By Myron Washchyshyn, P.ENG., Mulvey & Banani International
INFRASTRUCTURE: Hornes Brook BridgeEngineering
The Hornes Brook Viaduct, owned by the Cape Breton and Central Nova Scotia Railway, is located five miles west of Stellarton in Nova Scotia. The bridge was originally constructed in 1887 and is one of...
The Hornes Brook Viaduct, owned by the Cape Breton and Central Nova Scotia Railway, is located five miles west of Stellarton in Nova Scotia. The bridge was originally constructed in 1887 and is one of the few wrought iron bridges remaining in North America.
The bridge consists of nine wrought iron deck plate main spans with two timber approaches at each end, totalling 404 feet (123 metres) of track. When the railway decided to run heavier 286,000-lb. rail cars in 1996, a consultant found that the structure was overstressed by as much as 40% for the proposed loading. All the superstructure was deficient in carrying capacity. The railway responded by running alternate empty spacer cars at reduced speeds to avoid overloading the bridge, and in 2000 asked for proposals for strengthening the structure.
Delcan was one of nine companies that came up with proposals. The other eight proponents based their approach on the recommended conventional strengthening repairs, which involved swapping the deck sections as they were repaired. Delcan proposed a solution that was less expensive, faster, safer and less disruptive.
Their scheme was deceptively simple. The engineers added two new A-frame towers between the existing towers to effectively halve all the span lengths. They also designed new braces inside the towers to halve the length of their spans, and added new steel bents supported on concrete caissons to support the bridge’s end spans.
The design involved encapsulating, strengthening and enlarging the existing masonry tower footings to enable them to accommodate the additional A-frame tower legs. Environmental permits were obtained to allow the contractor to add 600 tonnes of riprap to the banks of the stream bed, which improved the channelling of the stream, and protected the tower foundations.
Since the strengthening generally involved adding more supports, most existing bridge members were left alone and the structural integrity of the bridge was never compromised during the repairs. Rail traffic was hardly interrupted whereas the span swapping method would have meant extensive track closings.
The total project cost $5,000 less than the original estimate of $800,000. It was completed in a compressed schedule between February and July 2000. At the completion, a train engineer commented that “the bridge had never felt so solid.”
Owner: CBCNS Railway
Designer and project manager: Delcan Corporation, Toronto (Stan Reimer, P.Eng.)
Site monitoring: Sobey & Associates
Soils: Jacques Whitford
Steel fabrication: Cherubini Metal Works
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