Since the opening of the original truss bridge in 1959, and subsequent widening with the box girder extensions in 1969, the Auckland Harbour Bridge has been one of the most iconic and busiest bridge structures in New Zealand. It not only connects water, telecommunication, power and gas services; but also carries an average of 180,000 vehicles that travel over the bridge each day. This makes it one of the country’s most vital infrastructure links not only between the North Shore suburbs and the rest of Auckland City, but between the Northland Region and the rest of New Zealand.

In 1998, Opus International Consultants Ltd (now WSP in New Zealand) partnered with Fulton Hogan and TBS Farnsworth to form Total Bridge Services (TBS), and since 2012 have been an integral part of the Auckland Harbour Bridge Alliance (AHBA), maintaining and managing the bridge. One of the latest developments was the recently completed lighting upgrade and installation, for which 118 new poles were designed by WSP, with the project delivered by the AHBA.

Lighting Upgrade and Installation:
WSP led the way in the lighting design and replacement of more than 140 High Pressure Sodium (HPS) lamps with low energy Light-Emitting Diodes (LEDs). For those involved, the project has been a massive undertaking. Andy Collins, Technical Principal - Lighting Design at WSP said there was a real need for the upgrade, which involved multiple engineering disciplines. "LEDs are more robust than the old luminaries, which themselves have been replaced a number of times over the decades. This solution encompasses the latest changes in lighting technology saving over 50% in energy consumption while maintaining the New Zealand Transport Agency's requirements," said Collins. The use of LEDs has had an immediate impact, lowering maintenance and operational costs, and significantly reducing light pollution from the bridge. Crucially, the new lights improve nighttime visibility for drivers and are more effective in the rain, making for a safer driving experience.  <p>

Designing for the Next 50 Years:
The project grew in complexity when the fatigue issues associated with the ageing lighting poles were taken into account. Raed El Sarraf, Technical Principal- Materials and Corrosion at WSP, said the upgrade required the installation of new poles, as the original poles were reaching the end of fatigue life due to vibration from the bridge traffic and wind loading. "Fatigue has been a concern for the past decade resulting in the annual inspections and monitoring to detect cracks at high stress locations on the poles for the past five years. All of which adds to the operating cost of the poles." Tony Raper, Principal Design Engineer at WSP, designed the new poles. He said, "Real time measurements of the pole's vibration were taken over a three month period, which were used to supplement the theoretical wind loading when designing the new poles. Having said that, we were not just designing for wind. The most active part of the bridge is the 220m navigation span, which is also 45m above the harbour. In addition to wind, we also needed to allow for traffic loading as well as the 'twisting' of the box girders during service. All of this meant that there were multi-directional vibrations applied at different loads and points on the pole. That is why the size and thickness of the new poles are slightly larger than the old to resist these differing loads."

Material Selection and Hot-Dip Galvanizing:
According to El Sarraf, the new hot-dip galvanized poles were designed to withstand the applied loadings and vibrations, while still combining the aesthetics of the original 1959 and 1969 poles. This ensured a visual continuation between the past and the future, while providing a low maintenance, durable solution for the next 50 years. "Steel was chosen for this project mainly due to its strength, versatility, ease of fabrication, the bespoke aesthetics, and its known fatigue design. This made steel an ideal material for this project in comparison to other materials, whether aluminum, fibre reinforced polymers, or even concrete," El Sarraf stated. "Once steel was selected, the next question was how to protect the poles. The original 1969 poles are welded onto the pedestal, thereby requiring site repair and maintenance of the protective coating system; next to live traffic lanes, all of which adds significant costs to the operational maintenance cost of the poles. When designing the new poles, we had to consider how to protect and maintain them. This was achieved by specifying hot-dip galvanizing and by bolting the poles onto a new welded flanged base connection onto the existing pedestals. The beauty of this connection is that we can easily and quickly remove a pole and replace it with a refurbished pole overnight. The plan is to replace the first number of poles with spare examples. The removed poles will then be re-galvanized and re-installed elsewhere on the bridge. This maintenance methodology provides a much more efficient and cost-effective solution than painting all the poles in-situ. Finally, when dealing with unsealed steel hollow sections, hot-dip galvanizing is the only way to protect the poles. Galvanizing delivered longevity and was the most cost-effective choice."

Designing and Detailing for Durability:
An important part of the design process was designing and detailing for durability. WSP undertook a 12-month study assessing the atmospheric corrosivity at different locations on the bridge. It was confirmed that some locations on the bridge were in different atmospheric corrosivity categories. For example, steelwork that is sheltered from the rain but exposed to the marine aerosol can be taken as being in a C5-M (Very High Marine) corrosivity environment. Meanwhile, the light poles that are rain washed, were found to be in a C3 (Medium) corrosivity environment. As such, the expected time to first maintenance for the rain washed galvanized poles is 25+ years, if not 50 years. 

So, the next question was: What was the actual amount of zinc deposited on the newly galvanized poles? According to El Sarraf, "Once the first pole was fabricated, it was directly sent to be galvanized. We wanted to check what thickness of the galvanizing could be expected, as well as the standard of finish. Because the steel chemistry of the pole components were different, we weren't sure whether the galvanizing finish would be mottled, shiny, or matte. The main concern was that the freshly galvanized poles would be too shiny, resulting in reflective sun strike off the poles that may distract motorists. When the poles came out of the galvanizing bath, we were pleased with the final look. It had a relatively uniform matte finish, hence sun strike was a non-issue. Also, the measured thickness of the galvanizing was up to three times the minimum specified in the standards. Therefore, while we allowed for the poles to undergo maintenance in 25 year's time, there is a high likelihood that the poles may meet the 50 years design lift with minimal maintenance ever required.

Planning for Success:
Considerable up-front planning and clear communication lines were vital to the smooth implementation of this project. According to El Sarraf, “There was a lot of work completed before the poles were delivered to site, from confirming the correct material was ordered and supplied through the fabrication process itself and welding inspection; because once galvanized, the poles were directly delivered to the job site to meet the tight installation timeframe. We worked directly with the galvanizers and followed our galvanizing ‘bible’, the Designing for Galvanizing resource published by the Galvanizers Association of Australia; to ensure that once the poles were double dipped in the bath, they emerged as expected with no surprises or issues. Once delivered, the pre-planned installation process, which required the removal of the old poles, preparing the pedestal for the installation of the new flanged base followed by bolting on the new pole, was an example of clockwork efficiency. A testament to the hard work the AHBA team going above and beyond to ensure that the Auckland Harbour Bridge is lighting the way into the 21st century,” said El Sarraf.