As mentioned in previous issues of Concrete magazine, the NZ Concrete Society is marking its 50th anniversary in 2014. We have already taken a look back at “Concrete Trailblazers”, individuals who have made a lasting impression on the New Zealand concrete industry. Now it is time to explore how many notable concrete structures have been realised by the technical and collegial support offered by the Society.
NOTABLE CONCRETE STRUCTURES
Informed by the work of the Society, innovation, more than any other characteristic, has marked the advancement of New Zealand’s concrete construction over the last 50 years.
The industry has evolved on multiple fronts and is best reflected in the diverse mix of structures built over the decades – each a unique response to pervading seismic, commercial or engineering imperatives.
These structures represent some of the most innovative solutions in the history of concrete construction, with several noteworthy examples summarised below.
NAC HANGAR – Christchurch
At the time of its construction the NAC Hangar’s 124.5m pre-stressed concrete span, which holds up the doors and roof, was the country’s longest such span.
National Airways Corporation (NAC) was the country’s domestic airline from 1947 until 1978, when it amalgamated with Air New Zealand. The new hangar was part of an improved facility for aircraft maintenance. Its design called for an internal, column-free area of 122m by 61m to allow the simultaneous servicing of multiple aircraft.
Four separate designs were considered and costed by the designers Beca. The alternatives included structural steel, reinforced concrete arches and an internal steel box girder spine beam.
The pre-stressed concrete box girder solution – spanning 124.5m across the hangar door – was selected because it would have the best appearance and offer superior performance in the event of a fire. Given the militant trade union activity of the period, there were also concerns about the reliability of steel supplies. Furthermore, the maintenance costs for concrete would be lower than those of steel.
The concrete girder was constructed by Downer in segments on conventional ground falsework. First, the end span was constructed, with the main span progressively built out for each side of the hangar. This construction sequence is similar to that used in cantilever construction of pre-stressed box girder bridges, the girder being sequentially pre-stressed to cantilever from the main columns. After the closure of the two cantilevers, the girder was made continuous with post-tensioned cables.The project received the NZ Concrete Society’s Award for Excellence in Prestressed Concrete at the Society’s annual conference at Wairakei in 1981.
Design: Beca, Carter Hollings & Ferner
Contractor: Downer
LOWER SHOTOVER BRIDGE
This vital piece of South Island roading infrastructure features an innovative pre-tensioned deck, and was designed to blend aesthetically with its surroundings.
Described at its 1975 opening as "mathematical precision combined with beautiful form" the bridge is a two-lane carriageway. Measuring 320m long, it’s made up of 169 precast, pre-tensioned concrete box girder sections. On the tourist route between Cromwell and Queenstown, the Remarkables mountain range forms the site’s spectacular backdrop.
It replaced the original 172 m Lower Shotover Bridge (1915) which for 60 years was the main route into Queenstown from the north and east.
Governed by aesthetics, the design is a slender, curving structure that blends easily with the landscape. Opting for a precast, box girder bridge was appealing because construction crews wouldn’t have to truck aggregate from a considerable distance to a tricky site.
The bridge’s cross section is 23 per cent smaller in area than for a standard 100-foot I-beam span. Its superstructure contains 35m3 of concrete, versus the I-beam design’s 45m3. This saving in dead load on the pier foundations reduced the number of piles needed for each pier from five to four – a significant cost saving.
The shape of the piers is functional as well as aesthetic, in that the height had to be adaptable from 17 feet to 25 feet, and wide enough at the top to collect the reaction from bearings under the outer webs. The load from the middle web was transferred at the piers to the outer webs by means of transversely pre-stressed diaphragms.
The bridge received a Merit Award from what was to become the NZ Concrete Society in 1977, and its aesthetic attributes were celebrated by NZ Post with the issuing of a 35c stamp in 1985.
Design: Ministry of Works
Construction: Fletcher Construction Company Ltd, Stresscrete New Zealand Ltd and BBR NZ Ltd.
MOTUNUI SYNTHETIC FUELS PLANT – New Plymouth
This was the first facility of its kind – internationally – to apply seismic design in accordance with recommended criteria laid down for petrochemical plant. This included continuous dewatering of a saturated sub-strata sand layer to reduce the likelihood of site liquefaction under a severe earthquake.
Opened in 1986, the Motunui Synthetic Fuels Plant was the world’s finest commercial production facility converting natural gas to synthetic fuels. It was also the world’s largest methanol production facility, and was built in response to the OPEC oil crisis, a period when New Zealand was 85 per cent dependent on foreign oil.
The twin reformer furnace/waste heat recovery duct structures are the plant’s major components, traditionally built in steel. Beca, the structural designer, opted to replace the conventional steel structure with large, reinforced concrete cantilevers detailed to sustain post-elastic rotations near their base (“plastic hinges”).
Simple rocker bearings isolate the trusses when the columns flex under lateral load, to increase the overall stiffness of the columns. Below hearth level smaller-scale reinforced concrete cantilevers with a similar plastic hinge capability at their base have replaced the steel columns. Rocker bearings provide a pin connection to the furnace hearth.
The project – completed within budget and on schedule – received much international interest and demonstrated the capability of New Zealand’s engineering resources.
Design: Davey McKee Corp, Foster Wheeler Energy Corp & Works Corp.
Construction: Bechtel Petroleum Inc. & New Zealand subcontractors.
Other notable concrete structure that have risen to grace the New Zealand landscape over the past 50 years include the first Newmarket Viaduct, Bluff Smelter, Jerningham Apartments in Wellington, Mangaweka Viaducts, Pakuranga (Waipuna) Bridge, Eden Park’s private boxes, Wellington's Ngauranga Interchange and Chaffers Marina, Otira Viaduct and of course the Sky Tower.
These structures, along with many more, are all indebted to the vision, foresight and courage embodied and fostered by the NZ Concrete Society.
A CELEBRATION OF 50 YEARS
A range activities are planned over coming months to help celebrate the Society’s 50th anniversary, including acknowledging its distinguished members. These activities will include a commemorative publication that examines a complete list of notable concrete structures.
Celebrations will culminate with the 2014 NZ Concrete Industry Conference to be held in the Society’s spiritual home of Wairakei, the venue of the first conference – 50 years ago.
This article appeared in Concrete magazine.