Award of Excellence John James Audubon Bridge

South-central Louisiana is now home to the longest cable-stayed bridge within Canada and the United States of America: the 482-m John James Audubon Bridge, which crosses the storied Mississippi River.

Providing a long-awaited means of transport across the river in this rural area, the bridge was completed in 2011 and is expected to enhance the local economy well into the future.

The Audubon Bridge Project as a whole consists of 19.3 km of roadway and structure that ties US 61 and Louisiana Route 10. It is the first major design-build project undertaken by the Louisiana Department of Transportation and Development.

Centre span is the centrepiece

The centrepiece of the project is the central 482-m long cable-stayed bridge across the Mississippi River. The cable-stayed structure consists of five spans, symmetrically arranged about the navigation channel. The central span is flanked on each side by 189-m side spans, supported by the stay cables, and 49-m transition spans.

Buckland & Taylor was asked to design a cost-effective and durable Main Bridge that is consistent with the importance of the structure and the aesthetic theme.

The structure is named after the famed naturalist John James Audubon, who once lived at Oakley Plantation near the bridge. The natural colours selected for the aesthetic theme of the bridge pay homage to Audubon. The effect of the burnt umber coloured cable-stays together with the soft-white 152-m tall towers, provides a remarkable and striking addition to the skyline of the region.

Durability for 100-years

Several key features were included in order to meet the stringent 100-year design life requirement of the contract.

• A corrosion protection plan was developed. It provided the concrete properties required to ensure that the full 100-year design life of the bridge would be achieved before the ingress of chloride ions into the concrete, which would initiate corrosion.

• The structurally critical concrete deck is protected by a relatively impervious 2-inch thick overlay composed of latex modified concrete. The overlay is sacrificial and can be replaced over the life of the structure as necessary to preserve the underlying concrete deck.

• The structural steel is resistant to atmospheric pollution and carrion. The steel develops a stable patina of oxide to protect the underlying steel from corrosion, thus eliminating the life cycle costs associated with repainting.

• The pedestals supporting the main towers are protected at the water line from the impact of vessels by ultra high molecular weight polyethylene panels attached to their outside faces.

• The anchorage of the cable-stays in the towers provides access for stressing and facilitates their future replacement. However, the need to replace them is unlikely as the cable-stay system provides three levels of corrosion protection.

Grouting technique reinforces shafts

The weak soil conditions required the use of an innovative tip grouting technique to reinforce the bearing capacity of the 42 drilled shafts. After casting the pile caps inside 15-m high coffer dams, the 152-m towers were erected using jump forms to speed up the process (4-m lift/5 days). Steel anchor boxes are cast inside the hollow tower section to anchor the stay cables.

Lock-up devices activated during high winds

Under normal service, the superstructure is longitudinally connected to the west tower only using steel fixed links for a rigid connection. But during high winds, lock-up devices are activated to also connect the superstructure at the east tower. This system is highly efficient, allowing both towers to share any short-term or dynamic wind loading, allowing the structure to “breathe” under slowly applied thermal movements without building up large restraining forces.

Composite deck

The composite deck uses a steel grillage with a precast panel system. Every 14 metres each cable-stay is protected against vibration by a friction damper. To facilitate their installation in the tower, the stays were stressed one strand at a time. The relative flexibility of the superstructure required the stressing of the stays in two stages: the first to lock in the correct deck forces and geometry when the concrete deck was constructed and the second to obtain the correct final cable forces and bridge geometry. cce

Project name: John James Audubon Bridge, Louisiana, U.S.A.

Award-winning firm (superstructure design, foundation

independent check, detailed erection engineering):

Buckland & Taylor, N. Vancouver, B.C. (Don Bergman, P.Eng., Armin Schemmann, P.Eng.)

Owner: Louisiana Department of Transportation and Development

Client: Parsons Corporation

Design-builder: Audubon Bridge Constructors (Flatiron

Construction, Granite Construction, Parsons Transportation)

Other key players: Dan Brown Assoc. (geotechnical); Leonhardt, Andrä (independent analysis check); Parsons (foundation design, independent check), Louisiana Timed Managers/Parsons Brinckerhoff, LPA Group, GEC (program manager); RWDI (wind); Touchstone (architecture).