Shared from the 4/12/2024 Oamaru Mail eEdition

Wood v concrete bridge decision looms

Using wood instead of concrete for Kakanui’s new bridge could save the Waitaki District Council hundreds of thousands of dollars.

On April 23, councillors will decide which design will be built, with estimated prices ranging from $8.59 million to $9.97m

If the council’s funding application to Waka Kotahi NZ Transport Agency(NZTA) is successful, 57% of that will be subsidised.

Council major transport projects manager Mike Harrison said the lifetime cost of maintaining the current bridge was much higher than building a new one.

The bridge would be built just upstream of the current one and would be about 2m taller, to protect against flooding.

The main differences between the options were the width of the bridge and whether it was made of wood or concrete, he said.

Pre-stressed concrete is considered the standard solution. It is considered low maintenance and has a proven track record, but has a large carbon footprint, requires more intensive machinery to build and is much heavier.

The alternative is a glulam bridge, made from timber and ply and designed to be lightweight.

Glulam bridges are uncommon in New Zealand, but are more prevalent in other countries.

The design is one quarter the weight of the concrete bridge, leading to smaller foundations and seismic loads, which means it fares better in an earthquake.

It also has the potential to be carbon neutral. Both designs have a 100 year lifespan. Mr Harrison said the preferred option was a 7.1m wide glulam bridge, as it was the most cost effective option at $8.59m He understood people would want two lanes on the bridge, but anything other than the minimum would not be subsidised by NZTA. ‘‘As an engineer I’d love to build it.’’ Adding a second lane to the bridge was expected to add a minimum of $3m to the cost. There were bridges in New Zealand that had much more traffic and managed with a single lane, he said.

The bridge averages about 1400 vehicles a day and has capacity for more than twice that.

The council was expecting to hear from NZTA by September.

All the signs had been positive that it would be approved, but there was always uncertainty when dealing with a new government, he said.

The existing bridge will not be required once the new bridge was operating.

The council estimates it would cost $750,000 to be deconstructed.

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