Blind identification of the Millikan Library from earthquake data considering soil-structure interaction

The Robert A. Millikan Library is a reinforced concrete building with a basement level and nine stories above the ground. Located on the campus of California Institute of Technology (Caltech) in Pasadena California, it is among the most densely instrumented buildings in the U.S. From the early dates of its construction, it has been the subject of many investigations, especially regarding soil-structure interaction effects. It is well accepted that the structure is significantly interacting with the surrounding soil, which implies that the true foundation input motions cannot be directly recorded during earthquakes because of inertial effects. Based on this limitation, input-output modal identification methods are not applicable to this soil-structure system. On the other hand, conventional output-only methods are typically based on the unknown input signals to be stationary whitenoise, which is not the case for earthquake excitations. Through the use of recently developed blind identification (i.e. output-only) methods, it has become possible to extract such information from only the response signals because of earthquake excitations. In the present study, we employ such a blind identification method to extract the modal properties of the Millikan Library. We present some modes that have not been identified from force vibration tests in several studies to date. Then, to quantify the contribution of soil-structure interaction effects, we first create a detailed Finite Element (FE) model using available information about the superstructure; and subsequently update the soil-foundation system's dynamic stiffnesses at each mode such that the modal properties of the entire soil-structure system agree well with those obtained via output-only modal identification. Copyright (c) 2015 John Wiley & Sons, Ltd.