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This two-story frame is a simple structure with two distinct lateral modes of vibration. Fixed-fixed columns provide lateral stiffness and steel floorplates provide a relatively heavy concentrated mass at each floor level. Multi-story structures are common in civil engineering and often have modes of vibration that are important in the detailed analysis for seismic and wind loading. This particular structure was measured with a mobile phone accelerometer at the first floor level as well as LiDAR measurement of the first floor. The results of these two measurements are provided here.
Frequency spectrum with phone at 1st floor with mode 1 displaced and released into free vibration. Natural frequency of first mode appears to be 4.35 Hz.
Frequency spectrum with phone at 1st floor with mode 2 displaced and released into free vibration. Natural frequency of second mode appears to be 10.47 Hz.
Displacement response and frequency spectrum with LiDAR patch at first floor with mode 1 displaced and released into free vibration. The natural frequency of the first mode appears to be 4.34 Hz.
Displacement response and frequency spectrum with LiDAR patch at first floor with mode 2 displaced and released into free vibration. Natural frequency of second mode appears to be 10.92 Hz.
Note the peak displayed at 4.11 Hz. This is an alias of the 10.92-Hz frequency that appears based on the up-sampling of LiDAR measurements in the iPhone from a true sampling frequency of 15 Hz to 60 Hz. This up-sampling is done as part of the data fusion process to match the frame rate (60 fps) of the RGB video camera on the phone.
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