Research On Equivalent Static Wind Load And Wind-resistant Bearing Capacity Of Long Crossarm Transmission Towers

With the expansion of power transmission line scale and the demand for conserving land resources,long cantilever transmission towers with multi-circuit lines on the same tower have been widely applied.Long cantilever transmission towers have extended crossarms,and their torsional effects are quite significant under strong wind forces.The existing standards lack relevant provisions for wind-induced vibration calculation methods.The static overturning method for transmission towers in engineering projects also lacks consideration of member instability factors.Therefore,research on the equivalent static wind load,torsional effects,and structural collapse analysis of long crossarm transmission towers is essential.This paper is based on the high-frequency force balance(HFFB)test of long crossarm steel tube towers,using the frequency domain method to calculate the wind-induced responses in the downwind and torsional directions of the transmission tower,and analyzing the influence of higherorder vibration modes on downwind response calculations.Subsequently,various equivalent theories,such as the background plus resonance method and the standard method,are used to calculate the downwind equivalent static wind load of the transmission tower and make comparisons.The results show that the influence of higher-order vibration modes on acceleration calculation is more significant than that on displacement.The downwind displacement calculated considering only the first-order vibration mode is already accurate enough,while the acceleration requires considering the first three vibration modes.The background plus resonance method provides the most accurate equivalent for the target response.The wind vibration coefficients of this method are uniformly distributed along the tower height,with larger wind vibration coefficients at the equivalent target locations.In contrast,the wind vibration coefficients of the standard GB50009-2012,DL/T5551-2018,and the inertia force method are 1 at the tower base and increase with height,which may underestimate the equivalent wind load at the lower part of the tower.Among them,DL/T5551-2018 and the inertia force method take into account the irregular distribution of structural mass,area,and vibration mode characteristics.The sectional estimation method,as an essential nonlinear vibration mode structural highfrequency force balance(HFFB)test modal force calculation method,can estimate the spatial distribution of structural fluctuating wind loads.This paper abandons the assumption that the dimensionless wind load spectrum in the sectional estimation method remains constant along the height and proposes a method that considers the load spectrum’s variation along the height.Using the Kaimal fluctuating wind spectrum as an example,this paper proposes and derives the modification factor for the dimensionless wind load spectrum’s variation along the height.With a true-scale transmission tower frame as the example,combined with the high-frequency force balance(HFFB)test,the influence of the modification factor on the structural response calculation is analyzed.The research results show that the difference in the background component calculation values of the displacement response before and after modification is small,but the resonance component difference is significant,with a deviation amplitude of up to 25%.The original assumption for the transmission tower in this paper remains valid.However,for taller and more flexible structures with a higher proportion of resonance components in the total response,the variation of the load spectrum along the height needs to be considered.For wind speed spectra that decrease or increase along the height,the existing sectional estimation method with constant load spectrum along the height will overestimate or underestimate the structure’s displacement response.The method proposed in this paper has higher accuracy and broader applicabilityThe torsional wind load of long crossarm transmission towers is caused by two factors: the incomplete correlation of fluctuating wind speed and characteristic turbulence.Taking long crossarm steel tube towers and angle steel towers as research objects,under quasi-steady assumptions,the torsional wind load of the tower frame caused by the incomplete correlation of fluctuating wind speed is calculated based on the theoretical fluctuating wind spectrum and coherence function.The torsional wind load caused by characteristic turbulence is calculated based on the base torque spectrum from the HFFB test.The results show that when calculating the torsional displacement and acceleration response using the frequency domain method,considering only the first-order torsional mode is accurate enough.The coherence function has a crucial influence on the torsional response calculation.The distribution of torsional loads caused by the two factors along the tower frame follows the same pattern,with crossarms and bracing angles accounting for the vast majority,reaching more than 95%.A static pushover method for transmission towers based on the failure criteria of member buckling under compression and tensile bending failure is proposed.On this basis,the weak members and failure modes of the two long crossarm transmission towers previously mentioned under design load conditions are analyzed,and the influence of the tower frame wind load application mode on the collapse analysis is studied.The main conclusions are: nonlinear buckling analysis under the overall defect mode cannot fully consider the reduction in bearing capacity caused by member instability,and the pushover method proposed in this paper is more suitable for tower frame structures.The weak members of the steel tube tower are the lower crossarm and the tower head diagonal members near the middle crossarm;the weak members of the angle steel tower also include the long diagonal members and leg diagonal members at the lower part of the tower body.Torsional wind loads are unfavorable for diagonal members and cross bracing.Considering the torsional wind loads,the tower frame failure wind speed is significantly reduced,with a decrease of 8% for steel tube towers and 10% for angle steel towers.The equivalent static wind load calculated by the transmission line load code is close to the background plus resonance method at the upper part of the tower body,but much smaller than the latter in the middle and lower parts of the tower body.For transmission towers with weak sections at the bottom,the equivalent wind load of the code will overestimate the tower frame’s bearing capacity.The load application mode distributed along the members results in a slight decrease in member axial force,an increase in bending moment,and an increase in member load-bearing ratio,leading to a reduction in the tower frame collapse load.For angle steel towers with eccentric connections of members,the contribution of bending moments to the load-bearing ratio is high,and the tower frame collapse load is significantly reduced,by about 5%.For steel tube towers with axial connections of members,the node loading mode is accurate enough.