EFFECT OF UNCONVENTIONAL SHAPES ON RESPONSE OF TALL BUILDINGS UNDER WIND LOAD

Abstract

With the advances in construction methods, materials, and technologies, more high rise buildings with unconventional shapes have been continuously built. A large number of tall buildings with irregular shapes have been built in past years in urban areas due to a shortage of land and demand for good architectural design. As the wind velocity increase with height, the top of the building may experience a higher wind. Tall buildings are the structures that are more sensitive to wind loads, and thus the response to the wind load is the main concern of designers while designing tall buildings. Most of the tall buildings are bluff bodies. As flow separates and reattaches around bluff bodies, the external shape of tall buildings plays an influential role in the generation of wind load on high-rise buildings. High-rise residential buildings are commonly built as twin towers. Twin tall buildings are subjected to the proximity effects due to small gaps between them. Under the influence of proximity, the wind load on tall buildings may differ from that on the isolated buildings. The codes and standards related to the wind loads are generally do not consider these proximity effects. Further, no analytical formula is available to evaluate the wind effect on irregular shape tall buildings and proximity effects between twin towers, which necessitates more experimental or analytical study for irregular shape tall buildings. In the present study, four different plan shapes, namely square, plus-1, plus-2, and H plan, are considered. The floor area and height of all four models are kept the same. The plus-1 and plus-2 plan shapes are prepared by providing the large-sized recessed corners in the rectangular plans of different sizes, whereas the H-plan shape is prepared by providing recessed cavities on the two opposite faces of a rectangular plan shape. The study has been conducted in three phases, namely (i) pressure measurements through wind tunnel study in isolated and interference conditions, (ii) force evaluation through pressure integration technique, and (iii) study of the response of prototype buildings to wind loads calculated on scaled models. In the first phase, to investigate the wind-induced pressure at the surfaces of the building, four rigid models scaled at 1:300 as described earlier are tested in an open circuit layer viii wind tunnel having a working section of 2 m x 2 m cross-section and 15m length. The wind flow characteristics inside the tunnel are simulated similar to the Indian sub-urban terrain with well-scattered objects with a height between 1.5 to 10 m according to the Indian standard IS 875 (part 3): 2015. The mean wind speed and turbulence intensity profile with a power-law index of 0.22 is simulated in the tunnel. The turbulence intensity near the floor of the wind tunnel and wind velocity at building height is 12% and 9.87 m/sec, respectively. The pressure models are prepared with a 4 mm thick transparent Perspex sheet with stiff faces to ensure sufficient rigidity and strength of the model. The pressure measurement study is conducted in two parts. In the first part, the pressure models are tested in isolated conditions for wind directions of 0, 300 , 600 , and 900 angles at an interval of 300 to assess the effects of wind direction on the surface wind pressure. In the second part, the pressure models are tested with an interfering building model present upstream of the pressure model at different locations. The interfering building models are made of wooden material with dimensions similar to the pressure models. Interference effects are assessed for three different positions of interfering building, such as full blockage, half blockage, and no blockage. From the time history of fluctuating pressure data, the mean, maximum, minimum and r.m.s pressure coefficients are evaluated at each pressure point on the surface of the model in isolated and interference conditions. In the second phase of the study, local wind force coefficients at various levels of the models and the overall base forces have been evaluated through the pressure integration technique. The local forces at each level of the model are calculated by integrating the local forces of each pressure tap at that particular level. The overall base forces and moments are calculated by integrating the force and moment of all levels. The forces in along-wind and across wind directions are presented as mean and r.m.s coefficients. In the third phase of the study, the responses of prototype buildings of the four models to the wind load calculated through wind tunnel study on the scaled models are studied through analytical study. The effects of various wind directions and various interference conditions have been assessed on the response of four buildings through stress parameters, including axial forces, the moment in X and Y directions, and twisting moments. At last, ix the effects of the various plan shapes on the response of tall buildings under wind loads have been investigated. The results show that wind flow direction has significant effects on the pressure distribution on the surfaces of the models. For the square model, the wind directions normal to the surfaces are critical direction. The wind direction of 600 generates the most critical positive and negative mean pressure on the plus-1 model. The effects of wind direction on the pressure on the plus-2 model are not as much severe as the plus-1 model. Pressure distribution on the front faces of all models is completely different in interference conditions from those in isolated conditions. The influence of interference on the square and H-plan model are beneficial, while it has negative effects on some face of the plus-1 model. The along-wind mean local wind forces at normal wind incidence are higher than those at oblique wind incidence. The along-wind forces in all interference conditions decreased significantly from those in isolated condition. The values of along-wind forces: drag and moment at normal wind incidence of 00 and 900 angles are larger than those at oblique wind incidence in isolated condition. The CFD and CMD values are likely to be reduced significantly in interference conditions. The effects of change in cross-sectional shape are significant on the across-wind forces. The effects of cross-sectional shape are dominant for wind flow at oblique angles in isolated condition while more at half blockage condition of interference. The axial force is independent of the building cross-section. All the buildings have the same axial force in central columns. The maximum twisting moment is observed in PL-2 Building in isolated as well as in interference conditions. The H building show the best performance in isolated condition.