BRI Research Paper


No.108

Dynamic Characteristics of A Full-Scale Seven-Story Reinforced Concrete Building. -Part of The U. S.-Japan Cooperative Research Program-.

Y.Kitagawa*1, M.Midorikawa*2, T.Kashima*3, T.Hamamatsu*4; May, 1984. 190p.

Abstract

This paper presents the structural dynamic properties, such as natural periods of vibration, mode shapes and critical damping ratios,which were obtained from vibration tests on a full-scale seven-story R/C building. Four types of dynamic tests were carried out five times throughout all test programs and consisted of: i) observation of natural earthquake responses, ii) observation of microtremor excited vibrations, iii) free vibration tests, and iv) forced vibration tests.

Free vibration tests were made by "pull-back and quick release"method. This test was repeated seven times with applied tension between 0.5 and 3.5 tons at the roof level in each free vibration test. Forced vibration tests were executed with the use of two rotating eccentric weight exciters so that the translational and torsional vibrations were generated. One was a big exciter and the other was a small exciter.The translational vibration was generated by the former in the lonaitudinal or transverse direction, while the torsional vibration was produced by the latter. In torsional vibration tests, the sweep up technique around the torsional natural frequency was adopted because of very low level excitations.

The pseudo-dynamic seismic tests comprised two phases. In the first phase, the building was tested under various earthquake conditions. The damaged building due to phase I tests was repaired, and a variety of nonstructural elements was installed in the repaired building. In the subsequent second phase, effectiveness of repair and interaction between the building and nonstructural elements was examined. Consequently, vibration test results throughout all test programs showed an interesting change of dynamic characteristics of the building in various stages.

In addition to vibration tests, inelastic dynamic response analyses were done by considering effects of strain rate and stress relaxation during the phase I pseudo-dynamic tests.

From Experimental and analytical results especially in the longitudinal direction, it is concluded as follows:

1) Before the pseudo-dynamic tests in phase I, that is, in the initial elastic range, the correlation among the free and forced vibration tests and the earthquake record was excellent with respect to the natural periods.
2) According to the free and forced vibration tests, the fundamental natural period of the building was 0.43 sec. before the phase I pseudo-dynamic tests. After the phase I pseudo-dynamic tests,repair, installing nonstructural elements, and final failure, the fundamental natural periods were 0.80`0.91 sec., 0.57`0.63 sec.,0.45`0.52 sec. and 0.86`0.96 sec., respectively.
3) An increase in the fundamental natural period of the building occurred gradually with the increase of response displacement except for the initial elastic range.
4) The critical damping ratio increased with the increase of response displacement except for the initial elastic range. It should be noted that the damping ratio after installing nonstructural elements was the largest among all vibration tests.
5) The equivalent story stiffness was distributed along a triangular shape before the phase I pseudo-dynamic tests. However, this distribution was changed to a uniform shape as the damage of the building progressed.
6) In case of discussion on earthquake performance of a structure from a dynamic response analysis, it is pointed out that the dynamic restoring force characteristics should be evaluated accurately.

*1Head, Structural Dynamics Division, Structural Engineering Department
*2Research Engineer, Structure Division, Structural Engineering Department
*3Assistant Research Engineer, Structural Dynamics Division,Structural Engineering Department
*4Engineer, Building Division, Government Buildings Department, Minister's Secretariat; Former Assistant Research Engineer, Structure Division, Structural Engineering Department




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