A loophole in the Eurocode 8 allowing for non-conservative seismic design

The prescriptions of the Eurocode 8 (EC8) concerning the static method of seismic analysis may lead to non-conservative design with respect to the more rigorous modal response spectrum analysis. The reason is the unconditioned reference the EC8 makes to the correct value of the fundamental period of vibration of the building. This can produce non-conservative results, especially when the method is applied to buildings that are not regular in plan. The present paper shows how this can happen by referring to the seismic analysis of a series of buildings with different eccentricities between center of mass and center of stiffness. The result should prompt appropriate changes to be introduced in the EC8. The paper also hints at some possible routes to avoid this shortcoming.