Atiamuri Bridge is a 123m long steel-concrete composite bridge, which carries SH1 traffic over the Waikato River on a single undivided carriageway. This bridge was designed by BBO to replace the existing bridge which had progressively suffered from severe fatigue issues for many years.
The challenge
The Atiamuri Bridge site is located immediately downstream from Atiamuri Dam (serving the hydroelectric plant operated by Mercury Energy). As such, the river flows are essentially managed, however, flows are highly variable on a daily basis. Part of BBO’s challenge was to design a bridge superstructure elevated well above extreme flood flows, with pier foundations and stems catering for the direct demands of flood flows, and the indirect impacts resulting from scour.
The carriageway geometry also needed to be consistent with the geometry of the realigned approach carriageways to the north and south, meaning significant curve widening was included on the upstream (northern) side of the bridge to cater for sight distance requirements. The key decision for bridge geometry was the length of the main span crossing the Waikato River, impacts on length of approach spans, and hence on overall bridge length.
This bridge was constructed as an Early Contractor Involvement (ECI) form of contract. The design needed to provide a technically defensible and cost-effective bridge structure, but also one which was practical to construct, tailored to the contractor’s skills, experience, and equipment. It also needed to minimise effects on the Waikato River.
The outcome
Pier positions are located beyond normal water level in the river channel, recognizing potential cultural issues associated with piers in the riverbed. This pier position also eased construction logistics and avoided acceleration of river flow velocities.
A 4-stringer system was adopted for the bridge deck, designed as a steel-concrete composite structure, located on a vertical grade and horizontal curve, and flaring out towards the northern end. Two sets of parallel stringers with optimum deck edge cantilevers and variable internal stringer spacing were adopted, to eliminate the need for curved beams. This saved significant fabrication costs.
As fatigue was identified as governing the design, a series of detailing and design limitations were implemented.
The Bridge is supported on piled foundations. Sleeves were provided for piles at abutments, to match piles stiffness at piers and abutment under lateral load, and to optimise horizontal load distribution under transverse seismic loads.
