Dynamic Soil-Pile-Structure Interaction of Pile Supported LNG Tank
Authors: Chang Dongdong , O&apos;Riordan Nick , Willford Michael , Powell John 
Published: April 2012

The complexity of dynamic soil-pile-structure interaction for pile supportedLiquefied Natural Gas (LNG) tank has been demonstrated through numericalstudies using different methods, which raises the importance of thoroughlyevaluating and comparing different methods to provide a more challengingappraisal of each method's limitations and general accuracy. In this paper, dynamic soil-pile-structure interaction analyses using two different numericmethods were undertaken to determine the seismic tank response and foundationdemands of a pile supported LNG tank in high seismic region. The tank is 60m indiameter and supported by 480 number F750mm circular piles. The dynamicsoil-pile-structure interaction was analyzed using both direct method andindirect (substructure) method.

In the direct method, nonlinear dynamic finite element (FE) analyses wereperformed in the time domain using FE software OpenSees with a fully coupledconstitutive model. The direct method FE model consisted of a finite mesh ofnonlinear soil, nonlinear soil-pile interaction springs, piles, and tankcomponents. The FE model was shaken with a series of recorded earthquakemotions that were spectrally matched to the design spectra developed from aprobabilisticseismic hazard assessment (PSHA) study.

In the substructure method, the soil-foundation and the superstructure weretreated separately, where frequency-dependent foundation impedances werecalculated and modeled as spring and dashpot. The two methods were evaluatedand compared for their ability to capture and present the essential dynamicfeatures and loading mechanism of the tank-foundation system, and sensitivitiesand limitations of the two methods are discussed.

The direct method is considered preferable when the inertia effects aresignificant and the soil response is significantly non-linear. However, withappropriate calibrations and assumptions, the substructure method is also ableto reasonably approximate the essential features of seismic response of thetank-foundation system.

Introduction

Dynamic soil-pile-structure interaction is significant in the seismic designand analysis of pile-supported LNG tanks, due to the large mass of the tanksand the resulting large inertial forces transferred from the tank to thefoundation to modify the seismic site response. The complexity of seismicsoil-pile-structure interaction for LNG tanks has been demonstrated throughnumerical studies using different methods (e.g., Willford et al. 2010, Kauselet al. 1978), which raises the importance of thoroughly evaluating differentdynamic numerical methods to provide a more challenging appraisal of eachmethod's limitations and general accuracy.

This paper describes and compares two different methods undertaken to carry outseismic assessment of a pile supported LNG tank in a high seismic zone, i.e., the direct method and the indirect method (substructure method). The directmethod solves soil, foundation, and tank in one step by performing anequivalent two-dimensional (2-D) nonlinear finite element (FE) analyses in thetime domain using software OpenSees. OpenSees is a validated software platformdeveloped by PEER and NEES (http://opensees.berkeley.edu/). The direct methodFE model consists of a beam-column model of the tank and nonlinear piles thatare attached to a 2-D nonlinear soil mesh by nonlinear soil springsrepresenting soil-pile interaction. The substructure method treats tank andfoundation separately by following a three step procedure proposed by Kausel etal. 1978, where equivalent linear frequency dependent foundation impedances arecalculated and modeled as a set of spring and damper in the structuralmodel.