An Iconic Wisconsin Landmark Rises Again and Takes Flight: The Eagle Tower Project

The original Eagle Tower, erected in 1932 in Peninsula State Park – Photo Credit: Yinan Chen – Friends of the Peninsula State Park

If you live in Wisconsin, chances are that you at least know of Eagle Tower. More likely, you—along with thousands of visitors from around the world—have had indelible experiences of taking in spectacular views of Lake Michigan, the surrounding islands, and Michigan’s Upper Peninsula. Eagle tower offered a captivating and much beloved panorama of Peninsula State Park.

Built in 1932, the observation tower was a 76-foot timber structure. But in 2015 the tower’s deteriorating state caused the Wisconsin Department of Natural Resources (DNR) to have serious concerns about its structural integrity and safety. The Wisconsin DNR asked Forest Products Laboratory (FPL) to assess the structure. “Nondestructive Assessment of Wood Members from a Historic Viewing Tower” is a detailed publication of their findings on Eagle Tower’s condition.

On May 20, 2015, Eagle Tower was closed to the public and structural assessments began in earnest.

Robert J. Ross
FPL Supervisory Research Gen. Engineer
Engineering Properties of Wood, Wood Based Materials and Structures

“Because of FPL’s longstanding stature as an unbiased source of technical information on wood and wood products, we were asked to provide scientific and technical assistance to the Wisconsin Department of Natural Resources during their assessment of the historic viewing tower, and subsequent construction of a new viewing tower,” explained Robert Ross, FPL’s Supervisory Research Gen. Engineer, who led the team assessing the tower.

During FPL’s initial visual inspection of the tower, Ross observed considerable deterioration of both structural and non-structural wood members. Of greatest concern was the condition of the tower’s main supporting columns. Numerous deep splits and cracks within many of the timbers and the main load-bearing support columns were immediately evident. Holes were also visible in the timber, most likely the result of bird nesting activities. Splits and deterioration in the vicinity of connections and where the support columns contacted the concrete pads were of significant concern. Ross also observed evidence of lateral movement from the upper sections of the tower. That is all to say, the old Eagle Tower was in rough shape and ready for retirement.

On September 19, 2016, Eagle Tower was dismantled.

FPL researchers then used several nondestructive techniques to assess the condition of the large timber poles from the tower. Stress wave timers were used to inspect the internal integrity of the poles. Sensors were positioned on opposite sides of the timber and a tap on the pole generated stress waves in the wood. Because stress waves take longer to travel between sensors in deteriorated wood, degraded timber can be easily identified.

Timber sections TL#1 through TL#4, with corresponding stress wave transmission test results. Test results indicated severe deterioration in sections TL#1 and TL#2.

Additionally, microdrilling resistance, a technique used to identify decay and voids in both lumber and live trees, was used to identify degraded beam areas. When wood decays it becomes soft, therefore drill penetration will meet little resistance from deteriorated wood.

Microdrilling resistance results indicated deterioration in the vicinity of a bird nest in the timber.

Both tests found that critical support beams were significantly deteriorated and could not be re-used for any structural purpose.

In 2019, after intense community and local government deliberation, a replacement was agreed upon, and Ayres Associates completed the new design.

Wisconsin DNR Eagle Tower reconstruction overall plans and 3D view created by Ayres Associates

Why is the new tower only 62 feet high?

In the initial design planning stages, the new tower’s height was debated, and technology ultimately helped settle the issue. Ross explained, “The DNR conducted an assessment of the view from the bluff upon which the tower rests—a drone was used to evaluate the best panoramic view at various heights above the bluff. Based on these tests they selected the height.”

Eagle Tower ramp construction – Photo Credit: Chris Holicek

The immense scale of the project is truly difficult to grasp unless you’re standing right next to the cement support pads that will bear the structure. Massive doesn’t quite capture the enormity of the tower’s design. The new Eagle Tower will have an ADA-compliant ramp that launches from the forest floor and flies through the tree canopy—much like the eagle it’s named for—taking visitors aloft into the surrounding forest. Approximately three wheelchairs wide, the ramp will give visitors a squirrel’s eye view of the forest, with educational stops at strategic corners, until it ends with the well-known stunning view from the location of the old tower.

New tower construction underway – Photo Credit: Chris Holicek

And every design care has been taken to build the new tower to look aesthetically similar to the original. Visitors wanting to climb spiraling stairs to get to the top will still have that opportunity.

FPL researchers are proud of their participation in this project.

“Based on technical input from FPL, the DNR chose to use modern, engineered wood products to construct the tower. In addition, to meet ADA requirements, the DNR decided to construct a sloping ramp, using engineered wood products, that would gradually wind from the site’s parking lot through the local section of the park, raise above the parking lot and out onto the top level of the viewing tower. FPL was asked, and provided technical assistance, on details of the engineered materials that were selected by the architectural firms responsible for the tower’s design and construction,” explained Ross.

Eagle Tower ramp construction – Photo Credit: Tom Blackwood

Eagle Tower is a full-circle Wisconsin story. But more than that, it’s a tribute to the beloved natural surroundings, trees, people, businesses, materials, and inclusivity the location has to offer. Although the old timbers from the last tower couldn’t be reused in the new structure, local artisans reclaimed them for their work. The trees cleared to make a path for the ramp were used to build benches and signage for the park. The timbers for the ramp and tower (glulam—glued-laminated timber) were supplied by Sentinel Structures, one of Wisconsin’s oldest lumber companies.

Work on the tower and ramp is progressing quickly. The large glued-laminated timbers crossing the park road are in-place and the observation deck installations will soon follow. The new Eagle Tower is expected to be completed by the end of this year and open to the public next spring.

Illustration of how the new Eagle Tower and ramp is expected to look when complete – Blake Theisen, Ayres Associates

Situated on a 180-foot limestone bluff called Eagle Bluff, Eagle Tower offers a truly magical experience. It’s a place where many a first kiss happened, where birdwatchers finally catch a glimpse of a rare native species, or a child discovers the wonder of nature and grows into an adult that in turn brings their own children. And now that Eagle Tower is ADA compliant, it can be magical for everyone.

Recent news coverage on the Eagle Tower project can be seen at https://fox11online.com/news/local/eagle-tower-starting-to-take-shape-at-peninsula-state-park

To find out more about the amazing advancements our scientists are making, visit the Forest Products Laboratory at https://www.fpl.fs.fed.us/