Buildings Undergoing Complex Motions: Accelerations and Human Comfort

The design of tall buildings, while concerned with efficiently and effectively carrying the anticipated loads imposed upon the structures, is largely concerned with serviceability and habitability issues. The latter issue may still require attention, even for buildings designed to meet lateral drift requirements (Kareem et al. 1999). As such, structural accelerations, as a function of return period, must be considered to ensure acceptable occupancy comfort in buildings. The international community, however, has not agreed as to whether RMS or peak values should serve as a universal descriptor of acceleration (Tall Buildings Committee/SEI: www.nd.edu/~dynamo). On one hand, negative reactions by occupants may be a result of a sustained motion, best described by an averaged effect like RMS. The other viewpoint asserts that perception and reaction to motion is most dramatically affected by large events better characterized by peak values. While humans are capable of adjusting to accelerations, any change in the acceleration will require additional adjustments for equilibrium. For this reason, ergonomical studies concerning vibration and comfort suggest that motions containing impulsive vibration or shocks may lead to higher discomfort. Furthermore the transient nature of such impulsive motions repeated at frequent intervals may compound this effect. As a result, basing perception criteria on a measure of jerk, or rate of change in acceleration, may better capture the stimulus defining perception thresholds under random motion. The adoption of such a criterion may altogether override the need for the peak acceleration criteria.