Fragility Analysis of pile supported wharf using performance based design

Abstract

Maritime transportation has played a vital role in deciding economy of a country through prehistoric times. Rapid development of international sea trade in the last few decades has drawn attention towards seismic safety of port structures. A number of pile supported wharves have suffered extensive damage due to seismic events in the past decades, causing extended economic losses to port [1].India currently has 13 major and 187 minor ports. Nearly 95% of foreign trade by volume and 70% by value takes place through ports. About 65% of country s land is under moderate to very high seismic risk and the country has witnessed several major earthquakes in the past three decades. Currently, there is no guideline in India for earthquake resistant design of port structures. The existing earthquake-resistant design standards IS1893 part-1(2002) and IS13920 (1993) are proposed for buildings that behave very differently from port structures during earthquakes. In the absence of specific seismic design code for port structures, it becomes essential to make vulnerability analysis of structure /component to understand its behavior and probability of failure (or probability of repair work after seismic event) for different intensity earthquake.Seismic fragility analysis is a vital tool to comprehend structure s performance and probability of failure for different intensity of earthquakes. In the present study, seismic fragility curves are developed for a typical pile supported wharf for some important port sites in Gujarat i.e. Mundra, Kandla, Navlakhi, Dahej and Hazira, thereby representing moderate to very severe level of seismic hazards (Zone V and III) as per IS1893 part-1:2002. Fragility curves are developed for three levels of ground shaking i.e. Serviceability Earthquake (SE), Design Based Earthquake (DBE), and Maximum Considered Earthquake (MCE). The structural model of wharf is prepared in SAP 2000 using Winkler model to represent soil structure interaction. Static nonlinear pushover analysis is performed to obtain the capacity curve of wharf. Damage states are defined as per PIANC (Permanent International Association of Navigation Congresses). Demand is obtained from site specific spectra constructed using geotechnical report of port sites and corresponding seismic events normalized, and scaled from 0.1g to 1.0g. Using Capacity Spectrum Method and linear Time History Analysis, maximum displacements at deck are obtained and response matrix is created. Based on the damage states and the response matrix, the fragility curves of the wharf are constructed.It is observed that the selected port sites have much higher ground motions than specified by the default spectrum of IS1893 part-1:2002. It is also revealed that the port sites Mundra, Kandla and Navlakhi are most susceptible to seismic risk. Dahej and Hazira ports are comparatively at lower risk. The Indian standard (IS1893 part-1:2002) thus underestimates the fragility of wharf at selected sites, stating it to be functional for DBE and MCE. The site specific spectrum obtained at selected sites indicates the wharf as deficient in terms of serviceability during its design life. Hence, site specific spectrum is necessary for seismic design of port structures. There is also a need to review the exisitng Indian standard in context to ground motions. newline