The views are spectacular, but at Western Australia’s recently opened Kalbarri Skywalks, the engineering is just as impressive.
The cantilevered steel structures, which extend over the edge of the Murchison River Gorge in Kalbarri National Park, allow visitors to wander 17 metres and 25 metres beyond the gorge edge.
Terpkos Engineering was the lead structural engineering firm on the project, and the team won a 2020 Australian Engineering Excellence Award for their work.
Engineers Australia member, Terpkos Director Paul Terpkos CPEng told create that the use of glass as the walkway surface, like other international skywalk structures, was considered but did not meet the brief requirements.
“The skywalk decking is largely constructed from fibre reinforced polymer (FRP) mesh flooring and rectangular hollow section joists,” he explained.
“The FRP was selected due to its low maintenance requirements and long term design life. A glass surface requires relatively high ongoing maintenance or would typically require full time supervision.”
A feat of strength
Another Engineers Australia member, Andrew Jens, was the senior structural engineer on the project.
“The Skywalk 1 cantilever is approximately four metres longer than the Grand Canyon Skywalk,” he said.
“The connection between the weathering steel box beam and the cast in sleeve plate at the end of the concrete beam required 27 M36 bolts to the top and bottom, and 30 M24 bolts to each side plate. The minimum tension in the M36 bolts to induce the required friction between the plates is 490kN per bolt.
“Rock anchors were used at the rear concrete beam footings to restrain the footing from uplift, the rock anchors extend 10 metres below the underside of footings.”
Jens added that the skywalks were pre-assembled in a workshop to ensure the boxes fitted and could be connected with ease.
“As part of the pre-assembly, the FRP decking and barrier was also installed,” he said.
“The decking and barriers were left in position during transportation to the Kalbarri National Park and for lifting. This created a safe working platform for the steel riggers to work on as soon as the box beam segments were bolted on.
“The eight box beam segments on Skywalk 1 were lifted and connected in three days using a 350 tonne crane,” he added.
Two key aspects of the brief were ensuring that the skywalk structures blended into the natural environment, and minimising ongoing maintenance over the design life.
“The weathering steel is high strength, with a yield strength of 340 MPa, and the unique chemical composition forms a protective layer of patina that reduces the material loss due to corrosion,” Jens said.
It is anticipated that the box plates will lose 2 mm of thickness over their 100 year design life, which was factored into the structure models.
“The use of weathering steel had implications for the detailing of the structure, it is critical to ensure that water can effectively drain from the structure,” Jens said.
“Falls were documented in all the box beams and the connections were detailed to limit the possibility of crevice corrosion.”
The engineers had to ensure the walkway was fully accessible, which presented a new set of challenges.
“It was agreed that the beams would be installed below ground to ensure a gentle grade from the natural ground onto the skywalk decks,” Jens said.
“This required considerable excavation of the gorge rock. The excavated rock was stockpiled at a nearby site and then re-used to form the considerable stonework landscaping, which has become a project feature in itself.”
Finally, Jens and his team had to model the dynamic behaviour of the structure, so the public felt comfortable standing at the tip of the walkway without vibration from crowd movement.
“After some modifications to the box spine beams to maximise the natural frequency, the maximum acceleration was found to comply with AS 5100 Bridge Structures for the critical footfall frequency,” he said.
“The public response we have received has been very positive.”
The Skywalks, which opened in June last year, also feature art by the local Indigenous Nanda people, including in nearby play areas and the walkway leading to the platforms.