Combined Asymptotic Method in Seismic Analysis of High-Rise Building: Specifics of Low-Frequency Structure

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Rubric:     Theoretical and experimental studies   
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Combined Asymptotic Method (CAM) has been developed and used for SSI analysis of stiff and heavy NPP structures. However, some of civil structures nowadays are comparable with NPP structures in terms of size and mass. The question is whether SSI is significant for such structures and whether it should be accounted for in the design process. In the previous two papers CAM was applied to the seismic analysis of the high-rise structure resting on a sample homogeneous half-space modeling soil. The overall impact of SSI or, in other words, the applicability of the fixed-base seismic analysis was studied. In the present paper two specific features are investigated which make horizontal/rocking seismic response of the high-rise building completely different from the vertical response of the same building and from the total response of the conventional NPP buildings. Maximal integral horizontal forces under the base are several times less than vertical force though maximal horizontal base accelerations are greater than vertical one. Transfer functions to the horizontal base response show little peaks at the lowest natural frequencies corresponding to the dominant modal masses. Rocking stiffness of the soil proves to be more important than the horizontal one. Due to the low first natural
frequencies, the response along the first modes is overcritical and out of phase – as a result, the corresponding modal mass is subtracted from the total “rigid” mass. Base rocking is also out of phase and leads to the decrease in deformations in the lower part
of structure.