1. IntroductionThe historic Pyrmont Bridge commenced construction in 1899 and officially opened in 1902. Over the past century, it has operated as the primary gateway to the Sydney CBD from Pyrmont. While originally designed for horse and cart, the Bridge has since provided a journey path for buses, trams and monorails. In more recent times the Bridge is used primarily for pedestrian thoroughfare offering a scenic walk across Sydney’s iconic darling harbour. The Bridge consists of 14 timber truss spans, each 25m long with a total span of approximately 370m. Arguably Pyrmont Bridge’s most impressive feature is its electric swing span, the first of its kind in the world. The swing span pivots about a central pier to provide access into the harbour for larger boats, the swing path opens over 300 times per year. Pyrmont Bridge is a heritage-listed icon and requires ongoing maintenance commensurate with a public timber bridge that is over 100 years old. Following a review by RMS, a significant percentage of structural components were identified, including timber trusses and piers, required replacement. The restoration was determined to be completed in two stages, Stage 1 would involve the replacement of the Timber Pier Sets, and Stage 2 would include the replacement of the timber trusses. Robert Bird Group (RBG) was engaged on behalf of Waterways Construction to provide the construction engineering methodology for Stage 1, which is now complete. Stage 2 has not yet comenced.

Pyrmont Bridge Restoration ProjectACSE 2020 Awards Unusual Projects Category

2. Project BriefRBG’s design brief was to develop a construction methodology in collaboration with Waterways Construction (WWC) such that bridge pier sets could be ‘de-loaded’ to allow removal of Bridge structural elements. Key challenges of the project brief include:• The Bridge is to remain open to the public throughout the entirety of the works. This period would

include times of very high pedestrian traffic, including ANZAC day and the Vivid Festival.

• Existing jacking equipment located at Pyrmont Bridge was adapted for reuse in the current project.

• Stability of the Bridge through all construction stages was to be demonstrated for wave, impact and wind loading.

• The construction methodology was to consider the permanent ‘out of balance lean’ of the Bridge; refer to sections below for more.

RBG used a Strand7 construction stage finite element analysis of the Bridge to ensure the Bridge was stable and all structural elements remained within allowable structural capacities.

3. The Method and Construction ProcessReplacing the Bridge Pier sets while keeping the Bridge live to the public was the critical project challenge.

RBG took into consideration the project brief and constraints to develop innovative structural solutions that would ensure successful delivery of the program.• Structural Modelling of Pier sets to understand load distribution of pier elements.

• Design restraint system to control pier sets position during de-loading process.

• Carry out advance analysis of de-loading process to assess the overall stability of system from lateral and vertical actions.

• Re-assess pier member capacity due to redistribution of load.

• Provide detailed design of the restraint system.

Structural drawing Analytical Model

Original stateRemoval of waler beams

Removal of right rakerRemoval of right raker and bracing De-loading stage

Removal of walers, bracing and left raker

4. Sustainability, Built Environment and HeritageThe Pyrmont Bridge is of State heritage significance. It is an iconic landmark in Sydney’s Darling Harbour, the Pyrmont Bridge restoration project further prolongs the life of the Bridge by revitalizing its structural integrity. All work was completed with respect for the existing structure and leaving as much of the original Bridge fabric intact as is practical. Where possible, deteriorated members were repaired rather than replaced, where replacement was required Ironbark timber of similar age and quality was selected to suit specific locations on the Bridge. Where structural timber connections required replacement, new connections were designed to replicate the existing connections such that the aesthetics of the Bridge were maintained. The project is an excellent example of structural engineering promoting sustainability. Restoring the Bridge, gives the Bridge a second (or even third) life and negates the need for a replacement bridge (with a large carbon footprint) to be constructed.

5. The Challenges and ResolutionsIt is a little-known fact that the Pyrmont Bridge has a permanent lean towards the City end once you are made aware of the lean it is evident to the eye. There are various theories as to why the lean exists but no definitive answers. What is known is that the lean increases towards the Pyrmont side and is generally getting worse with time. One of the key challenges of the Pyrmont Bridge restoration process was to predict the behaviour of the Pier sets once they were deloaded. There was a general concern, particularly with Pier Set 7 which shows the most significant lean, that the pier set would ‘spring back’ into position once the weight of the Bridge was taken off it. While this would result in a re-straightening of the Pier Set, there was a fear that the Bridge would not be able to be supported on it’s intended location and cause destabilization of the de-loading equipment. To resolve this challenge, RBG applied Advanced Finite Element Analysis to simulate the ‘locked-in’ energy in the leaning Pier Sets so that ‘spring back’ movements could be quantified. RBG then developed a steel cable restraint system that ‘tied’ the Pier Sets in place while de-loading occurred. The finite element analysis simulated the contact pressures and subsequent friction between the headstock and the de-loading equipment to ensure the preloading of the tie-back system was not sufficient to cause destabilization of the jacks. Along with valuable comments from the Project Principal Engineer Sid French at Advisian Ltd and other engineers at WWC Ltd, RBG’s solution resulted in all bridge decks being lifted off safely with no interruption to the bridge operation.