A Comparison of Analytical Methods With Experimental Data for Earthquake Ductility Assessment
Authors: Adelson B.L. , Steinmetz R.L. 
Published: October 2008

A two-part study was conducted to help evaluate analytical methods used to assess the ductile behavior of offshore structures subjected to severe earthquake ground motion. Part I of the study consisted of a comparison between analytical time-history responses predicted using the Inelastic Tower Response Analysis (INTRA) program and experimental data gathered during a shake-table experiment performed by the University of California at Berkeley in March 1981. The experiment involved a series of tests on a scaled, two-dimensional model of a prototype offshore platform subjected to various levels of simulated ground motion. Part II of the study consisted of a comparison between the ductile behavior predicted using simplified analysis methods (static pushover and impulse velocity) and the ductile behavior demonstrated by the experimental and analytical time-history results.

Results of Part I indicated that the analytical time-history analyses predicted within reasonable accuracy the response of the test structure as measured during the experiment. The results of Part II demonstrated that the simplified methods produce conservative estimates of the test structure's energy absorption capacity and ductile behavior during intense ground motion.

INTRODUCTION

In recent years, there has been a growing need to install offshore structures in seismically active areas such as southern California, Alaska, and Japan. Industry practice, as set forth in the provisions of the API-RP2A [1], has been to design such structures to meet specific strength and ductility requirements. The strength requirements are intended to provide sufficient resistance for the structure to experience moderate earthquake shaking without sustaining significant structural damage. The ductility requirements are intended to provide sufficient energy absorption capacity to withstand rare intense earthquake motions without collapsing. As demonstrated by the performance of buildings during earthquakes, structural ductility is an important aspect of developing a safe, yet cost-effective, earthquake resistant design.

Much progress has been made recently in developing analytical tools to assess the ductile response of offshore structures to severe earthquakes. In 1979, the Inelastic Tower Response Analysis (INTRA) computer code was completed during Phase II of the Soil-Pile-Structure-System (SPSS) Program sponsored by 17 companies in the offshore industry [2J. Program INTRA is a finite element code capable of performing three-dimensional, dynamic, nonlinear analyses, including the inelastic and instability effects of the structural members, piles, and soil. Industry efforts are now being directed toward verifying and calibrating such nonlinear tools [3]. Since no field data of an offshore structure response to severe earthquakes exist, experimental data must be utilized. During 1979-82, a series of experiments was conducted on two-dimensional scale models of an offshore structure at the University of California (UC) at Berkeley. One of the experiments was conducted on their shake-table [4]. This experiment is significant in that it is the only shake-table experiment designed specifically for studying the response of offshore structures.