Scenic Caves Suspension Bridge

The 126-metre long pedestrian suspension bridge that opened last June at Collingwood Scenic Caves Nature Adventure park in Blue Mountains, Ontario overlooks a 100,000 square kilometre panorama that includes Georgian Bay.

Originally conceived by the park’s owner, Rob Thurburn, the bridge had to be integrated into the surrounding environment with a minimum of disruption to local topography and tree cover. Environmental agencies also placed limits on the tower height, bridge materials and foundation fooprint. No trees could be removed and there was to be no interference with a small stream below.

Since the trees in the valley floor grew higher than both the proposed landing areas, a traditional sag type suspension bridge that Thorburn had originally envisioned would not fit the site. Instead, R.J. Burnside’s team proposed a suspension bridge where the walkway would arch in a parabolic shape upward over the valley. The design minimized interference with the trees and helped address the issue of wind loading. Thorburn did not want stiffening trusses or unsightly wind braces, and he was willing to close the attraction when wind conditions dictated.

Burnside took advantage of the upward parabolic shape by introducing two pre-tensioned bottom cables that would trace the shape of the walkway and provide resistance to wind loads.

The interaction between the upper primary cables and the bottom pre-tensioned cables proved to be an interesting exercise. For every load case, the interaction between the cables had to be satisfied and many iterations were required to balance the effects and distribute the loads.

Suspended between steel towers 11.5 metres high, the steel and timber footbridge is 125 metres between the towers and 1.5 metres wide. The main cables are 187 metres long, anchored in 1.8 metre open bridge sockets. Hangers are 12.5-mm diameter wire rope.

Flying arrows

To install the main cables and hangers, specialists Remote Access Technology assembled a crew from as far away as Nova Scotia.

The team started by shooting an arrow and line over the valley. The line is still visible and has become part of the lore of the building of the bridge. The crew pulled larger lines across until curtain lines were in place. Then, helped by constructors Owen King, they lifted the main cables to the curtain line with a small crane, and attached pulleys to the curtain line. The main cables were pulled across like an inchworm. The crews took three days to place the first cable, but benefiting from experience, took only one day to place the second.

Once the main cables were placed, the team used an overhead buggy and platform to deliver and attach by hand the pre-measured, pre-assembled suspender cables and floor beams. As work progressed, crews began bolting timber decking to the floor beams.

Once all hangers and timbers were placed, the crew fed the bottom cables through specially fabricated reinforced boxes in the floor beams and then tensioned the cables in place.

Open year-round, for hiking and snowshoeing, the $550,000 bridge has helped to increase attendance in the park by 36% during its first year of operation.

Client: Collingwood Scenic Caves Nature Adventure

Prime consultant: R.J. Burnside & Associates, Collingwood, Ont. (Steve Riley, P.Eng., Victor Segula)

Construction: Owen King

Cable installation: Remote Access Technology