Award of Excellence Strandherd Armstrong Bridge Erection

A new multiple space-truss arch bridge crosses the Rideau River in Ottawa. The Strandherd Armstrong Bridge is located south of the airport, connecting the communities of Barrhaven and Riverside south.

The bridge has a concrete deck supported by steel framing that is in turn suspended by cables. The total span of the completed bridge is 143 m, while the arches rise above the deck level approximately 21 m at their peak. The crossing has eight full traffic lanes, two bike paths and two pedestrian sidewalks.

River navigation on

a UNESCO site

A major challenge was the requirement for the navigation channel to remain open from May to October. No overhead work could take place Friday through Monday of each week during this time frame. It was inconceivable for a construction site to sit idle for four days of every week during the summer months. Therefore a construction and erection method that significantly limited the amount of work completed over the waterway was essential.

In addition, the Rideau River is a designated UNESCO world heritage site which heightened the need to limit the project’s environmental impacts at the site.

Innovative construction method and launching

Horseshoe Hill Construction, the contractor, required a practical and affordable erection plan and approached Harbourside Engineering Consultants. The engineers designed an innovative construction method that allowed 90 per cent of the steel superstructure to be erected on temporary supports on the east approach. The superstructure was then launched into place across the river. After the arch segments were welded and the deck was suspended, the ends of the arches were tied together with a “bow string” horizontal post-tensioned cable system and supported on railcars. The steel superstructure could then be rolled across the temporary launch structure.

The launch structure consisted of three parallel steel box trusses that were aligned transversely with each of the bridge arches. They were founded on temporary piled bents consisting of drilled, then driven, steel pipe piles and a reinforced concrete pile cap. The pile bents were strategically located outside the navigation channel and the trusses were designed to be positioned above the navigation channel throughout construction.

Stability during installation with

shoring towers

Stability of this type of structure through all phases of its construction becomes challenging since the final structure relies on the entire deck weight, the parallel strand system cable hangers that tie the arches laterally to the deck, and the fixed anchorages at the thrust blocks. Having three independent arches constructed across the river without the deck providing stability through the hangers or stay cables would leave the arches susceptible to high winds.

The use of shoring towers on land made construction of the arches much easier and more accessible. The arch segments essentially became bending members, spanning between the vertical supports of the towers located at each segment field splice. The towers also provided the transverse and longitudinal support which ensured stability of the arch throughout its construction, even during high winds when the deck was hung from the arches.

Environmental benefits

The construction and erection plan greatly minimized disruption to the water course and habitat by keeping the majority of the construction materials contained on land.

The bridge provides the missing link between the southwest and southeast transit corridors in the city of Ottawa. It greatly reduces the transit time for residents, thereby reducing emissions. cce

Project name: Strandherd Armstrong Bridge Erection

Award-winning firm (erection and construction engineer):

Harbourside Engineering Consultants, Charlottetown,

P.E.I. (Robbie Fraser, P.Eng., Greg MacDonald, P.Eng.,

Kyle Boudreau, EIT, Nick MacEachern, P.Eng., Stephen

O’Brien, EIT, Ronald Keefe, EIT, Sarah Foster Hardy, P.Eng., Daniel Grant, EIT, Chris Mantha, EIT, Paul Burke, P.Eng.)

Owner: City of Ottawa

Client: Horseshoe Hill Construction

Other key players: Stantec (geotechnical); Cherubini Metal Works (fabricator); Montacier International (steel erector);

Freynssinet International (PSS cable supplier);

Parsons/Delcan (bridge designer).