Field Measurement, Wind Tunnel Tesing And Theoretical Analysis Of Typhoons-induced Wind Loads On Low-rise Buildings

Typhoons are one of the most destructive natural disasters in the world. The southeast coastal regions of China are exposed to strong tropical cyclones with an average number of eight or nine making landfall each year. As a result, the wind-related disasters cause significant property losses and heavy causalities in this region almost every year. Based on post-storm evaluation of damage and losses, buildings and structures in the typhoon-prone region, in particular residential houses and light industrial structures might easily be damaged or destroyed in extreme wind storms. Hence, there is an urgent need to understand the extreme wind loads, which are the primary causes of damages of low-rise buildings subjected to typhoons or strong wind storms. In this study, the wind loads on low-rise buildings during typhoons were investigated through field measurements, wind tunnel tests and theoretical analysis. The major objective of the study is to further understand near ground typhoon-generated wind characteristics and wind loads on low-rise buildings under extreme wind conditions during typhoon landfalls. The major scope and achievements of this study are as follows:(1) Mean wind speed and wind turbulence characteristics of typhoon winds in surface boundary layer were studied based on three-dimensional wind data, which were recorded from ultrasonic anemometers at10m height at sites near seashore during landfalling of typhoons. The mean values of longitudinal turbulence intensity ranged from11%to14%for the coastal terrain under strong wind conditions. The mean values of lateral turbulence intensity ranged from6%to11%. The mean values of vertical turbulence intensity varied from3%to5%. The relations among turbulence parameters such as turbulence intensities and gust factor, turbulence intensities and turbulence integral scale length were analyzed. Gust factor increases with increasing turbulence intensities. The integral length scales decreased with increasing the longitudinal turbulence intensity. By fitting the results of the turbulence parameters, the relations among the parameters were proposed.(2) A full-scale instrumented flat roof low-rise building has been constructed and implemented to monitor wind velocity field and building surface pressures during typhoons. The near ground wind turbulence characteristics such as turbulence intensity and integral length scale, gust factor and wind velocity spectra were quantified in different terrain exposures and different portions of a typhoon based on the field measurement of wind data. The observed turbulence intensities (TI), turbulence integral length scale and gust factor values in the outer convective Rainband region of a typhoon were larger than those near the Eye-wall region of the typhoon. The normalized power spectral values of the longitudinal and vertical wind components for typhoon winds within different typhoon portions show similar energy distributions. However, the energy content of the lateral wind component in Eye-wall region was slight higher than that in the Rainbrand convective region.(3) The mean wind speed and wind turbulence characteristics of typhoons in surface boundary layer were studied base on the wind velocity data recorded from a full-scale instrumented gable roof low-rise building and100-m meteorological tower during landfalling of typhoons. The results revealed that in the near-surface range (<100m) vertical distribution of mean wind speed on seashore can be well described by a logarithmic law and a power law. The variation of the mean longitudinal turbulence intensity with height approximately followed a power law. Compared with those measured in monsoon wind climates, there are apparent increases in shear stress velocity, surface roughness length and exponent of power-law profile. The average values of turbulence intensity observed during typhoons were more than20%higher than those obtained during monsoons.(4) The results of extreme suction pressure coefficients and the mechanisms of the generation of the peak pressure coefficients on a corner of the flat roof instrumented low-rise building were studies based on the field measurements of wind data and associated building surface pressures during typhoons. Detailed analysis of the mean, standard deviation and negative peak pressures measured on the roof corner zone were conducted to investigate the pressure distribution characteristics in conical vortex region under oblique approaching wind directions. The measured maximum peak suction pressure was-4,240Pa and the associated minimum negative peak pressure coefficient was-13.5. The observed minimum negative peak pressure coefficient on the windward leading edge of the corner area exceeded that recommended by ASCE7-10Standard. Moreover, the effects of spatial and temporal average on the peak pressures on the corner zone were discussed. The effects of turbulence integral length scale and turbulence intensity on the roof pressures in conical vortex flow regime were evaluated. The effects of fluctuations of incident wind direction and vertical wind angle on the peak suction pressures generation on the corner zone were estimated using non-convection pressures coefficients.(5) Wind loads on the gable roof of a low rise building were studied based on the wind velocity data and pressure data recorded from the instrumented low-rise building. The results revealed that high local suctions on ridge corner and eave corner were observed under oblique flows. Probability distributions of fluctuating pressures on the ridge corner and eave corner were non-Gaussian. The extreme value analysis was performed using an automated procedure to determine the peak suction pressure coefficients. The estimated extreme peak pressures were compared to those stipulated by ASCE7-10Standard, the observed and estimated suction pressure coefficients on the windward leading edge of the ridge and eave corner zone exceeded that recommended by ASCE7-10Standard.(6) Wind tunnel tests were conducted to assess the wind loads on the two experimental buildings. The model test results were compared to the field measurements to evaluate the accuracy of the wind tunnel simulations. Further comparisons of the full-scale measurements and wind tunnel simulations of negative peak and fluctuating pressure coefficients near the roof corners, roof ridges and leading roof edges showed distinct discrepancies. Wind tunnel test results underestimated the field measured negative peak and fluctuating pressure coefficients in separated flow regions and conical vortex regions. Additional wind tunnel tests were also conducted to investigate the effects of different terrain conditions and varying gable roof angles on the wind pressures on the roofs of the low-rise buildings. Furthermore, wind tunnel tests were performed to evaluate the effectiveness of parapets mounted at the roof-edge on mitigating these high suctions.(7) Field measurements of wind velocity and pressure data have been used to evaluate the performance and the effectiveness of the Quasi-steady Theory, which was employed to predicted pseudo-steady pressure coefficients and pressure fluctuations on the roof corner zones. The measured mean pressure coefficients were compared with those predicted pseudo-steady pressure coefficients and distinct discrepancies were observed. The theory underpredicts the rms pressures on the corner zones for conical vortex region. The measured pressure spectra at several locations on the windward ridge and eave corner agreed with the predictions of the Quasi-steady theory in low frequency range quite well. However, the predicted spectra decayed significantly faster than the measured pressure spectra in high frequencies. Taking into account of the annual occurrence frequency of strong tropical cyclone, the Compound Poisson-Gumbel extreme value distribution was adopted to model the extreme wind speeds, which were obtained from anemometers at the Waglan Island meteorological station by Hong Kong observatory. The results indicated that the suggested model is reasonable, and is suitable to describe the probability distribution of extreme wind speeds in typhoon-prone regions. Finally, the extreme wind pressures on the roofs of the low rise buildings were evaluated with the application the pseudo-steady pressure coefficients and extreme design wind speeds.The outputs of the field measurements and wind tunnel test study on the wind loads on the two instrumented low-rise buildings during typhoons are expected to provide useful information and reference on revising the wind loads design standard for the wind-resistant design of low-rise buildings in typhoon-prone regions.