Poster #233, Earthquake Engineering Implementation Interface (EEII)

Structural healing of Millikan Library over 20 years of continuous seismic monitoring

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Poster Presentation

2020 SCEC Annual Meeting, Poster #233, SCEC Contribution #10562
Since 2001, the Southern California Seismic Network has archived continuous waveform data from strong motion station CI.MIK in Caltech’s Robert A. Millikan Memorial Library, a nine-story reinforced concrete building. A simple spectral analysis of this 20-year record reveals that the fundamental frequency of Millikan Library’s east-west resonance has increased gradually and monotonically by 6.8%, corresponding to a ~14% increase in structural stiffness. This finding is unexpected, as previous analysis of forced vibration tests and earthquake strong ground motion records has shown that between 1968 and 2003 this same frequency decreased by 22%. The historical weakening of Millikan Library ...has been attributed to minor structural damage and changes to the soil-structure system during the 1971 M6.6 San Fernando, 1987 M6.1 Whittier Narrows, 1991 M5.8 Sierra Madre, and 1994 M6.7 Northridge earthquakes. Today, the library’s apparent structural stiffness is comparable to what it was in 1986, before the Whittier Narrows earthquake. While some of the incremental changes in frequency may have been caused by interior renovations to the building, in particular a reduction in mass during its conversion from a library into an office space around 2003, the quasi-linear healing trend over the past two decades is largely a mystery. In this poster, we examine the 20-year record of ambient and forced vibrations in detail and discuss competing evidence that the healing trend is either due to changes in soil-structure interaction or the integrity of the structure itself. We also look at the response of Millikan Library to the 2019 M6.4 and M7.1 Ridgecrest earthquakes, during which the E-W fundamental frequency dropped by 17%, followed by a period of rapid log-linear recovery.