Investigation on wind loads on angle-steel cross-arms of lattice transmission towers via direct force measurement

Abstract

Cross-arms are key structural components of transmission towers usually located horizontally on the upper part and therefore bear large wind loads that contribute a lot to the base bending moment. In wind tunnel tests of wind loads on cross-arms, indirect force measurement (IFM) method is usually applied, which obtains the wind loads on the cross-arms by subtracting the forces measured on tower-body models with and without the cross-arms. This method over-simplifies the interference of the flow field between the cross-arm and the tower-body and therefore cannot provide an accurate evaluation on the exact wind forces on the cross-arm. A novel wind tunnel test method – direct force measurement (DFM) method, which directly measures the wind loads on a cross-arm attached to a tower model, is proposed in this article. DFM method and IFM method are both employed to investigate wind loads on cross-arms of a lattice transmission tower. Drag coefficients and cross-wind force coefficients of cross-arms are then identified. Wind tunnel test results and standard-calculated values of skewed wind load factors are compared. Calculation methods of wind load distribution factors from various standards are derived and wind tunnel test results and standard calculations are compared. It is concluded that drag coefficients of cross-arms obtained with the DFM method is very close to the standard-calculated values and are larger than those obtained with IFM method. A fitting formula for the skewed wind load factors is proposed which well fits the test data. Comparison between wind load distribution methods illustrates that ASCE (2010) standard-calculated values match well with all wind tunnel test results which show significant differences from other standards. It is indicated from wind tunnel test results that the calculation formula for skewed wind factor proposed in this article is simple but effective.