Two iconic bridges in New Zealand portrays gracefully the skills of a generation of bridge engineers at Beca who have worked on a range of projects that have now become part of the country’s heritage.

I couldn’t help but note with pride when I saw the recent KiwiRail Scenic Journeys TV commercial featuring stunning shots of their trains crossing two Beca-designed iconic bridges in New Zealand’s North Island – the South Rangitikei Viaduct (famous amongst engineers because of its earthquake-stepping piers) and the Hapuawhenua Viaduct. It also portrays gracefully the skills of a generation of bridge engineers at Beca who have worked on a range of projects that have now become part of the country’s heritage.

This trip down memory lane prompted me to write about the special technical features of these bridges which were so innovative at the time. These iconic bridges would not have been possible without the input from Technical Director Rob Jury who led the design of the Hapuawhenua Viaduct and Chief Structural Engineer Ian Billings who was involved with the South Rangitikei viaduct. Also, Chris James continues to be our technical guru in all matters to do with aerial photography, digital orthophotography and design using digital terrain modelling.

Hapuawhenua Viaduct
The 414 m Hapuawhenua viaduct, completed in 1987, was built as part of a 10 km deviation (i.e., a new alignment) of the main line between Ohakune and Horopito through the Tongariro National Park. The deck has a 420 m radius horizontal curve and a significant slope from one end to the other. As well as the usual challenges of earthquake resilience, the design forces included those from the emergency-braking of a freight train in full flight downhill on a curve. These try to twist the deck about its longitudinal axis. An innovative system of highly-stressed steel bars at each abutment holds the deck down and the lateral loads are carried by a specially designed 'anchor' pier located approximately one-third along the length of the bridge.

Prior to design there was much concern as to what the visual impact of the bridge structure would be. Using what was leading-edge software at the time, we produced multiple perspective views of different bridge structure configurations (many short spans, fewer long spans, varying pier thicknesses, constant pier thicknesses) set in an accurate contoured model of the site to assist the decision. Although rendered only in black and white, these views signalled a new era with respect to the realism, accuracy, speed and versatility of production.

We also designed the railway deviation itself. In the early 1980s we led New Zealand in the use of digital terrain modelling using the revolutionary software MOSS – initially running this ‘in the cloud’ through a dedicated connection to the CDC Cybernet system (long-disappeared) in Melbourne. MOSS has morphed over the decades to the MX Roads we continue to use. The primary data for the digital terrain model, which was 10 km long and half a kilometre wide, came from existing contour plans which were digitised using an optical/mechanical line-following device by a Western Australian professor. Our New Zealand Railways client showed great faith in us and the technology by accepting that they would not see the traditional deliverables of sheets and sheets of cross-sections until nearly the end of the process. In parallel, we were using stereo aerial photography by NZ Aerial Mapping to capture in digital form more detail in some locations.

As the project progressed, the advantages of the digital approach showed our client that their faith was well placed. Our original model was augmented with field survey using tape, compass and Abney. Our surveyors commenced the set-out with traditional theodolites, but then introduced the new ‘total stations’ which could pass data to/from the digital terrain model. The productivity jump was so dramatic that the client banned from site the traditional instruments. The locations of each top-of-cut and bottom-of-fill for the carefully optimised earthworks for the new railway line were marked on the ground with yellow tape by the surveyors, and our client was able to walk the proposed line with environmental lobby groups. In one location, a significant tree which was the habitat for an endangered bird was identified as likely to be felled. Over the weekend, we were able to propose an acceptable shift to the alignment. Without fuss and with no delay in the production of the construction drawings, all was resolved. Our client suggested that the traditional manual process would have taken some months for the changes to be made.

During construction, the client agreed with the earthworks contractor that the work in progress for a change of rates would be measured by aerial photography at midday on a particular day. Our surveyors and aerial photographers learnt some unusual lessons that day on where not to place visible benchmarks on the ground. Some white crosses were painted between the existing rail lines, and others were of white cardboard in surrounding pastures. On their first pass overhead, the aerial photographers found that a train was over one of the crosses, so they turned an re-flew the site only to find that the train had moved on to cover the other cross. A cardboard cross in the pasture was then found to be missing – devoured by a cow.

South Rangitikei Viaduct
At 78 metres high, and 315 metres long, the South Rangitīkei viaduct is one of three that replaced a steep, winding section of the North Island main trunk line. It incorporated an earthquake resilience feature that was unique in New Zealand and probably first in the world. In a large earthquake, the bottom of the shafts of each pier can lift up to 13 cm to allow pressure to shift from one shaft/leg to the other for motion transverse to the line of the bridge. What is more, revolutionary “rusty hinge” energy absorbers at the stepping interfaces dampen the motion. Forty years on, our structural engineers still receive international enquiries about this bridge.

Construction by the Italian contractor Codelfa Construction Ltd began in 1974. In 1975 falsework (temporary structure) for the first span collapsed under the weight of the wet concrete. Coincidentally, falsework collapsed under the ramp to a major motorway bridge in Auckland the next day. These events resulted in major changes to falsework design by contractors in New Zealand. After a four-year hiatus, construction began again in 1979, and the bridge was opened in 1981. The original falsework was made from the launching girder used in Auckland on the Pakuranga (known locally as the Waipuna) bridge also designed by Beca. These were technically challenging times. The contractor was still reliant on calculating deflections of the falsework by traditional graphical methods, whereas Beca was using 3D analysis software much the same as we use now. There was a big effort put in to make sure the replacement falsework would fare better. Our site team lived in the nearby small town of Mangaweka from where anecdotes emerged of engineering mixing with excellent pasta and red wine. A number of papers were published at the time describing the leading technology.

Although my involvement with these three railway projects was minor, it warms my heart to see these iconic bridges highlighted after all these years. Whether or not the general public sees them as I do, the TV advertisement showcase Beca innovation and striving for excellence which has been a hallmark of the nearly 40 years I have worked with my colleagues across multiple disciplines.

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Richard Sharpe

Senior Technical Director - Earthquake Engineering

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