Asymmetric hysteretical curves appear due to the asymmetry of the mechanical properties of the tested element, of the geometry or of both. Song and Der Kiureghian (2006) observed that the hysteresis loops are often affected not only by the signs of the velocity u ˙ ˙ ( t ) and the hysteretic displacement z ( t ) but also by the sign of the displacement u ( t ) , because the hysteretic behaviour of a structural element in tension can be different from that in compression. Therefore, Song and Der Kiureghian (2006) proposed the following function for modelling those asymmetric curves: