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The Manawatū Tararua Highway (or Te Ahu a Turanga) in New Zealand is currently being constructed to replace an existing road that had to be closed due to landslides in this hilly and seismically active region. The main bridge construction challenge on the highway was posed by a four-span structure, the balanced-cantilever Parahaki Bridge, which required the superstructure to be temporarily supported during the construction stage. mageba’s team in Sydney provided the bridge designers with valuable assistance already at an early stage in the project, proposing a solution that would avoid the need, as previously anticipated, to construct temporary piers in the river – which would have been considerably more expensive, with longer lead times. With this approach, temporary bearings were required to fit onto the relatively small piers in a way that avoids any clashes with any other temporary or permanent parts or components of the bridge, with anchorages to suit the permanent bearings that would replace them. The solution as subsequently designed sees the superstructure supported at each pier by six free-sliding RESTON®POT bearings of the superior HP (“high-performance”) type, with horizontal loads resisted by two specially designed RESTON®FORCE shear keys.
The design of the bearings and shear keys is strongly influenced by the seismic conditions that required to be considered, including the rocking movements and impacts that might arise at each pier. Depending on the earthquake, the seismic loading may oscillate, and the system must be free to accommodate the associated rotations throughout an earthquake’s cycle, since the piers could not withstand the loading if the system ‘locks up’. With uplift movements of up to 35 mm being possible at one side of a pier during an earthquake, resulting in all vertical and horizontal forces being transferred to other bearings and shear keys, the components needed to be designed for very high loads and also to accommodate these uplift movement conditions.
The free-sliding pot bearings were designed with a vertical load capacity of 44,000 kN each, and with the ability to accommodate an uplift movement of 35 mm during seismic loading. The shear keys – and their connections to the superstructure and substructures – were designed to resist enormous horizontal loads of 13,700 kN each. All of the components were designed in accordance with Australian design standards as specified, allowing for the seismic load to oscillate for the duration of the earthquake in order to protect the piers against excessive loading.
The bearings and shear keys were manufactured in Australia, in accordance with Australian standards, in just 14 weeks – a considerable challenge considering in particular the need to source many very thick steel plates locally, and the special welding processes and testing requirements for the extra-large fillet welds required. Weighing in at 3,200 kg each, the high-performance, uplift-capable pot bearings are very large, but the shear keys, each 12,500 kg, are even larger – making the solution quite extraordinary for such a temporary one.
Client: HEB Constructions
Client’s engineer: Bamford Consultants and Kina Consulting
Owner: Waka Kotahi (NZTA)