Energy Radiation from a Multi-Story Building

Damping limits the resonance of vibrating systems and thus higher anelastic damping is generally favored for engineered structures subjected to earthquake motions, because it means that a structure can dissipate a larger percentage of its energy per oscillation cycle. However, there are elastic processes that can mimic the effects of anelastic damping. In particular, buildings lose kinetic energy when their motion generates elastic waves in the Earth, which is referred to as radiation damping. Unlike anelastic damping, strong radiation damping may not always be desirable, as reciprocity can be used to show that buildings may be strongly excited by elastic waves of similar characteristics to those generated by the building's forced vibrations. As a result, it is important to quantify the radiation damping of structures to be able to improve their design. Several experiments using Caltech's nine story Millikan Library as a controlled source were performed to investigate the radiation damping of the structure. The building was forced to resonate at its North-South and East-West fundamental modes, and seismometers were deployed around the structure in order to measure the waves generated by the library's excitation. From this "local" data set, we determine the elastic properties of the soils surrounding the structure and estimate what percentage of the total damping of the structure is due to energy radiation. Using Fourier transforms, we were also able to detect these waves at distances up to 400 km from the source using the broadband stations of the Southern California Seismic Network. This "regional" data set is used in an attempt to identify arrival times and to constrain the type of waves being observed at regional distances.