Bridges

Whether you believe sea levels will rise in the future due to global warming or not, can bridges be easily designed to accommodate changing sea levels? This paper describes how a bridge on New Zealand’s state highway network has been designed with an adjustable clearance mechanism to accommodate any potential increase in sea levels. Punganui bridge is a three span, two lane road bridge on SH16, New Zealand, located in a 300 metre long dip in the State Highway, where the existing narrow reinforced concrete bridge had significant decay and required replacement. The bridge spanned a tidal stream and experienced continual wetting of the span soffits during high tides, which presented a significant durability issue as well as posing a long term sensitivity to rising sea levels. The design considered a super-structure that could be easily lifted up 600mm from the substructure, without compromising its resistance against earthquakes. As the lifting mechanism may not be used for many years, the design needed to be robust to withstand harsh tidal environments and remain operational for a future date. This paper discusses the design details of the bridge, as well as the context of the decision to design and construct a liftable bridge for a future potential event. Other options are discussed, including upgrading the road approaches, or constructing a bridge that may well need to be demolished within its 100 year life if sea levels rise.