2019 CSCE Annual Conference - Laval (Greater Montreal) Conference
Dr. Bujar Morava, RWDI Inc.
Modern tall buildings are often slender, lightweight, and possess low inherent damping, which can lead to excessive wind-induced motion. This excessive motion can cause occupant discomfort on the upper floors of the building due to high accelerations, or decrease the longevity of the partition walls and façade system due to large inter-floor drifts. Dynamic vibration absorbers (DVAs) in the form of tuned mass dampers (TMDs) and tuned sloshing dampers (TSDs) are being commonly employed to increase the effective damping of structures, thereby reducing their resonant responses under wind excitation. In its simplest form, a DVA can be represented as an auxiliary spring-mass-dashpot oscillator that is coupled to the primary structure. When the structure experiences motion, the DVA mass will move out-of-phase with the structure, thereby opposing its motion.
While the behaviour and performance of structure-DVA systems has been studied extensively theoretically as well as experimentally (at scale-model) in the laboratory, very few studies have reported on the as-built performance of full-scale implementations. The performance of a DVA is typically quantified using the concept of “effective damping”, which is the amount of damping that a structure appears to have after being equipped with a DVA. Until recently, it has been difficult to measure the effective damping of a structure, since the coupled structure-DVA system is excited through unknown, ambient excitation. The popular traditional random decrement technique is only valid for simple structures that are not equipped with DVAs. Other system identification methods have been proposed to estimate the dynamic properties of the structure and DVA, and then back-calculate the theoretical effective damping. However, these methods are quite complicated, and there is a desire for a practical performance evaluation technique.
In this study, a method is presented that enables the inherent structural damping (that is, the damping of the structure without the DVA), as well as the added effective damping (the damping that the DVA appears to add to the structure) to be estimated from full-scale ambient measurements of a structure-DVA system. The method requires the DVA and the structural generalized masses to be known and structural and DVA responses to be measured. With this basic information, the motion reduction achieved by the implementation of the DVA is estimated. After the method is briefly presented, the remainder of the paper focuses on real life applications where the efficacy of DVAs implemented in full-scale tall buildings is presented.