In this paper, a multitangent-plane surface method is suggested for ca
lculating the reliability of complex structures of which the limit-sta
te surfaces are not known explicitly. In this approach, multiple tange
nt planes are used to approximate the actual limit-state surface. Also
radial importance sampling is used in conjunction with probabilistic
finite-element (PFE) analysis. Numerical results suggest that the new
method can be used to improve the accuracy of solutions obtained from
first-order second moment (FOSM) analysis and also require considerabl
y less CPU time when compared with Monte Carlo simulation (MCS) analys
is. This method is only suitable for the case of a nonlinear concave o
r convex limit-state surface.