The Research On Wind Effects Of Power Transmission Tower With Long Cross Arms

In recent years, long cantilever transmission towers have been widely used in order to save the line corridor. This kind of tower has a characteristic of complex wind effects. Its highly flexible structure makes wind load control the design process.A numerical analysis of a four-circuit angle-steel transmission tower under conventional terrain B wind field and typhoon wind field was performed for investigating wind-induced response. A FEM model was established, and the dynamic response was calculated with the fluctuating wind field simulated by harmonic wave superimposing method. Based on data of numerical analysis, wind-induced response under each wind field was discussed. Essential conclusions are as follows:high turbulence and strong variability of typhoon wind field has great influence on transmission towers. RMS of acceleration under typhoon wind field is larger than that under terrain B wind field. Under the two types of wind field, the average ratio of wind load factor is about1.25.Therefore, the design of transmission towers in typhoon-prone areas must take the fluctuating wind load magnification effect into consideration. Furthermore, an aerolastic model wind tunnel test of the transmission tower was performed to study its wind-induced response under different velocity. The test results were compared with theoretical values and the accuracy of numerical analysis was verified.By means of the numerical analysis and the phenomena of the wind tunnel test, structural optimization for wind resistant design of the tower mentioned above was carried out. Essential conclusions are as follows:due to the gas-solid coupled effect, strong flexure-torsion vibration was observed during the wind tunnel test, which would lead to the premature instability of angle steel members. Furthermore, subjected to the fluctuating wind excitation, several local vibrations appeared in the very first several modes, which resulted in the large out-of-plane deformation and eventually the reduction of the ultimate bearing capacity. Further research shows that the torsional vibration could be apparently reduced by increasing the rigidity of diagonal bracings between the arms of the tower. Also, adding diaphragms with appropriate arrangements could optimize the dynamic characteristics remarkably, suppress the local vibrations, and ensure the global stability of the tower.Due to the complex structure of tower head and long cantilever, the asymmetry of wind pressure distribution generated by the inhomogeneity of incoming turbulence could not be ignored. Furthermore the signature turbulence around the members of tower head would produce a certain dynamic torsion wind loads and exacerbate the torsion effects. In light of basic scale laws, a rigid model of500kV long cross-armed pipe transmission tower with high frequency and light weight was designed and produced. By means of freq-domain numerical analysis and high frequency force balance technique, the contribution to the torsion effect of the dual factors was studied. Essential conclusions are as follows:compared to the standard code method, the axial stress of diagonal bracings in various parts of the tower was significantly increased, and apparent relative displacement on both sides of the tower arm end was observed, showing the necessity to consider the torsional effect caused by the inhomogeneity of incoming turbulence as well as torsion wind load generated by signature turbulence.