Seattle City Light
Seattle City Light (SCL) has a transmission and distribution line crossing of the Lake Washington Ship Canal at Warren Avenue. The crossing includes two lattice steel towers that were designed in the 1920s. The towers are 190′ in height with arms at the 190′ and 176′ elevations. The top arm supports three phases of transmission. The lower arm supports six phases of distribution.
Commonwealth was tasked to review the adequacy of the existing lattice steel towers and to design new tubular steel poles to replace the existing wood structures that are an integral part of the crossing. Commonwealth analyzed the towers based on available data using PLS-TOWER. The analysis concluded that minimal repairs and modifications were required for the proposed conductor upgrade to 945 kcmil ACSR Rail conductor. Based on this analysis, drawings were developed for the required modifications. Commonwealth also designed new tubular steel structures to replace the existing wood poles.
During construction, it was noted that the arms, due to their asymmetrical nature and the vertical loading, had twisted excessively. This previous damage had not been noted by a climbing inspection performed by others. Commonwealth recommended that new, more traditional symmetrical truss arms be designed, fabricated, and installed prior to installing the new conductor. SCL requested that Commonwealth perform the design of the new arms and they were fabricated by SCL for installation in the spring of 2011.