The first passage time of discretized plate structures subjected to transversal and in-plane intensive transient loadings treated as nonstationary random processes is studied. The mean rate of various crossings for the nonstationary random response is evaluated using the definitions of Vanmarcke and Roberts. The definition of Roberts is extended to include clump size and clustering effects in similar spirit to that of Vanmarcke. Approximated first passage probabilities based on these definitions of mean rate of various crossings are obtained for some square steel plate structures whose response statistics are derived by an approach employing the Ito's calculus.