1. Introduction
1.1 Objectives
This project aims to provide an in depth seismic risk analysis of Vancouver’s downtown core, which will include three main objectives: (1) to develop a relational database that can be easily updated and use information from many different attribute tables to perform complex analyses, (2) to assess the amount of structural and non-structural damage to each building and estimate from this the expected monetary losses in the study area for Modified Mercalli Intensity (MMI) events VI-XII and, (3) to develop a Canadian methodology that can be used to estimate the expected number of injuries and casualties for three distinct times of the day, indoors and outdoors of each building in the study area for an MMI VI - XII earthquake scenario. This method will be modeled off the U.S. HazUS methodology but adjusted for southwestern B.C building prototypes and Canadian Census variables. The results will be modeled in a Geographic Information System (GIS) on a per block basis.
1.2 Background
Modified Mercalli Intensity (MMI)
The Modified Mercalli Intensity scale will be used throughout this methodology as a measure of different seismic event case scenarios. It is a scale of different intensity levels, represented in roman numerals ranging from VI-XII, which is widely used because it defines levels of structural damage. Table 1.2.1. defines the MMI scale as a description of effects occurring at each intensity level.
Table 1.2.1 The Modified Mercalli Intensity Scale
Building Inventory
In order to perform the seismic risk analysis, information on the building stock is required. The building inventory is a single attribute table consisting of 10,350 buildings in the study area. The data for each building contains the following information: name, block#, street address, material, prototype, primary use, year built, stories, subfloors, footprint area, and comments. GIS was used to integrate a relational database in which the building inventory was used with many other attribute tables to estimate and map the extent of structural and non-structural damage, monetary losses, and number of injuries and casualties per block.
ATC - 13
In order to assess the amount of structural damage to a building in California, a building classification with damage matrices for each class was developed by the Applied Technology Council (ATC-13, 1985). In their methodology, they used a damage probability matrix to relate expected damage to ground motion intensity for each building type. “ATC-13 is an influential study which uses MMI, a subjective scale, to measure the level of ground shaking” (Blanquera, 1999). The study contains 40 building prototypes, which were chosen based on their prevalence in the California structural inventory and also because of their unique seismic risk characteristics. The damage estimation methodology used in this project follows that of ATC-13 but was modified for building prototypes constructed in Southwestern British Columbia and their associated damage probability matrices.
BC 31 Building Classification
Building prototype is divided into 31 classes based on the most common construction practices in Southwestern B.C. The prototypes were classified by their structural framing type, construction material and building height. Some prototypes have been cross-referenced with corresponding ATC-13 building types. “Some building types do not correspond to any of the ATC-13 [prototypes] due to the decision of the local expert to expand the classification system to include all building types predominant in Southwestern British Columbia and which may not have been prominent in the California building inventory during the development of ATC-13”(Blanquera, 1999). Damage probability matrices are associated with each of the 31 prototypes and will be used to relate ground motion to building damage in order to estimate the seismic damage of the building inventory.
1.3 Study Area
The study area includes 1,461 blocks in the downtown core of Vancouver, 1,335 of which contain data on the building stock. The boundaries are at MacDonald St., W King Edward Ave., and Victoria Dr. (Figure 1.3.1.).
Figure 1.3.1. The Study Area: Downtown Vancouver Core
Building material and prototype were counted for each block and are displayed in Figure 1.3.2. (prevalent material) and Figure 1.3.3. (prevalent prototype). Vancouver’s downtown core mainly consists of blocks with wood as the prevalent material (66%). The second and third most prevalent materials are masonry, with 20%, and concrete with 12% of all the blocks. Spatially, blocks with wood as the prevalent material are mostly concentrated in the residential neighborhoods of Kitsilano, Fairview, Shaugnessy, Mount Pleasant, and a few in the West End and Chinatown. Most of the masonry blocks are located in Vancouver’s downtown eastside, Central Business District, Gastown and along Main St. The majority of the concrete blocks are located in the northwestern portion of Vancouver’s downtown, representing the concentration of many high rises, but there are a few others scattered throughout the study area. Of the 66% wood material, 63% is Wood Light Frame Residential (WLFR), the most prevalent building prototype in Vancouver’s core. There are two other wood prototypes that are less widely built but still significant; Wood Light Frame Low Rise Residential (WLFLR) with 22% of the wood blocks, and Wood Light Frame Low Rise Commercial/Institutional (WLFCI) with 14% of all the wood blocks. The most prevalent prototype made from masonry material is Unreinforced Masonry Bearing Wall Low Rise (URMLR) with 72% of all masonry blocks, making it the second most prevalent prototype in Vancouver’s downtown core. Reinforced Masonry Shear Wall Low Rise (RMLR) makes up most of the remaining masonry blocks with 24%. The most prevalent prototype from the concrete material is Concrete Frame with Concrete Walls High Rise (CFHR) with 66% of all concrete blocks. The remaining 34% consists of mainly Concrete Frame with Concrete Walls Medium Rise (CFMR), with 14%, and Concrete Frame with Concrete Walls Low Rise (CFLR), with 12%.
Figure 1.3.2. Prevalent Building Material per Block of the Study Area
Figure 1.3.3. Prevalent Building Prototype per Block of the Study Area
The geology of the study area is shown in Figure 1.3.4. 86% of all the blocks rest on glacial till which does not expect to amplify in a seismic event (Onur, 2001). The area at the end of false creek however is on landfill which includes sand, gravel, till, crushed stone and refuge. This area has the potential to amplify the ground motions of a seismic event. Only 22 blocks rest on this landfill but spatially it covers a large area, roughly 1km2. In recent years, a large amount of development has occurred in this area and should be given particular interest in a seismic risk analysis.
Figure 1.3.4. Geology of the Study Area per Block
1.4 Data Source
The building inventory data used in this project was collected in 1996 and was provided by the Greater Vancouver Regional District to Dr. Carlos E. Ventura, P.Eng., P.E., the Director of the Earthquake Engineering Research Facility, in the Department of Civil Engineering at the University of British Columbia. Structural and non-structural damage probability matrices and mean damage factors were supplied by a graduate student, Tuna Onur, as part of her thesis paper “Seismic Risk Assessment in Southwestern British Columbia” (2001).
Population data was derived from 2001 census tract variables provided by Statistic Canada. Population distribution relationships for three time scenarios and casualty rates were provided by the HazUS Technical Manual. Kate Thibert, a civil engineer graduate student in the Earthquake Engineering Research Facility at UBC, provided data on the translation of the HazUS prototypes into the BC 31 Building Classification and maximum occupancy rates for population estimation.