e-mosty December 2018 e-mosty December 2018 | Page 56

III. SPECIMEN DESIGN VS. TENDER DESIGN
The Water of Leith Bridge project was a Design and
Construct contract to deliver a cycle-bridge for
Dunedin City Council. As part of the tendering process a proprietary
composite decking manufactured from 90% recycled
materials (HDPE plastics and bamboo) was proposed.
The original Specimen Design released with the
tender documents was for a 45m span steel truss
bridge with steel decking covered in epoxy non-slip
surfacing. Early estimates predicted a steel weight in
the order of 50 tonnes would be required to traverse
the 45m span. The product is a highly durable non-slip surfacing
integrated into the top surface with incredible
sustainable credential compared to steel and/or FRP
equivalents.
The Edifice Contracts D&C Team featuring bridge
Architect and Engineer Dan Crocker of DC Structures
Studio and Eli Maynard of Geosolve Consultants
developed a striking but economical alternative cable-
stay bridge concept.
The proposed design reduced the amount of
steelwork to circa 30 tonnes by promoting the use of
glulaminated timber and composite decking.
By preferring the use of timber, stainless steel, and/or
man-made plastics the design provided enhanced
durability and lower overall maintenance compared to
the steel truss in the Specimen Design.
The client team also noted the iconic nature of the
cable-stay was a governing factor in the selection
process compared to the industrial look of the truss
that they were expecting.
A key benefit of using the composite decking
(compared to concrete or steel decking options)
comes from its lightweight properties and proven
performance in marine environments.
IV. SUBSTRUCTURE AND FOUNDATIONS
DESIGN
Bridge abutments are formed from reinforced
concrete abutment beams supported by steel driven
H piles founded in the gravels.
A range of piling options were considered including
bored piles, grouted micro-piles, and driven piles
(concrete, H-piles, timber etc.). Driven steel H-piles
were selected for constructability and performance
reasons.
They are installed behind the existing Water of Leith
Sea Walls. Piles are 12m long at the west abutment
Figure 5: Deadman anchorage prior to pouring concrete
Figure 4: Steel driven piles being installed
4/2018