A simplified risk-targeted decision model for the verification of the seismic performance of critical infrastructure components to the operational limit state

A simplified risk-targeted decision model for the verification of the seismic performance of infrastructure components to the operational limit state is presented. It assumes linearity between the intensity measure and the engineering demand parameter and requires the evaluation of the displacement demand for a hazard-targeted design earthquake and of the displacement capacity. The latter is defined as the ratio between the operational limit-state displacement and a risk-targeted safety factor, which depends on the seismicity of the relevant location, on the randomness of the intensity measures causing the operational limit-state, and on the acceptable (target) probability of exceeding this limit state. In the paper, the theoretical background of risk-targeted safety factor is first explained, followed by a discussion on the sensitivity of the risk-targeted safety factor to the input parameters. The design procedure involving the simplified risk-targeted decision model is then introduced and demonstrated by means of an example of a simple pipe rack located at a petrochemical plant. Within the scope of the presented example, the operational limit-state displacement of the pipe rack is estimated on the basis of the limit-state strains of a pipe attached to the frame. The risk-targeted safety factor is then evaluated, and the frame is designed. It is shown that the application of the proposed procedure is straightforward once the operational limit-state displacement has been determined. The advantage of the procedure is that the target risk is accounted for by the risk-targeted safety factor, which can be calculated with respect to the importance of the equipment of the critical infrastructure. Additionally, the proposed decision model can be easily adopted by engineers, because the seismic demand is calculated by analogy to the Eurocode. However, at this stage of the research, the application of the proposed decision model is limited to structures that do not exhibit any significant nonlinear seismic response at the operational limit state, and to structures where the equal displacement rule applies.