Commissioned by the NSW Governments’ development agency, Landcom, the pedestrian and cycleway bridge is believed to be the first of its type in the world.
The bridge is a 178m overall length, variable diameter helical steel truss bridge, curved on plan, spanning from the new Lachlan’s Line Residential Precinct on its western extremity to the North Ryde Commercial Sector and Metro Station on its eastern extremity. It traverses both Delhi Road and the M2 Motorway.
Fabricated from 200 tonnes of locally produced structural mild steel and 80 tonnes of high grade imported Duplex 2205 stainless steel, the bridge comprises 4 segments with a maximum span of 50 metres. The substructure comprises reinforced concrete abutments and 3 piers, each in turn supported on bored reinforced concrete piles. Bridge approaches to both ends are landscaped and paved in keeping with the free-flowing theme of the structure.
Daracon Landscaping were awarded the soft landscaping package for the Lachlan’s Line Bridge in early July 2019 to enable the specified plant species, numbers and sizes to be available for when planting works would begin in April 2020.
Scope of Works
Arenco Daracon JV was responsible for the novated design and construction of the new bridge. The steel superstructure consists of 78 chord members, 160 diagonal members and 82 connecting nodes (320 members in total), creating a singular longitudinal truss arrangement. The fabrication of each member required individual cutting to size and bending of approximately 4,000 flat steel pieces prior to welding to form the individual box shaped members. Approximately 14 km of welding was undertaken and every member was unique in shape and plate thickness.
Members for each segment were trial assembled in the fabricator’s workshop using specially designed support jigs, tack welded in position and surveyed to verify dimensional accuracy. From there, segments were dismantled, grit blasted and painted for delivery to the purpose-built site fabrication facility. There, they were reassembled using the same support jigs but fully welded whilst again being surveyed for dimensional conformity. The decking elements were fabricated entirely off site, prior to delivery and fixing into each segment. Paint touch-up was completed concurrently and tensile stainless steel mesh protective screens enveloped the external form. As one segment was completed, the adjoining segment would begin, replicating the same process whilst ensuring the splice connections between those adjoining segments matched perfectly.
Detailed structural design analysis enabled the curved fabricated box sections to be optimised with varying plate thicknesses and thus ensured the absolute minimum quantity of steel was required. The diameter of the bridge was adjusted to achieve greater structural depth at locations of higher demand, as opposed to increasing material use at a shallower depth. Robust materials and finishes, the use of closed structural sections and smooth connections between elements minimises maintenance and ensures long-term durability.
Advanced design and modelling software was required to assess each structural segment in its temporary state during handling, lifting and final positioning.
Extensive planning was undertaken with the Transport Management Centre (TMC) for all 4 segment installations to understand and mitigate the effect each would have on the Sydney traffic network. The key objectives were the absolute minimisation of impacts on traffic and the community. Traffic engineers prepared detailed analyses to assess projected traffic queuing and compare potential traffic detours. This process was a balance between traffic optimisation and mandatory construction aspects including personnel safety and bridge installation complexity.
Using limited road closures, the 4 x individual bridge segments were successfully erected into position using a single crane lift methodology. The heaviest lift was Segment 2 with a gross lifting weight of 135t involving a 600t crawler crane and closure of both Delhi Road and the M2 Motorway.
Project success relied heavily upon detailed planning especially in temporary works design, understanding of site restraints (utilities, M2 Motorway proximity, Delhi Road Bridge and traffic), crane selections, resourcing and contingency planning