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Dynamic behavior of a high-rise diagonally braced steel building

Rea, Dixon; Shah, A. A.; Bouwkamp, Jack G.

UCB/EERC-71/05, Earthquake Engineering Research Center, University of California, Berkeley, 1971-08, 97 pages (545.1/R35/1971)

A building of novel design featuring a diagonally braced steel framework has been the subject of forced vibration tests. The building is 27 stories (379 ft) high and braced diagonally between the 2nd and 26th floor levels. Between the foundation and 2nd floor levels, there are vertical columns stiffened laterally by shear walls. In plan, the building is symmetrical about two axes (approximately E-W and N-S) with outside dimensions of 210 ft by 109 ft. Elevator shafts and stairwells are contained in a central core area of dimensions 101 ft by 42 ft. The building was vibrated in its first two translational modes in both the E-W and N-S directions as well as in its first two torsional modes by means of eccentric-mass type vibration generators. Resonant frequencies determined were: E-W, 0.453 and 1.405 cps; N-S, 0598 and 1.685 cps; torsion, 0.895 and 2.350 cps. The damping factors evaluated from the associated resonance curves were: E-W, 0.9 and 2.0%; N-S, 1.1 and 2.3%; torsion, 0.7 and 1.3%. A mathematical model of the building for E-W translational motion was formulated from the structural drawings. Various parameters in the model were adjusted until resonant frequencies and mode shapes matched the corresponding experimental values. Due to the geometry of the framework a model with twelve effective degrees of freedom was found adequate to define the behavior of the 27 floors of the building. Although various combinations of parameters were capable of matching the resonant frequencies, the final choice of model was governed by the mode shapes. One of the significant conclusions of the study was that in order to match the mode shapes accurately, the effects of the shear walls, the central core, and foundation compliance had to be included. The final phase of the investigation was the determination of the response of the mathematical model (with various amounts of damping up to 5% of critical) to the El Centro (1940) earthquake ground acceleration. In all cases the linear analysis indicated that yielding would occur in some diagonal members. However, a nonlinear analysis would be required to determine if yielding would also occur in the corner columns.

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