First-of-its-Kind Timber-Steel Hybrid Bridge Proposed for West Seattle
A proposal for the reconstruction of the failing West Seattle Bridge has been put forward that utilises a hybrid timber and steel arch structure. B+H Architects, SMEC and Robert Bird Group (all owned by Singaporean government-owned infrastructure behemoth Surbana Jurong) have put forward the unsolicited design proposal which they say will provide a sustainable, lightweight structure that is easy to maintain, and aesthetically eye-catching whilst being adaptable to future transportation changes.
The proposed design for the approx. 800 m bridge consists of compression arches made of steel tube sections, with 150 mm hangers consisting of mass timber with a steel or carbon fibre core suspending the deck.
“We envision the West Seattle Bridge as a functional, beautiful monument to the innovative and conscious populace it serves,” the proposal says.
The timber hangers would range from 8.5 to 15 m long, and the end grains capped with steel to prevent water and moisture ingress. Steel connectors, approx. 1 m in length, would connect the hangers to the deck and the arch.
A sinuous form of arches continuing above and below the deck enable the 180 m and 115 m spans of the structure. The underdeck arch would incorporate longitudinal stainless-steel cables or carbon fibre tensile strands. The deck would comprise of transverse steel trusses and be supported by concrete piers.
The proposed use of hybrid mass timber-steel hangers responds to typical, asymmetric loading conditions of a bridge where tensile steel hangers experience periodic, momentary compression forces resulting in various bending moments in the arches.
The proposal says that the use of mass timber-steel composites can “reduce the stress range in the hangers and potentially improve their fatigue performance”.
“In environmental terms, timber serves as a carbon sink with a comparatively low embodied energy footprint; it is the only renewable building material. [Using timber will] put Seattle on the map for fuelling an economy based on renewable resources.”
B+H says by using timber, the bridge’s existing concrete foundation and piers can be reused, further contributing to a lower carbon footprint of the bridge.
The West Seattle Bridge opened in 1984 and is the city’s most used bridge, carrying an average of over 100,000 vehicles a day. It is the primary connection between West Seattle and the rest of the city.
The bridge was closed in March 2020 due to worsening cracks in the post-tensioned concrete box girders. Cracking in various areas on the bridge had been known for a while, but recently the cracks had extended 60 cm in one month, sparking the mayor of Seattle to declare a civil emergency and order the immediate closure of the bridge.
The cracks are suspected to be a result of excessive long-term concrete creep causing steel stressing cables to slacken, reducing the load-bearing capacity of the girders.
Stabilisation works have commenced whilst a feasibility study is undertaken to determine the fate of the bridge. Officials say they are considering 6 options, 3 of which involve remediation and 3 involving replacement.
Recently, the world’s tallest hybrid building was unveiled for Atlassian’s new HQ adjacent to Sydney’s Central Station.
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