Mitigation of low-velocity, wind-induced vibration of an architectural spire

Abstract

This paper presents the results of analytical and experimental studies conducted on an architectural spire, which experienced wind-induced vibrations shortly after its construction. The circular spire is attached to the corner of a 30-story building along the upper ten stories then cantilevers up for 21.44 m (70.33 ft) with a pipe cross-section of 0.508 m (20 in)-diameter for the first 3.86 m (12.67 ft) and 0.406 m (16 in)-diameter for the last 17.58 m (57.67 ft). Experimental studies showed that the spire's first natural frequency was 0.88Hz with a damping ratio of 0.1-0.2 % of the critical value. Average wind speed during vibrations, recorded on ground within 8 km (5 miles) of the site was 1.79 m/s (4.0 mph). Vortex-shedding was indicated as the preliminary cause, since the projected wind velocity at the spire elevation was low and compatible with the Strouhal relationship based on the 0.406 m (16 in)-diameter region of the spire, which yielded a vortex-shedding frequency of 0.88Hz (=0.2*1.79/0. 406). Due to time limitations, only two mitigation techniques were considered. First, a chain damper was installed inside the pipe, which increased the damping ratio to at least 2.0%. Finally, the mass of the spire was increased by sand-infilling, which reduced the first-mode frequency to 0.69Hz and the wind speed to 1.39 m/s (3.1 mph) for possible "lock-in"; most importantly, an increment in the Scruton number was calculated. Hence, the susceptibility to vortex-shedding decreased.