Across-wind force excitation mechanism of two tall buildings under interference

Abstract

Wind-induced dynamic wind loads and building responses are crucial factors in the design of tall buildings for structural safety and serviceability requirements. In an urban circumstance, it is very common to find a tall building surrounded by other buildings in its vicinity, leading to significant modification of wind loads for the tall building. Thus, it is necessary to take this interference effect into consideration during the building design. As there are many parameters effecting the modification of wind loads induced by interference from surrounding buildings, possible combinations of these parameters are too large to be covered exhaustively. Therefore, a physically-based approach, such as investigating the underlying mechanism of interference effect, would be worth adopting to solve the problem. This study attempts to understand the relationship between the generation of wind forces on the building and the characteristics of wind velocity field around it by making simultaneous measurements of fluctuating wind forces on a tall building with and without surrounding buildings, concurrent with PIV measurements of the unsteady wind flow field around it in the wind tunnel. The project confirms the relationship of the development of vortex shedding with the phase of the across-wind force for a tall building without surrounding buildings. The use of building models at geometry scale 1:1000 due to PIV limitations is validated by comparison with other studies using larger geometry scales two tall buildings under interference. The minimum Reynolds number requirements for wind effects on a single building with sharp corners can also apply for building groups under interference. Excitation mechanisms for interference of two tall buildings have been established respectively for the tandem, side-by-side and staggered arrangements. For the side-by-side arrangement, the main effect of the presence of the interfering building is to reduce the fluctuating across-wind load on the principal building, as a result of the impairment of the regular vortex shedding. For the tandem arrangement, regular vortex shedding on the downstream building is impaired. The fluctuating energy of the across-wind force on the downstream building is mainly provided by the quasi-periodic oscillation of the upstream building wake. Sketches with five quasi-periodic phenomena responsible for the across-wind force excitation are proposed describing the across-wind force excitation mechanism. For the staggered arrangement, a synchronization process among the five phenomena same as the tandem arrangement is also identified. According to relative locations, a classification of interference between two buildings and sketches of corresponding flow structures of each region is proposed. The related problem of shielding effect from a group of upstream medium-rise buildings to a tall building is also investigated. The degree of shielding is found to be governed by the combinations of several arrangement parameters of the sheltering buildings. Multiple regression empirical equations are proposed to predict the shielding effect. The project successfully unveils the excitation mechanisms for dynamic across-wind forces on an isolated tall building and two interfering tall buildings in proximity.