A Twenty-Story Instrumented Building Response to M7 Anchorage, Alaska Earthquake

Date: 4/26/2019

Time: 02:00 PM

Room: Cascade II

We examined the dynamic responses of a twenty-story steel moment-resisting frame building in downtown Anchorage, Alaska, during and after the moment magnitude 7 Anchorage earthquake. This regular-plan mid-rise structure (Atwood Building) is instrumented by the USGS with a 32-channel accelerometer array at ten levels. We computed impulse response functions (IRFs) based on waveforms recorded from the main shock and six aftershocks. The aftershocks have moment magnitudes ranging from 4.2 to 5.7. We determined the building’s fundamental frequencies and mode shapes by using a singular-value decomposition of multiple reference frequency-response functions. The traveling waves, identified in IRFs with a virtual source at the roof, are used to estimate the shear-wave velocity (V_S) in the building. The median V_S from the IRFs of the seven earthquakes is 179 m/s for the east-west (EW), 200 m/s for the north-south (NS), and 165 m/s for the torsional responses. The intrinsic attenuation associated with the fundamental modes are identified by the IRFs with a virtual source at the ground. The building’s average intrinsic-damping ratio is estimated to be 4.6% and 3.9% in the 0.2­­–1 Hz band for the EW and NS directions, respectively. These results are generally in agreement with our previous findings, which were based on earthquakes between 2005 and 2014 (Wen and Kalkan, 2017), indicating that the response of the building to the mainshock remained in linear-elastic range—a conclusion, also confirmed by visual inspections after the earthquake.

Presenting Author: Erol Kalkan

Authors