Newmarket Viaduct

The Newmarket Viaduct, sometimes considered 'one of the most distinctive engineering features' of New Zealand, is a seven-lane State highway viaduct in Auckland, the country's largest city. Carrying the Southern Motorway over the Newmarket suburb area southeast of the CBD of the city, the 700 m long viaduct is up to 20 m high in places. Due to concerns about its ability to withstand earthquakes and its increasing inability to cater for peaktime traffic demand, it is being replaced with a stronger and wider structure over several years, starting in 2009.

The viaduct was opened on 3 September 1966 at a cost of NZ$2.26 million, and was constructed as New Zealand's first pair of balanced cantilever bridges. With its long spans, the s-curvature and the varying superelevation of the motorway, it is a complex structure even today, and at the time was ground-breaking design for New Zealand.

While considered a successful design, it was built to far lower earthquake standards than which other New Zealand structures now fulfill, being built to survive only a 1 in 500 year return period earthquake. It could have received significant damage in an earthquake as common as once in 200 years. This made the structure the weakest strategic transport infrastructure link in the Auckland area, with a potential collapse cutting off all motorway transport to the South. Also, during design, temperature stresses were not yet understood to a sufficient degree, leading to increased wear on structural elements.

The limited earthquake stability, together with increasing traffic demands (over 200,000 vehicles a day, more than on the Auckland Harbour Bridge and the most traffic on a section of road anywhere in New Zealand), led to the planning for a new viaduct structure, which will be able to withstand an earthquake with a 2,500 year return period. Other factors were the very low safety barriers (which once allowed an out-of-control truck to fall onto its roof 15 m below, in 2004), which are also insufficient to prevent debris from falling onto the properties beneath, and the fact that the existing viaduct is a prohibited route for overweight vehicles, forcing more trucks through the city streets.

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Wikipedia