Measurement of the natural frequencies of a stack through vibration generated by the environment

Fig. 15-19: A multiflare stack

Environmental effects such as air movements around a structure can also induce vibrations, though such vibration may be very small. These types of vibration may not be exactly free vibration as they are caused by disturbances due to external effects, such as wind. However, the concept of free vibration can still be used to identify the natural frequencies of a structure.

Fig. 15-19 shows a 97.5m tall multiflare stack which can be used to burn off excess gases. A laser test system was set up approximately 100m from the stack to monitor the stack vibrations. Several velocity-time histories on selected measurement points of the stack were measured when wind was blowing.

The natural frequencies of 0.67 Hz and 0.73 Hz were measured which corresponded to the fundamental modes in the two orthogonal horizontal directions.