Lin, S.R.; Moratalla, J.M.; Uma, S.R.; Lukovic, B. 2023 Development of vulnerability functions for multi-storey (3 storeys and above) residential buildings in New Zealand. Lower Hutt, NZ. GNS Science report 2022/49. 61 p.; doi: 10.21420/W4J1-8M79
Abstract
This report details the development of a ground-motion-damage database for multi-storey buildings (3 storeys and above) on flat land in New Zealand using Toka Tū Ake EQC’s claim information and damage assessment reports obtained from the 2010–2011 Canterbury Earthquake Sequence and 2016 Kaikoura earthquake events. The database is then used to derive vulnerability functions for multi-storey residential buildings in New Zealand. The vulnerability functions in this report describe the relationships between shaking intensities and damage ratios (repair cost as a ratio of replacement cost of a building).As the first step, a database of multi-storey residential buildings was required to be compiled. Unfortunately, there is no single database that provides information of multi-storey residential buildings in New Zealand in a complete and comprehensive manner. Hence, we had to rely on multiple sources that covered different information, including construction period, material of construction and number of storeys, to build the database. In consultation with Toka Tu Ake EQC portfolio experts, we proposed a set of procedures to identify potential multi-storey buildings. This exercise required a manual process of checking property by property, as the information from multiple sources was not complete and not able to be mapped directly, thereby requiring ‘Google Street view’ verifications to identify potential multi-storey buildings. Obviously, it was a time-consuming process dealing with incomplete information, and it was not possible to fully identify an exhaustive list of multi-storey buildings within the limited time and budget available for this project. Around 400 buildings from Christchurch and Wellington were identified and listed in the database. After high-level scrutiny, it was only possible to make broad groups of building typologies based on storey height and construction period. From the database, depending on the available sample sizes of the building typologies, vulnerability functions of the following groups of buildings were derived and presented in this report: (i) post-1976, 3 storeys; (ii) post-1976, 4–7 storeys; and (iii) post-1976, 4 storeys and above. The sample size of pre-1976 buildings was too small and spread only betweena narrow bandwidth of peak ground acceleration (PGA) of 0.15–0.2 g to develop reasonably well defined functions. PGA is the shaking intensity used to derive all three sets of vulnerability functions. In addition, we have also considered spectral acceleration (Sa) at 0.3 s for three-storey buildings and spectral acceleration (Sa) at 1.0 s for buildings with four storeys or above to derive vulnerability functions, thereby reflecting their structural-period effect. Buildings subjected to liquefaction damage deserved separate attention. In our damage database, a substantial proportion of three-storey buildings recorded liquefaction damage and hence two sets of vulnerability functions, namely, shaking only and shaking and liquefaction combined were able to be derived. However, for buildings with four storeys and above, the damage information for both shaking and liquefaction is limited. In addition, little or no details of these multi-storey buildings characteristics (e.g. foundation design) are available. Due to these limitations, only combined (i.e. shaking and liquefaction) vulnerability functions were developed. For loss-modelling purposes, a two-step approach is proposed in this report. Step 1 is to determine the likelihood of a building being undamaged or demolished or to sustain damage that is repairable. For this step, we use fragility functions representing probabilities of exceeding certain discrete damage states. Step 2 is to calculate damage ratio for the building corresponding to the damage sustained. For this step, we use vulnerability functions relating damage ratios to hazard intensity that are derived from the present study. However, the fragility functions to be used in Step 1 are sourced from published literature, and their relevance is discussed. These modelling procedures, the derived and proposed functions, are summarised later in the report to facilitate their implementation into risk assessment platforms, such as RiskScape™ and Toka Tu Ake EQC’s PRUE. Limitations, such as inherent small building sample sizes, uncertainties of liquefaction being present and demolition being required, are discussed and recommendations for future improvement are summarised.
