Evaluation of infrastructure vulnerability at selected gas regulatory station of Dhaka city

Abstract

This study focuses on the infrastructure vulnerability at selected gas regulatory station of Dhaka city which includes all of the stations and important feeding stations around Dhaka. For this purpose, vulnerability of 23 gas stations are determined by comparing characteristic site period of soil and structural periods of buildings constructed within 300 m radius of gas stations. Shear wave velocities are determined by microtremor array method by analyzing recorded data with computer program Geopsy. Shear wave velocities of gas stations are taken as input parameter to determine site periods. Shear wave velocity determined by this method is verified by SPT N values of eight gas stations. Average shear wave velocity of top 30m (VS30) is determined by BNBC 2017 (proposed) and IBC 2009 methods from which Characteristic site period of gas stations are estimated. The periods of buildings are determined based on empirical formula provided by BNBC 2006, NEHRP 2003, ATC 3-06, 1978, NZSEE (2006) and different research conducted on Unreinforced Masonry buildings (URM) based on information collected from google map and from field investigations. Three structural types: Moment Resisting Frame (MRF), Dual System and Unreinforced Masonry are considered in this study.

The VS30 values of the stations have been found to vary from 222 m/s to 248 m/s. Shear wave velocity determined by two methods (based on SPT and microtremor array) vary from 8 to 27%. The rigid rock and elastic rock assumptions yield more or less same characteristics site period (0.485 to 0.572 sec.) which indicates that all stations overlie more or less same geological formations up to upper 30 m. The depth of engineering bedrock (corresponding to Vs≥500 m/s) varies from 35 m to 65 m. Significant variations of periods determined based on this method has been found varying from 0.575 sec to 0.842 sec. Building having 4-5 storeys (30% out of Total 1991 studied) may collapse due to resonance based on VS30. Mostly 8-14 storied buildings (6.2% of total investigated) may collapse due to resonance based on engineering bedrock consideration which indicates mainly building with dual structural system are vulnerable to resonance. The URM buildings observed in the study area are limited to 2-4 storied, whose periods do not fall in the range of site period.