Bald Hill Tower Replacement near West Salem, Oregon

Location: Bald Hill, near West Salem, Oregon
Client/Owner: Silke Communications
Partners: Tower manufacturer (tower design); county building department
Year: 2011, built
Size/scale: New 200-foot self-supporting steel tower on a concrete mat foundation 36 feet square and 4 feet thick, replacing a 200-foot guyed tower
JBE services: Climbing inspection and structural assessment of the old tower, site visit, foundation design and construction drawing, structural observation of the finished tower for the building department, and a written method for taking the old tower down
Sector: Towers & Telecom

Old guyed tower and new self-supporting tower standing side by side on Bald Hill near West Salem

The challenge

Silke Communications asked us to look at rust on a 200-foot guyed tower on Bald Hill, in the hills northwest of Salem. The tower dated from the early 1970s and carried a microwave link at its top.

We climbed it in February 2011. By the second section there were holes in one leg up to two inches across, with the steel eaten away from the inside. Higher up we found a split in a leg six inches long, and flakes of metal that had dropped down inside from the sections above. We stopped climbing near the 95-foot level. Our letter to the owner said the tower was not safe and should be removed.

The microwave link still had to work, and it needed the full 200 feet of height. So the old tower could not come down until a new one was standing beside it.

Project photos

What we did

We designed the foundation for a new 200-foot self-supporting tower, placed on the north side of the equipment shelter so it would clear the old tower and its guy wires. Bedrock is close to the surface on the hilltop. Instead of cutting into the rock, we designed a mat foundation 36 feet square and 4 feet thick. Uplift and overturning governed its size, so we spread the footing wide and kept it as thin as the anchor bolts allowed.

The tower itself was designed by its manufacturer for a 95 mph wind with a quarter inch of ice. We took the manufacturer’s footing reactions and showed them on our foundation drawing.

When the tower was up, in September 2011, we climbed it and checked the bolted connections at the section splices and bracing, as the building department required. We reported that the tower had been properly erected and the bolts properly tightened.

Then we wrote out the method for bringing the old tower down: which guy wires to release and in what order, so it would fall downhill to the southwest and away from the shelter that had to stay.

The result

The new tower was standing and certified by mid-September 2011, and the microwave antennas had been taken off the old one. A tower that was close to collapse was ready to come down in a controlled way, with its replacement already standing beside it.