The Kumutoto challenge
An exciting new walkway bridge on Wellington’s water front, presented lots of challenges for the design and construction team, says JAMIL KHAN, a senior structural engineer for Beca.
The Kumutoto Walkway Bridge is now an iconic element on Wellington Water front, connecting the historic Tug Wharf to the refurbished Kumutoto promenade.
A project between Beca, Wellington Water Front, Studio Pacific Architecture and Brian Perry Civil, the 33 metre walkway bridge is made up of a post-tensioned deck supported on the boat cradle at one end and a crane frame at the other. The central span is 20 metres with about six metres cantilevers at each end.
The crane frame consists of two post-tensioned precast cantilever columns with varying cross section. One of legs is planar while the other is kinked in a dogleg shape. The bridge deck at this location is vertically supported by the crane frame on one side and hung by cables on the other. The tops of the cables are supported by cast-in-situ post-tensioned outrigger beams at the top of the crane frame.
The central core of the cradle provides vertical support to the deck. The boat cradle shell was pre-cast in pieces, assembled and installed at the top of the cast insitu pile cap. The structural connection between the deck and cradle is only through the central core.
Constraints brought about by aesthetic requirements and the construction of the sub-structure below sea level and tide construction programme also presented some challenges to both the construction and the design teams. Not the least of these was control of torsion in the bridge deck; control of creep and shrinkage in the crane frame; protection against lateral spreading of the soil; and the structure’s durability in a marine environment.
The cables hanging from the cantilever crane frame are flexible supports. As a result there will be differential deflection, in the transverse direction, between the two edges of the bridge deck beam, which will induce high torsion in the bridge deck beam. To overcome this issue a pre-stressing strain is induced in the hanging cables in such a way that for dead load conditions there is no differential deflection, in the transverse direction, between the two edges of the bridge deck beam. The turnbuckle assembly in the cables provides the means of adjustment in the deck level. The bridge deck is detailed to remain level (i.e. tension in hangers is adjusted to give no twist) under dead load conditions to avoid high torsions in the deck.
The crane frame is a cantilever in the transverse direction, and creep and shrinkage in concrete will increase the deflection of the crane frame. This will cause a deferential deflection of the bridge deck, in the transverse direction, at the deck supports. To reduce the long-term creep and shrinkage effect in the crane frame, a pre-stressing force is induced to achieve long-term balanced compression stresses under dead load and SDL. To have a balanced section means the tendons are offset towards the tension face of the cantilever columns and stressing is induced in stages as the load increases on the cantilever columns.
Lateral soil spreading
Protection against lateral spreading has been achieved by ensuring that the piles can work in double bending with plastic hinging in the piles at the desired locations. A pseudo non-linear pushover static analysis with Winkler springs was carried out with the static lateral force applied at the bottom of pile cap. The mobilised reaction at each spring was checked against the dependable soil lateral load resistance capacity.
If the spring reaction at any location along the pile length exceeded the soil dependable capacity, the spring was replaced by a force equal to the soil lateral capacity applied in the opposite direction of the lateral load. The structure was re-analysed, until all spring reactions were within the assumed dependable soil lateral capacity.
The vibration of the footbridge superstructure was investigated for the serviceability limit state. Two independent design checks were carried out using BS5400 and Austroads Bridge Design Code. The vibration of the bridge deck is limited to prescribed limits of these codes.
As the bridge is in a marine environment, all exposed structural steel is specified to be either Grade A316 stainless steel or
Grade 350 structural steel protected by Zinc Spray of at least 400-micron thickness. All bolts, nuts and washers are of Grade A4-80 stainless steel.
The cable hangers were hot dipped galvanised prior to the rolling of threads and all steel components have been designed to be replaceable.
Award winning result
The Kumutoto Public Space project has netted Brian Perry Civil the Category 3 trophy, for projects with a value between $5 million and $20 million, at the Contractors’ Federation/Hirepool Construction Awards.
Judges commented that a high standard of finish is apparent, belying the fact that the physical site constraints, weather and ground conditions proved difficult at times.
The George Malcolm Award for Landscape Design, one of two Supreme Awards at the NZILA Resene Pride of Place Landscape Awards went to the Isthmus Group for its part in the project.
“Working with a simple, bold, contemporary look that acknowledges other recent developments on the waterfront promenade, the Kumototo development manages at the same time to achieve seamless integration with the working heritage of the historic waterfront,” says Simon Smale, a landscape architect with the Department of Conservation and one of the three design judges.
The project also won the award of excellence at the IES Lighting Awards.
Contractor Vol.32 No.11 December 2008 - January 2009
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