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Structural Modal Testing - ANCO MK-18 Eccentric Mass Vibrator
Structural Modal Testing - ANCO MK-4600 Eccentric Mass Vibrator

In order to measure structural damping values appropriate for use in wind induced vibration fatigue design, ANCO performed forced vibration tests on a 125 foot Braden Manufacturing steel exhaust stack. This stack serves a Westinghouse 100 MWe combustion turbine at the South Carolina Electric and Gas Company Hagood plant in Charleston Heights, South Carolina.

General industry guide lines (ASME STS-1-1992) suggest that the first mode damping in lined steel stacks can be as low as 0.3% to 1.0%. Braden, of Tulsa Oklahoma, has supplied scores of these stacks, and suspected that stack damping would be higher, and hence have lower wind vortex shedding response and higher fatigue design margin. ANCO performed forced vibration tests on the stack using a sinusoidal eccentric mass vibrator mounted near the top of the stack. The vibrator produced forces up to 5 tons and stack response levels up to 0.4 g.

The tests showed that the Braden stacks had first horizontal mode damping (in each direction) in excess of 2.0% at response levels above about 0.3 g. Consequently, Braden is now confident of higher wind fatigue margins in their stack designs, which result in a more robust and economical structure.