Build. Span. Connect.

Henry David Thoreau Suspension Bridge

  • Bridge Specs: Span = 134′ (165′ with approaches): Travel Width = 5′ 6′: Deck = Glulam Beam: Mainlines = 1-1/4″ dia.: Anchors = Helical Pile 150 KSI Anchors.
  • Services: Design/Engineering Review, Installation.

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The forests and streams of the Shepaug River Valley in the Berkshire Mountains of western Connecticut have had the good fortune to be the focus of conservation efforts for more than 120 years. This legacy began with the acquisition of 100 acres of forestland in 1889, and continues to this day behind the land management and preservation efforts of the Steep Rock Association based in Washington, CT.

The Steep Rock Association, led by Gray Organschi Architects in coordination with Robert Silman Associates (engineering), reached out to Seattle Bridge in May, 2013, seeking support for a bridge project to span the Shepaug River and link unconnected trail systems in the Steep Rock Preserve on either shore. The initial design was bold, visionary, and technically interesting; we admired it, but had misgivings. As we began working through the issues presented by this novel design we addressed our concerns, communicated them to the Organschi team, and found in them partners willing and able to steer a middle course between the vision and the practicalities of an installation such as this.

The design of the Thoreau Suspension Bridge was unusual in several respects. First, it is a suspension bridge with a single tower–the mainlines are anchored directly into a hillside on the far shore. Second, the mainlines do not run in parallel–they radiate from a single point atop the bridge tower to two anchor points 25′ apart at the hillside anchorages, and the suspender cables drop to the deck at compound angles that varied from bay to bay. Third, the bridge superstructure–glulam beams pinned together with all thread–was more rigid and resistant to tuning than the dynamic, fixed-pinned stringer bridges that are our usual stock in trade.

After iterating through design and erection-procedure questions, Seattle Bridge and the Steep Rock Association entered into a formal agreement for installation of the Thoreau Suspension Bridge in October 2014. Anchor installation by Helical Drilling Inc. began in Fall 2014, and was completed after melt in late Spring 2015. At the same time, Seattle Bridge began mobilizing installation equipment and fabricating suspender assemblies and temporary floor beams.

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Led by Martin Walz III, the installation crew (Ken Gilman, Austin Thompson, Steve Howell and Aaron Nelson) deployed to Connecticut on July 14 and took up residence in a house that was literally a stone’s throw from the project site. Our crew had ample time to scout the site, assess the (excellent) state of material deployment and anchor-setting, explore Washington Depot, and fine-tune the erection procedures for the single-mast tower.

In accordance with our old boss Carroll Vogel’s 6P principle (“Prior Planning Prevents [very]-Poor Performance”), all of the careful consideration of installation was time well-spent. On July 20, just one day after final material delivery, the bridge tower/mast was moved into position, raised, and pinned. Cables were deployed one day later. Time and again, we found ways to apply our thousands of hours of bridge installation experience to new and unusual bridge-erection problems. Step by step a slender, curving span of bridge came into being where days before there had been nothing.

By August 3 the field services crew was well into drilling and setting floorbeams, driving and drilling through-bolts, and beginning installation of handrails. Seattle Bridge’s field services crew packed up for shipping tools back to Seattle on August 18, and returned to Seattle on August 19.

The Thoreau Suspension Bridge opened to the public in January, 2016. The structure is a testament to fantastic cooperation and shared vision among multiple parties, with particular credit to Parker Lee and Alan Organschi with Gray Organschi Architects for going to great lengths to maintain coordination and keep the project rolling inexorably forward. Building this bridge was a delight, and that is manifest in the end product: a structure that is an homage to the poet whose name it bears and to the care the Steep Rock Association takes with the land it holds in trust for future generations.

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