  Global variance-based reliability sensitivity indices arise from a variance
decomposition of the indicator function describing the failure event. The
first-order indices reflect the main effect of each variable on the variance of
the failure event and can be used for variable prioritization; the total-effect
indices represent the total effect of each variable, including its interaction
with other variables, and can be used for variable fixing. This contribution
derives expressions for the variance-based reliability indices of systems with
multiple failure modes that are based on the first-order reliability method
(FORM). The derived expressions are a function of the FORM results and, hence,
do not require additional expensive model evaluations. They do involve the
evaluation of multinormal integrals, for which effective solutions are
available. We demonstrate that the derived expressions enable an accurate
estimation of variance-based reliability sensitivities for general system
problems to which FORM is applicable.
