1 Introduction

Critical infrastructure assets typically comprise a number of interconnected components that work in concert to deliver a service. In the context of natural hazard vulnerability, the components have differing susceptibilities, require different resource levels and time to repair, and have a range of criticalities to the overall service delivery. The vulnerability of the asset, then, is a product of their constituent components, their properties and interactions.

SIRA stands for ‘Systemic Infrastructure Resilience Analysis’. It comprises a method and software tools that provide a framework for simulating the fragility of infrastructure assets to natural hazards, based on assessment of the fragilities and configuration of components that comprises the asset. To date system has been used to work with earthquake hazards only. SIRA enables the vulnerabilities of each element to be within a facility or a network to be integrated into a holistic assessment of the direct system losses, service disruption and cost.

SIRA is designed for simulating vulnerability of high-value infrastructure systems to natural hazards. The infrastructure types can be individual sites or facilities (e.g., an electrical substation, or a water treatment plant) or networks (e.g., an electricity transmission network, water transmission network. etc.). Earthquake ground motion has been the primary focus. Uncertainties are captured through a Monte Carlo sampling process. The tool facilitates quantification of infrastructure assets’ vulnerability, and also enables the most vulnerable components to be identified in terms of repair cost, time to recovery, and implications for service continuity. The tool has also been designed and tested to assess risk, but that functionality would typically require that the simulation be run in high performance computing environment.

The outcomes of this tool can support identification of retrofit options, and their relative efficacy in reducing risk. SIRA can be used in generating information for cost-benefit analyses of retrofit options, which can support asset managers in decisions around the most cost-effective utilisation of limited retrofit resources.

Vulnerability of a facility is modelled by assigning fragilities to the individual components that make up a facility or a network. The program accounts for variability in component fragilities by sampling probability distributions for the each fragility curve median and beta values. Once values have been selected for each curve it checks that fragility curves do not overlap and if they do, re-samples the median and beta probability distributions until non-overlapping fragility curves are produced.

Damage scales for most facility types, along with the recovery time estimation method, has been taken from HAZUS [6]. Although, where deemed more appropriate, custom damage scales have been used, e.g. for electrical substations. Repair cost (and hence damage index) and recovery times for each component are customised for each asset type, based on consultation with assets operators. The threshold values of spectral acceleration for each of four damage states are sampled by randomly sampling the fragility curves described above.

Please note that hazard modelling is done externally by relevant experts using other applications, and used in this tool as input.