Seismic Analysis for a 12m Antenna Structure

Single point response spectrum (SPRS) analysis in ANSYS®

Figures Courtesy of VERTEX Antennentechnik GmbH 

Your Contact Person: Dr. Marold Moosrainer Phone 08092-7005-45 E-Mail mmoosrainer@cadfem.de

Task

A seismic analysis is frequently required for large buildings or other tall structures particularly at locations in seismic active regions. For that purpose FE analysis is applied to proof the integrity of the structure during an earthquake. Here, the 12m ALMA (Atacama Large Millimeter Array) telescope (Fig. 1) has been analyzed.

Solution

A response spectrum for the given earthquake location is defined e.g. by government authorities (Fig. 2). Now, the analyst’s task is to perform a modal analysis first to get the eigenfrequencies and mode shapes of the structure. After that the mode shapes are scaled according to the given response spectrum. A mode combination method like SRSS or CQC is applied to get the final result, for instance in terms of maximum displacement (Fig. 3). This way a response spectrum analysis does not compute the whole transient time history of the earthquake excitation. Rather it is a simplified method to get the maximum response. Large models with more than some million degrees of freedom can be analyzed this way. A special filtering technique may be used in order to suppress negligible modes. Usually a certain percentage of the overall structural mass is required to participate in oscillation. For that purpose modal participation factors and modal effective mass output from ANSYS may be used.

Horizontal Response Spectrum ALMA_ENV6_MLE ξ=1.5% log-log scale

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Frequency [Hz]

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Fig. 2: Specified response spectrum specified in a log-log scale. This is the input for an earthquake analysis.

Fig. 1: ALMA 12m telescope.

Fig 3: SRSS displacement for horizontal earthquake excitation.