Damping Strategies for Structural Systems

The direction of the evolution of tall building structural systems, based on new structural concepts with newly adopted high-strength materials and construction methods, has been towards augmented efficiency. Consequently, tall building structural systems have become much lighter than earlier ones. This direction of the structural evolution toward lightness, however, often causes serious structural motion problems – primarily due to wind-induced motion. From the viewpoint of structural material’s properties, due to the lag in material stiffness compared with material strength, the serviceability of the structure potentially becomes a governing factor in tall building design when high strength material is used. For instance, today, structural steel is available from 170 to 690 MPa (24 to 100 ksi). However, its modulus of elasticity remains nearly the same without regard to the change in its strength. The change of production process or heat treatment influences its strength but not the modulus of elasticity. Regarding concrete, increase in its strength results in increase in its modulus of elasticity, albeit increasing its brittleness. However, this increase in the modulus of elasticity is relatively small compared with the increase in strength. Thus, the lighter structures produced by high-strength materials can cause motion problems. The control of this structural motion should be considered with regard to static loads as well as dynamic loads. Against the static effect of wind loads, stiffer structures produce less lateral displacement. With regard to the dynamic effect of wind loads, not only the windward response but also the across-wind response of the structure should be considered. Generally, in tall buildings, the lateral vibration in the across-wind direction induced by vortex shedding is more critical than that in the windward direction.Regarding both directions, structures with more damping reduce the magnitude of vibration and dissipate the vibration more quickly. With regard to the vibration in the across-wind direction, a stiffer structure reduces the probability of lock-in condition because as a structure’s fundamental frequency increases, wind velocity that causes the lock-in condition also increases. Since the natural direction of structural evolution towards lightness is not likely to be reversed in the future, more stiffness and damping characteristics should be achieved with a minimum amount of material (Moon, 2005). Achievement of more stiffness in tall buildings is related to the configuration of primary structural systems, which were discussed in previous sections. For example, more recent structural trends such as tubes, diagrids and coresupported outrigger structures in general achieve much higher stiffness than traditional rigid frame structures. Obtaining more damping is also related to the choice of primary structural systems and materials. However, the damping achieved by the primary structure is quite uncertain until the building construction is completed. A more rigorous and reliable increase in damping, to resolve tall building motion problems, could be achieved by installing auxiliary damping devices within the primary structural system. The effect of such damping can be estimated relatively accurately. Thus, when severe wind-induced vibration problems are expected,