How to get an unbiased estimate(importance sampling) of reliability index in mat lab

Dear All

I am doing a reliability analysis of a dam structure using matlab and ansys. i have done monte carlo simulation and now i have to do importance sampling. for monte carlo simulation we need lot of simulations but in case of importance sampling since i know the region of failure i need to find the rare event that is the probability of failure of rare event using importance sampling.For i need to know how to get the unbiased estimate of reliability index in matlab!! how can we go about this?? In this case i am varying my material properties the probability of failure is very less. Its a case of load and resistance of a structure.