Physica A 274 (1999) 341–348 www.elsevier.com/locate/physa Strong motion duration eects on base isolated systems R. Greco a ; ∗ , G.C. Marano a , D. Foti b a Istituto di Progettazione, Facolt a di Architettura, Politecnico di Bari, Via Orabona, 4, 70123 Bari, Italy b Dipartimento di Ingegneria Strutturale, Facolt a di Ingegneria, Politecnico di Bari, Via Orabona 4, 70123 Bari, Italy Abstract In this paper the non-stationary response covariance of a base isolated building subjected to a seismic excitation modelled through a zero mean Gaussian non-stationary stochastic process, uniformly modulated in the time with a boxcar-shape temporal function, is determined. The results are utilised to give an estimation of the strong motion duration eects on the temporal peak of the root mean square value of the response of the system and on system failure probability for rst threshold crossing. c  1999 Elsevier Science B.V. All rights reserved. PACS: 91.30.P; 07.07.T; 02.50; 05.40 Keywords: Strong motion; Base isolated systems; First threshold crossing 1. Introduction In the last few years many methods for qualifying earthquake severity have been proposed, with the aim to evaluate their damage potential on structures. Some of these methods considered the seismic peak acceleration, the strong motion duration, the fre- quency characteristics and the temporal envelope shape of the seismic ground motion. The seismic peak acceleration is the simplest parameter to identify and compare dif- ferent earthquakes, but it cannot be necessarily correlated to the maximum structural response, especially if the excitation acts on the structure for a very short time and for systems with high fundamental period and a low damping. Arias intensity [1] represents a measure of the seismic excitation which considers at the same time the peak, the * Corresponding author. 0378-4371/99/$ - see front matter c  1999 Elsevier Science B.V. All rights reserved. PII: S0378-4371(99)00311-8