The estimation of structural modal parameters from the responses of the Gisborne Post Office building in recent earthquakes

  • J. X. Zhao GNS Science, Lower Hutt, New Zealand

Abstract

Two complete sets of records obtained from a strong-motion accclerograph array in the Gisborne Post Office building during earthquakes of 1982 and 1993 were analyzed by using systems identification techniques to estimate the modal parameters. Nonlinear responses of the soil-structure system in both horizontal and vertical directions were indicated by significant decreases in the estimated first mode frequencies for moderate to strong ground motion in 1993 compared to those for the small amplitude ground motion in 1982. The analysis of second floor and remote site records from a 1985 earthquake indicated that nonlinear responses also occurred then. Soil-structure interaction was found to be significant for both vertical and horizontal responses. Using a numerical model of the foundation-soil system, the foundation rocking frequency and first fixed-base modal frequency were estimated. It was found that frequencies of the building's first fixed-base mode, foundation translation and rocking all were reduced in the 1993 earthquake compared with those of the 1982 earthquake, indicating that nonlinear responses occurred in both the building and the soil in the 1993 earthquake. Possible evidence for a cutoff frequency at the soil site was also identified. Because of soil-structure interaction, the fixed-base horizontal response in the N44E direction was reduced by possibly 40-50% in the 1982 earthquake, and about 30% in the 1993 event, compared with the recorded first modal response of the soil-structure system.

References

Cousins, W.J., Hefford, R.T., McVerry. G.H. and Downer, R.M. (1887), Computer analyses of New Zealand earthquake accelerograms 1981-1985, Volume 2, PEL. DSIR, Lower Hutt, New Zealand

Cousins, W.J., Perrin, N.D., McVerry. G.H., Hefford R.T., and Porritt, T.E. (1996), Ground conditions at Strong-motion recording sites in New Zealand, Science Report 96/33, Institute of Geological & Nuclear Sciences.

Barker, P. (1994), A Report on Cone Penetrometer and Seismic Cone Penetrometer Testing at Selected Sites in Lower North Island, Prepared for Dr. G McVerry, Institute of Geological and Nuclear Sciences, April.

Cousins, W.J., Porritt, T.E., Hefford, R.T., Baguley, D.E., O'Kane, S.M. and McVerry, G.H. (1994), Computer analyses of New Zealand earthquake accelerograms, Volume 8, The Tikokino, Fiord/and, and Ormond Earthquakes of 1993, Institute of Geological & Nuclear Sciences, Lower Hutt New Zealand.

Read. S.A.L. and Cousins. W.J. (1994), The Ormond Earthquake of 10 August 1993: an overview of ground damage effects and strong motions, Proceedings of Technical Conference and AGM. New Zealand National Society for Earthquake Engineering. Wairakei, 18-20 March, 166-175.

Wolf. J.P. (1985). Dynamic soil-structure interaction, Prentice-hall, Englewood Cliffs. New Jersey.

McVerry, G.H. (1980). Structural identification in the frequency domain from earthquake records. Earthquake Engineering and Structural Dynamics, 3. 161-180 DOI: https://doi.org/10.1002/eqe.4290080206

McVerry. G.H. (1984). Comparison of remote site and basement records as excitation of the Vogel building, Bull. of N.Z. Soc. for Earthquake Eng. 17. 3-14

Novak, M. and Hifnawy, L. EL (1983). Effect of soil-structure interaction on damping of structures, Earthquake Engineering and Structural Dynamics, 11. 595-621.

McVerry. G.H. (1979), Frequency domain identification of structural models from earthquake records, Report No. EERL79-02, Earthquake Engineering Research Laboratory. California Institute of Technology. Pasadena. California.

Zhao. J.X. McVerry. G.H. and Cousins W. J. (1995), Horizontal responses of Gisborne Post Office building in two earthquakes. Proceedings of Technical Conference of New Zealand National Society for Earthquake Engineering. Rotorua. New Zealand. March. 69-76.

Nakamura. Y. (1989). A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface. Quarterly Review of Railway Technical Research Institute. 30. 25-33.

Zhao. J.X. (1995). Vertical soil-structure interaction of the Gisborne Post Office building, Proceedings of Pacific Conference on Earthquake Engineering. 3, 227-236.

Zhao. J.X. (1976). Effects of frequency dependent foundation-soil compliance on building vertical responses identified from earthquake records, Soil Dynamics and Earthquake Engineering. 16, 273-284.

Gazetas. G. (1991). Formulas and charts for impedances of surface and embedded foundations, Journal of Geotechnical Engineering. ASCE. 117, 1363-1381.

Zhao, J.X., Carr. A.J. and Moss. P.J. (1997), Calculating the dynamic stiffness matrix of 2-D foundations by discrete wave number indirect boundary element methods. Earthquake Engineering and Structural Dynamics. 26, 115-134. DOI: https://doi.org/10.1002/(SICI)1096-9845(199701)26:1<115::AID-EQE626>3.0.CO;2-0

Published
1998-03-31
How to Cite
Zhao, J. X. (1998). The estimation of structural modal parameters from the responses of the Gisborne Post Office building in recent earthquakes . Bulletin of the New Zealand Society for Earthquake Engineering, 31(1), 51-63. https://doi.org/10.5459/bnzsee.31.1.51-63
Section
Articles

Most read articles by the same author(s)