Abstract
Results are reported of earthquake simulation tests on a model frame with a partial base isolation system that includes energy-absorbing devices. The isolation system was modeled on a stepping bridge concept developed for the New Zealand Railways, and the energy-absorbing devices, based on the plastic torsion of rectangular mild steel bars, functioned only when the frame base lifted off the foundation. Two series of tests using scaled accelerations from the El Centro N-S 1940 and Pacoima Dam 1971 earthquake ground motion records were used as input to the shaking table on which the tests were performed. Results from these tests are compared to those from earlier tests on an identical frame with the foundation (1) anchored as in conventional design, and (2) permitted to uplift freely. The response of the frame with the energy-absorbing devices installed was improved over that of both the fixed frame and the frame allowed to uplift freely for the El Centro accelerations. Although the results are not as favourable for the Pacoima Dam input, the feasibility of the energy-absorbing devices associated with a partial base isolation system is established as an alternative to anchored frames and frames allowed to uplift freely.
Highlights
Structural elements that support pseudo-static loads must absorb dynamic loadings from lateral forceproducing phenomena such as earthquakes
Two earthquake records were used to produce the ten ground motion simulations used in the testing program: six scaled signals based on the El Centro N-S 1940 horizontal component, and four based on a horizontal component of the Pacoima Dam record of the 1971 San Fernando Valley Earthquake
EXPERIMENTAL RESULTS results of tests run using the scaled accelerations from the El Centro and Pacoima Dam ground motion records are compared for the three model frame base conditions considered in this report: fixed, free to uplift, and free to uplift with energy-absorbing devices installed
Summary
Structural elements that support pseudo-static loads must absorb dynamic loadings from lateral forceproducing phenomena such as earthquakes. The mechanical devices absorb energy during uplift by mechanical moment arms coupled to the base of each pier This partial isolation system can be applied to general structural frames not designed to be bolted to foundations, but allowed to step off their footings. A partial isolation system that can accommodate energy-absorbing devices[3,4] is the stepping support foundation system, generalised from the stepping bridge concept developed by Beck, Skinner, et al, for a viaduct of the New Zealand Railways ^ (the South Rangitikei Bridge under construction) In a recent series of tests, a three-storey, four column model steel frame[3,4,12,13] designed to uplift from its footings was fitted with energy-absorbing devices and subjected to several earthquake simulation loadings on the twenty-foot square shaking table at the Earthquake Simulator Laboratory at the Earthquake Engineering Research Center, University of California, Berkeley. It is proposed that the use of the energyabsorbing devices be considered as a compromise between the concepts of a fixed base frame and a frame totally free to uplift
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