A system is described, aiming at reducing the seismic excitation of structures, based on pure struc- tural solutions. The basic idea is to exploit the phase lag of the incident seismic waves along the foundation and, accordingly, to design it in order to possess the adequate stiffness and strength. The longer the foundation is, the larger the phase lag becomes. It is, therefore, well understandable that under this requirement, foundations of the maximum possible length must be designed. The presented methodology might be proved quite valuable for exist- ing structures and especially for monuments, where, in most cases, it is not possible to proceed to the necessary strengthening interventions in the structure above its foundation. As a technical support of the present investigation, the size of the foundation as a two dimensional elastic beam and the velocity of the propagation of the ground motion are examined as basic parameters. Two strong ground motions have been used, each one with quite differ- ent characteristics compared to the other one: an artificial time history of rather high frequency, fitting to EC8, Type 1, ground class A and a natural ground motion of the Edessa, Greece 1990, M = 5.9, earthquake. The Edessa earthquake is characterized by much longer predominant periods of vibration compared to the artificial one. Vari- ous lengths of the foundation beam have been examined in combination with the velocity of the propagation of the ground motion along the longitudinal direction of the beam. The achieved motions at the center of gravity of the beam as well as the pertinent response spectra are calculated. These spectra are compared to the free field ones. At the beginning of the paper, it is tried to explain the inconsistency between macroseismic observations and earth- quake code requirements concerning the effects of the size of the building foundation. At the end of the paper, the results of the described methodology are demonstrated in several practical case studies.
