Wind Tunnel Investigation On Wind Load Characteristics For Latticed Towers Constructed From Cylindrical Members

Latticed towers constructed from cylindrical members are more and more widely used because of their superiority that having smaller body shape coefficient and larger turning radius compared to angle steel tower. Most of the latticed towers are lightweight,high-rise flexible and lightly damped and thus sensitive to wind action. Therefore, the study of wind loads on latticed towers has great realistic significance. High frequency force balance test with a scale of1:130and aeroelastic model wind tunnel test of the same latticed towers constructed from cylindrical members with a scale of1:45were carried out to analyse wind load characteristics.First, the effect of Reynolds number on the wind forces on latticed towers with cylindrical members, which are obtained by the force balance in wind tunnel is investigated. Six towers with different shapes were tested both in the uniform flow and the boundary layer simulated category B. The base shear forces and base bending moments were used to calculate the total body shape coefficients of the towers. The correction factors due to Reynolds number effect were obtained by comparing the total body shape coefficients obtained in the two flows. Results from this study show that total body shape coefficients of steel lattice tower models with cylindrical members need not to be corrected when tests are carried out in the boundary layer simulated category B. On the contrary, the total body shape coefficients should be corrected when tests are carried out in the uniform flow. Based on the study above that total body shape coefficients of steel lattice tower models with cylindrical members need not to be corrected when tests are carried out in the boundary layer simulated category B, the base shear forces obtained from the tests of the four symmetry models were used to calculate the body shape coefficients of the total tower, the main body and the crossarms. The body shape coefficients of the main body were compared with some existing codes and standards. And the rationality of the distribution factors of main body and crossarm due to the yawed wind is analyzed. Results show the body shape coefficients in the along-line direction of the total tower, the main body and the crossarm are arrived at their maximum values at15°wind azimuth, instead of at0°wind azimuth, indicating that15°wind azimuth is the most unfavorable wind azimuth in the along-line direction. The body shape coefficients of main body at0°wind azimuth are close to the data regulated in IEC60826and ASCE/SEI codes, and bigger than the data regulated in Chinese codes. Besides, wind loads on the triangular shaped crossarm obtained from the tests are50%bigger than the loads calculated by the codes at45°wind azimuth.Second, aeroelastic model wind tunnel test of the same latticed towers constructed from cylindrical members in a scale of1:45with and without crossarms were carried out to get acceleration time histories. Empirical mode decomposition methods and ensemble empirical mode decomposition methods are used to decompose acceleration time histories to get intrinsic mode functions. It finds out that ensemble empirical mode decomposition methods gets better decomposition results. Then improved random decrement technique and hilbert spectral analysis are used to analyse damping of the aeroelastic tower model and the same tower model with no crossarms. Results show that damping of the tower gets bigger as wind speed increases,while structure frequency becomes smaller as wind speed increases. Damping in across-line direction exitsts maximum value in a specific wind speed because of crossarms.And it makes big differences of damping in along-line direction and across-line direction.Finally, damping ratio values of3%in along-line direction and4%in across-line direction are suggested to calculated the latticed tower model.Third, methods to calculated the torsion load of the aeroelastic tower model are proposed.Then the torsion load of the aeroelastic tower model in wind tunnel test is calculated.Results show that the torsion load of the aeroelastic tower model is close to the conversion base torsion of the1:130model in high frequency force balance test.