Forced vibration of a 22-story steel frame building

Abstract

The 22-story steel frame structure that houses the offices of the San Diego Gas and Electric Company is a well-knovrn feature of the skyline of San Diego and is a good example of modern, high-rise office construction. Designed by the architectural firm of R. G. Wheeler and Associates, with structural design by Ferver, Dorland and Associates, the building was completed in April 1968. The building, shown in Figure 1, is owned and occupied solely by the San Diego Gas and Electric Company. Because this building typifies many buildings in the major cities of the seismic west coast, and because a multistory office building may hold a thousand people or more during business hours, it is important that the dynamic properties of structures such as this be measured. Experimentally determined dynamic properties are required both to improve the techniques by which such properties are calculated for purposes of design, and to permit interpretation of the measured response of the building in the event of a strong earthquake. As information on the dynamic properties of tall buildings and their earthquake response accumulates, it should be possible to develop better procedures for determining the appropriate levels of earthquake loading for design. The need for special attention to the tall buildings becomes clear when it is realized that the seismic provisions of building codes are not as applicable to the newer tall buildings as they are to older types of construction. In the tests reported below, the San Diego Gas and Electric Company Building was excited by two eccentric-mass vibration generators located on the 20th (equipment) floor and measurements of the structural response were made throughout the building from the roof to the second basement. From these measurements, which are more extensive than in previous tests of this type, it has been possible to determine frequencies, mode shapes and damping values for the first 18 modes of the structure; six each in N-S translation, E-W translation and in torsion. In another portion of the testing program the dynamic properties of the building also were determined by analysis of the measured response of the building to ambient vibration (Trifunac, 1970a). The results of these two experiments and the analytical determination of the building properties (Gobler, 1969) complete the research efforts devoted to the structure. The experimental program was a cooperative effort between the research groups in earthquake engineering at the California. Institute of Technology and the University of California at Los Angeles. The testing, conducted on weekends to minimize disturbance to the occupants, extended over a period of approximately two months and required a crew varying between four and eight.

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