Since late March the excavation works for the new ETH building in Leonhardstrasse include regular blasts (explosions), which are heard and felt in the surrounding buildings. The SED has installed 13 seismic sensors in the NO building to monitor the response to these blasts. NO houses the SED as well as the Department of Earth Sciences, and is located some 10’s of meters from the construction site. Nine sensors were placed in the northeastern edge of the building, one at each floor, and four sensors were distributed across the building in the B, F, H and J floors. The sensors continuously monitored the structure over a two week period. The objectives of the survey are to monitor the amplitude of the building motion and compare its dynamic behaviour at different amplitude levels. With these measurements, it is possible to characterize the response of the building to various different ground motions.
Preliminary analysis of the explosions indicate they induce noticeably higher velocity amplitude than do ambient vibrations. The measured maximum velocity at the basement of the building during a blast is typically 1 mm/s, whereas at the top of the building it is of the order of 0.5 mm/s. For comparison, under normal conditions (ambient motion) the velocity at the basement of the building is 0.003 mm/s and at the top 0.01 mm/s. Despite these high values the blast does not significantly excite the building resonance frequencies since the amplitudes at the roof are smaller than that recorded at the basement. The displacement of the top floor, obtained by integration of the velocity recordings, barely exceeds the ambient vibration amplitudes. The maximum inter-story drift is in the order of 10-7 m/m, whereas cracking typically appears only once this value exceeds 10-4 m/m (1000 times larger motions).
As determined from its response to ambient vibrations, the fundamental resonance frequencies of the structure are between 3 and 4 Hz. In contrast the blast produces energy above 20 Hz, which explains why it doesn’t excite the resonance frequencies of the building.