Field Vibration Testing & Analyses of an Existing RC Building

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

The lack of adequately high spatial resolution data sets from field testing of full-scale structures under dynamic loading conditions significantly limits the ability of earthquake engineers to improve their understanding of the seismic performance of structural and geotechnical systems. For example, field observations of highly nonlinear structural response following major seismic events have been largely supported with qualitative observations. Instrument recordings from buildings have been an invaluable resource over the past few decades; however, the density of instruments (with ~12 per structure being a common allocation) is too low to enable detailed evaluations of system or component responses. To address this shortcoming, the Civil Engineering Department at UCLA is in the process of developing equipment to substantially enhance field testing capabilities for forced-vibration testing as part of the NSF Network for Earthquake Engineering Simulation (NEES) program. The project, entitled Field Testing and Monitoring of Structural Performancece, was initiated in January 2000 and continues through September 2004. Field testing equipment being purchased consists of state-of-the-art vibration sources (linear inertial and eccentric mass shakers), sensors and a wireless data acquisition system (accelerometers, LVDTs, fiber optic gage readout unit, digitizers, routers, workstations), a CPT truck and in situ soil vibration sensors, and networking equipment (data concentrator, satellite uplink station) to allow for real-time data acquisition, processing, and world wide web broadcasting of experimental results. The project involves only purchase and integration of the equipment; no specific research activities are being conducted with the project funds. Through collaboration with a practitioner-based Implementation Advisory Board (IAB) for the UCLA/NEES project, a four-story office building in Los Angeles has been identified as a candidate for forced vibration testing. The building is of general interest because (1) it has a very common structural system consisting of a perimeter moment frame and an interior post-tensioned slab-column frame, and (2) it was significantly damaged during the Northridge earthquake, causing it to be abandoned. Previous analysis-based post-earthquake studies of the building have generally been unable to explain the observed damage. The owners plan to demolish the structure; however, for a variety of reasons, will not do so for approximately two years and have granted the PIs permission to conduct testing. The proposed research involves forced vibration testing of the building using the NEES equipment as well as detailed system identification and numerical modeling studies. Both earthquake-type and harmonic force histories will be applied to the building at the roof level, and waveforms will be recorded to enable evaluations of the complete structural system response, structural component behavior (e.g., slab-column connections and pile caps), and the response of non-structural components (e.g., partitions, piping systems) as well as the influence of these components on the system response. A dense instrumentation array will be utilized that will include accelerometers, displacement transducers, and strain gauges. A wireless local area network will be installed to acquire and record waveform data,. The detailed data set generated from the tests will provide valuable insight into the performance of this common type of structural system and the reasons for the poor performance of the subject building during the Northridge earthquake. The testing will also provide invaluable experience to the NEES program with respect to equipment use protocols and data archiving models. Gaining experience on these issues within the present funding period is particularly useful as the NEES equipment development work is ongoing, and lessons learned can be implemented prior to the NEES system going online in 2004. Also relevant to the timeliness of this proposal is the fact that this important building will be razed in two years.