Variability in the natural frequencies of a nine-story concrete building from seconds to decades

Ethan F. Williams, Thomas H. Heaton, Zhongwen Zhan, & Valere R. Lambert

Submitted September 11, 2022, SCEC Contribution #12402, 2022 SCEC Annual Meeting Poster #273

Since 2001, the Southern California Seismic Network has archived continuous waveform data from strong-motion station CI.MIK in Caltech Hall (formerly Millikan Library), a nine-story reinforced concrete building in Pasadena, CA. Simple spectral analysis of this twenty-year record reveals that the building's fundamental frequencies have gradually increased by 5.1% (E-W) and 2.3% (N-S), with larger long-term variability up to 9.7% (E-W) and 4.4% (N-S). This finding is unexpected, as previous analysis of forced vibration tests and strong-motion records has shown that between 1968 and 2003 the fundamental frequencies decreased by 22% (E-W) and 12% (N-S), largely attributed to minor structural damage and soil-structure system changes from earthquakes. Today, the building's apparent stiffness is comparable to what it was in 1986, before the Whittier Narrows earthquake. Using data from earthquakes and forced vibrations, we also document the building's nonlinear dynamic elasticity, which is characterized by a rapid softening (decrease in apparent frequencies) at the onset of strong motion, followed by a slower, log-linear recovery trend over the scale of minutes. This nonlinear behavior does not appear to have changed with time.

We conclude: (1) linear, time-invariant elasticity is an exceedingly poor assumption, as power-law nonlinear elasticity persists down to the level of ambient vibrations; and (2) continuous waveform recordings are necessary for structural health monitoring and post-earthquake damage assessment, as passive variations in apparent structural stiffness exceed the changes induced by significant historical earthquakes.

Key Words
Engineering seismology, strong motion, structural health monitoring

Williams, E. F., Heaton, T. H., Zhan, Z., & Lambert, V. R. (2022, 09). Variability in the natural frequencies of a nine-story concrete building from seconds to decades. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Engineering Implementation Interface (EEII)