Experimental Study of Wind Loads on Tubular Crossarms of Transmission Towers

The actions exerted by yawed wind on the crossarm substantially differ from those on the tower body due to the presence of the angle between the crossarm’s cross-section and the incoming flow, and this phenomenon deserves a better understanding. Focusing on the tubular crossarms with round-section members, this paper aims to determine insights into wind actions, as affected by various factors, such as the tower type, the mean width to mean height ratio, the solidity ratio, and the wind incidence angle, and thereby derive the wind loads for design. Accordingly, a succession of wind tunnel tests was conducted by using quadrilateral and triangular crossarm models with various solidity ratios and mean width to mean height ratios. The method of direct force measurement (DFM) is employed for the high frequency force balance (HFFB) test to attain the transverse and longitudinal forces on the crossarm simultaneously. It was found that the wind load coefficients increase with increases of the mean width to mean height ratio and decreases of the solidity ratio. Moreover, in the case of the incoming flow perpendicular to the crossarm face, it is concluded that compared to the quadrilateral crossarm, the triangular crossarm possesses a relatively smaller drag coefficient, provided a moderate mean width to mean height ratio of 1.19. Furthermore, an analysis of the experimental results shows that the skewed wind load factor specified in the relevant codes would overestimate the effects of the wind incidence angle and the variations in mean width to mean height ratio, which is evident in the tests but not considered in the codes. It is worth noting that the experimental results show that the crosswind force is pronounced, which is usually ignored in the skewed wind load factor-based approach. Comparatively, using wind load distribution factors seems to be more rational. Thus, the corresponding estimating equations are proposed for the wind load-distribution factors of the crossarms, which agree well with the experimental results. The developed equations would greatly facilitate the determination of wind loads on tubular crossarms.