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Carillon bell tower: Quake strengthening work options weighed up

Last Post ceremony at the National War Memorial in Wellington.

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The Carillon bell tower in Pukeahu National War Memorial Park, Wellington.
Photo: RNZ / Alexander Robertson

Expensive and intrusive earthquake strengthening work on the landmark Carillon bell tower is being weighed against cheap and easy, but less durable, options.

The bell tower in Pukeahu National War Memorial Park in Wellington has been shut by seismic risks for three years.

The Culture and Heritage Ministry has till 2027 to fix the quake-prone tower.

Newly released consultants’ reports show the frames holding tonnes of bells are the weakest point, at as low as 20 percent New Building Standard (NBS). Anything under 34 percent is earthquake-prone, the lowest category.

A concept report outlines repair options ranging from fully replacing the two bell frames, to spot fixes; a separate quantity surveyor report estimates the frames costs at a combined $2.8m, to as little as $800,000.

As for the tower itself, of the four options that engineers Holmes explored, the priciest at $8m is for a fully new internal steel frame.

But this would be “most disruptive to the internal layout”, the engineers said.

Another $1.9m option that includes rod anchors driven into the ground would “cause significant disruption … around the heritage entrance way”.

In many cases, the threat of corrosion of any steel bracing complicates things.

A “quick, cheap, and easy fix” option for the lower or upper bell frames “does not address durability issues (steel maintenance)”, the report said.

Investigations last October found “corrosion was … significant within the structure” already.

The cost estimates guessed at up to 15 percent of steel having to be replaced; any more, and it would cost more.

Holmes recommended the ministry carefully consider the state of the steel when it came to choosing an option.

The investigation also found the different parts of the tower are likely to deform in different ways due to different stiffness.

Bars near the top at the corners of concrete columns were the number one worry, rating just 34 percent NBS. Some other parts had much better ratings.

Engineers were surprised to find “the building was not designed to rock on its foundation, rather it is connected with vertical bars that continue deep into the ground”.

This forced them to redo their calculations last November.

Years of strengthening work up till 2018 was later revealed to have failed to fix the historically significant, 50m high tower.

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