As widely experienced in many research activities, both concrete and steel exhibit different behavior under dynamic and static load conditions. It is possible to reach higher levels of ultimate stresses and strains under dynamic loads; this effect becomes more pronounced as strain rate increases. In case of structural analyses under particular load conditions, such as explosions or impact loading, both inducing strain rates from 10¹ s^-1 to 10³ s^-1, this aspect cannot be ignored. In order to correctly evaluate mechanical behavior of materials under such conditions, several numerical models are proposed that are mainly based on updating of static constitutive models. However, also in case of seismic actions which pertain to a medium range of strain rate from 10^-2 s^-1 to 1 s^-1, the differences with static conditions are quite significant. In this study,  the focus has been on the medium range of strain rate. The constitutive relationships for concrete and steel have been updated, according to available formulations. As a numerical example, an existing RC structure has been considered and the induced strain-rate level has been related to the curvature rate. Moment-curvature relationships have been derived as a function of axial load level and curvature rate and a non-linear static analysis has been performed. Finally, a seismic assessment has been performed through an Incremental Dynamic Analysis of the equivalent SDOF system, conducted both in case of strain rate sensitivity and in case of quasi static material properties; hence the seismic fragility curves have been derived, obtaining significant differences between the investigated structural systems.