. From these investigations, it is concluded that seismic response, viz. the base shear, the sloshing displacement and the impulsive displacement, of liquid storage steel tanks during near-fault ground motions can be controlled within a desirable range with the installation of the VFPI. Under strong harmonic excitations, the base shear of the VFPI reduces than that of the FPS whereas the sloshing displacement, the impulsive displacement and the isolator displacement of the VFPI exceeds than that of the FPS. It is also found that the isolation by the FPS and VFPI isolators has almost the same effect in the tank to the far-field ground motions. The triaxial ground motions have noticeable effect on the response of the VFPI-isolated liquid storage tanks relative to unilateral ground motion and if ignored, the sliding displacement and base shear will be underestimated.

[1]  Ting-Yu Hsu,et al.  Experimental Study of Isolated Building under Triaxial Ground Excitations , 2000 .

[2]  Babak Alavi,et al.  Behavior of moment‐resisting frame structures subjected to near‐fault ground motions , 2004 .

[3]  Goodarz Ahmadi,et al.  Response of frictional base isolation systems to horizontal-vertical random earthquake excitations , 1988 .

[4]  R. S. Jangid,et al.  Seismic analysis of base-isolated liquid storage tanks , 2004 .

[5]  Iradj G. Tadjbakhsh,et al.  Effect of vertical motion on friction‐driven isolation systems , 1986 .

[6]  J. Kelly,et al.  Shake Table Test of Cylindrical Water Tanks in Base-Isolated Structures , 1990 .

[7]  Eren Uckan,et al.  Parametric analysis of liquid storage tanks base isolated by curved surface sliding bearings , 2010 .

[8]  Ravi Sinha,et al.  VFPI: an isolation device for aseismic design , 2000 .

[9]  Yen-Po Wang,et al.  Seismic isolation of rigid cylindrical tanks using friction pendulum bearings , 2001 .

[10]  Praveen K. Malhotra.  New method for seismic isolation of liquid-storage tanks , 1997 .

[11]  Harry W. Shenton,et al.  Seismic Response of Isolated Elevated Water Tanks , 1999 .

[12]  Chin-Hsiung Loh,et al.  Effect of near-fault earthquake on bridges: lessons learned from Chi-Chi earthquake , 2002 .

[13]  Juan Carlos de la Llera,et al.  Modelling aspects of structures isolated with the frictional pendulum system , 1998 .

[14]  R. S. Jangid,et al.  Performance of sliding systems under near-fault motions , 2001 .

[15]  Medhat A. Haroun,et al.  Seismic Design of Liquid Storage Tanks , 1981 .

[16]  R. S. Jangid,et al.  Non-linear seismic response of base-isolated liquid storage tanks to bi-directional excitation , 2002 .

[17]  Praveen K. Malhotra,et al.  METHOD FOR SEISMIC BASE ISOLATION OF LIQUID-STORAGE TANKS. TECHNICAL NOTE , 1997 .

[18]  Shih-Po Chang,et al.  Effect of viscous, viscoplastic and friction damping on the response of seismic isolated structures , 2000 .

[19]  Andrew S. Whittaker,et al.  Seismic Analysis of Conventional and Isolated LNG Tanks Using Mechanical Analogs , 2008 .

[20]  Goodarz Ahmadi,et al.  Multi-story base-isolated buildings under a harmonic ground motion — Part II: Sensitivity analysis , 1990 .

[21]  T. C. Liauw,et al.  Structures on sliding base subject to horizontal and vertical motions , 1988 .

[22]  Medhat A. Haroun.  Vibration studies and tests of liquid storage tanks , 1983 .

[23]  R. S. Jangid,et al.  Base isolation for near‐fault motions , 2001 .

[24]  R. S. Jangid,et al.  Response of base-isolated liquid storage tanks to near-fault motions , 2006 .

[25]  G. Housner.  Dynamic pressures on accelerated fluid containers , 1957 .

[26]  A. Fuladgar,et al.  Response of pure-friction sliding structures to three components of earthquake excitation , 2003 .

[27]  R. S. Jangid,et al.  Variable friction pendulum system for seismic isolation of liquid storage tanks , 2008 .

[28]  Nam-Sik Kim,et al.  Pseudodynamic test for evaluation of seismic performance of base-isolated liquid storage tanks , 1995 .

[29]  G. Housner.  The dynamic behavior of water tanks , 1963 .

[30]  R. S. Jangid,et al.  Earthquake response of isolated elevated liquid storage steel tanks , 2003 .