Span Wire Sag and Clearance After Tensioning
A span wire is to be installed between an existing wood utility pole and a new steel strain pole across a 78-ft span. The assembly supports three signal heads and one street-name sign. The specification requires 3/8-in 7-strand EHS messenger wire, sag no more than 5 percent of the span, and at least 17 ft of clearance above the traveled way at the lowest point. After tensioning, the crew measures 5.5 ft of sag and 16 ft 2 in of clearance under the lowest signal head. The utility pole attachment is at 24 ft, and the utility company requires 3 ft of separation from its facilities. The new pole top is at 30 ft. The crew wants to add a second guy to the utility pole and tighten the span.
1. Calculate the allowable sag and evaluate the measured 5.5 ft.
Read the worked answer
Five percent of the 78-ft span is 0.05 times 78, or 3.9 ft. The measured 5.5 ft sag exceeds the allowable 3.9 ft, so the installation does not meet the specification. The 16 ft 2 in clearance also fails the 17-ft minimum by 10 in. The crew must increase tension or change the span geometry, but before tightening they must verify that the messenger size is correct and that the poles, anchors, and guys can accept the additional tension. Simply pulling harder without a load check can overload the existing wood pole.
2. The lowest signal head is 16 ft 2 in above the traveled way. What must be done to meet the 17-ft minimum, and what are the consequences for the existing pole?
Read the worked answer
Tightening the span will reduce sag and raise the lowest signal head, but it increases tension and the load on the existing wood utility pole. The utility pole owner must approve the added load; the pole may require a guy, a stub pole, or replacement. Adding a second guy can help, but it changes the load path and must be designed, not improvised. Another option is to reduce the span by adding a pole or relocating an attachment. After adjustment, re-measure sag and clearance at the lowest point and document the results. Do not accept the 16 ft 2 in clearance as a field tolerance unless the owner approves a variance in writing.
3. What approvals and checks are needed before attaching the span to the utility pole?
Read the worked answer
Obtain the utility company's written attachment approval or permit, including the attachment height, separation from its facilities, and the added load from the span and signals. Provide a pole loading calculation and verify the pole class and condition. Check clearances to other utilities and the roadway, confirm the guy and anchor design, and plan traffic control for the work. The 3-ft separation requirement must be maintained. Do not attach until the utility approves in writing and the inspector has reviewed the attachment detail.