where sign denotes the signum function, and A , β β > 0 , γ γ and n are dimensionless quantities controlling the behaviour of the model ( n = ∞ ∞ retrieves the elastoplastic hysteresis). Take into account that in the original paper of Wen (1976), β β is called α α , and γ γ is called β β . Nowadays the notation varies from paper to paper and very often the places of β β and γ γ are exchanged. Here the notation used by Song J. and Der Kiureghian A. (2006) is implemented. The restoring force F ( t ) can be decomposed into an elastic and a hysteretic part as follows: