ICE Bridge Month: Bridge #6 – Falcon Street Pedestrian and Cyclist Bridge

Post No. 6 for #ICEbridgeMonth in February 2019. In August 2009, the Falcon Street Pedestrian and Cyclist Bridge opened to the public. The bridge was initiated following the completion of the Lane Cove Tunnel project which included the addition of four new off and on ramps onto the Falcon Street road bridge impeding pedestrian movements between Neutral Bay/Cremorne/Mosman area and the Crows Nest and North Sydney CBDs. This major change to the Falcon Street Road Bridge affected the experience and safety for pedestrian and cyclists.

The new bridge would provide a designated safe route for cyclists and pedestrians and in addition would provide a key link in the regional cycling route connecting Chatswood and North Sydney. Separated regional cycling routes continue to be a key focus of Transport for NSW in 2019, 10 years on after the bridge opened.

The new bridge is a curved, continuous steel box girder structure which allows unimpeded access for pedestrians and cyclists across nineteen lanes of the Warringah Freeway north of the Sydney Harbour Bridge. Extensive stakeholder consultation drove the optimum location and alignment to suit community needs. Specific project constraints included issues surrounding construction access, shallow grades for mobility impaired users, vertical clearance over newly constructed traffic ramps, meeting specific bridge landing points and providing a visually attractive facility within the existing urban environment. The constraints dictated a continuous long span steel box girder structure, curved both in plan and elevation as the best solution.

The bridge is comprised five spans of approximately 193 m in total length. The span configuration is 15.7 m, 36 m, 66 m, 65 m and 10.3 m. The bridge has a 3.0 m clear width between handrails. After an extensive review of options, a trapezoidal steel box girder option was selected by the project stakeholders based on the following criterion:

• The ability for the steel box girders to be used economically in long spans as opposed to a concrete or composite section
• The speed and ease of erection required
• The aesthetic appeal and reduced maintenance specified by the client
• The fact that steel box best facilitates the curved horizontal and vertical alignment

The superstructure is a 1500 mm deep fully welded trapezoidal orthotropic steel box girder. The deck slab has an anti-skid corrosion protection coating that accommodates pedestrian and cyclist traffic across the bridge.

Provision was made in the design for the installation of Tuned Mass Dampers (TMDs) to negate dynamic effects due to the slenderness of the bridge structure. Minimising the overall structural depth was a key design consideration to meet the project constraints.

I asked John Hilton, Design Director, Bridges at Aurecon and Mark Alexander, NSW Infrastructure Leader at Aurecon some questions to gain insights into the design phase. John was the Aurecon Project Director and Mark was the Design Manager. I was also fortunate to work on the project as the Construction Stage Services Engineer and got great experience for the construction of complex steelwork.

Q: How many options were there to cross the busiest road in Australia?

Mark said “There were many options considered, these were narrowed to six based on many factors and from community feedback. Some were further North, some further South and some landing over in St Leonard’s Park. There was a need to match the pedestrian desire lines very closely and to keep the bridge at an elevation in keeping with the surrounding infrastructure.”

 Q: What was the most challenging part of the construction?

John said “In providing for disabled access the maximum structural depth was limited to 1500 mm, which was a real challenge for the 65 m span requirements. Logistical constraints ruled out cable stayed or through truss options.  As a result, the decision was made to adopt a steel box solution, with the box top flange acting as the wearing surface. In keeping with shallow structural depth requirements, the superstructure was made continuous, which also assisted in controlling live load deflections and dynamic effects.”

 Q: What was the most satisfying part of the design?

John said “Girder segments were lifted in during partial closures of the Freeway at night using temporary erection towers. Geometry control was critical. The erection sequence required some segments to be erected between previously erected units with a 2 mm accuracy on position.  As segment length varied with temperature a means was provided during construction to move segments longitudinally to allow new segments to be installed to the required accuracy. This all worked like a charm. Excellent work by the steel fabricator, Adua, and the main contractor, Reed Constructions. Aurecon undertook all temporary works for the project.”

 Q: For this design, the architecture is very striking. How did the interface with the architect work? 

Mark said “Aurecon engaged KI Studio and the design was given great respect by the RMS Project Manager, Martin Jones. The design evolution was collaborative with the architect and we very much enjoyed working with Miguel Wustemann. I remember the first time I saw the precast concrete blade piers, just 600 mm wide. I thought, “this will be a challenge”, but with some clever post-tensioning they worked and look fantastic. I also love the look and feel of the safety screens on the bridge.”

 The client was Roads and Maritime Services who engaged Aurecon to undertake the detailed design. KI Studio was the urban designer with Reed Constructions engaged under a Construct Only contract to deliver the project. Adua Engineering manufactured the steel box girder.

The project won the National Award for the Infrastructure and Mining Category of the 2010 Australian Steel Institute Awards. A great project to be involved in for me personally but also I love using the bridge every week to cross one of the busiest roads in Australia.