Quake risks for Wellington's Carillon bell tower left unresolved in report

A peer review of a seismic assessment shows uncertainty remains over how the tower would perform in a quake. Photo / RNZ

By Phil Pennington, of RNZ.

Questions around how the Carillon bell tower in Wellington would fare in a major earthquake have been left unresolved despite years trying to fix it, a report shows.

The Ministry for Culture and Heritage has released a review of a seismic assessment into the quake-prone tower at the National War Memorial.

It finds a detailed seismic assessment (DSA) by engineers Dunning Thornton released in mid-2020, was "unfinished" and asks if it has not identified all the hazards properly.

The review, by Holmes Consulting, resolves just 15 serious engineering questions and leaves another 20 unresolved, even as the Ministry for Culture and Heritage faces a May 2022 deadline to fully strengthen the tower.

The Carillon tower was recently rated under 34 per cent of the New Building Standard (NBS), and therefore quake-prone.

The peer review shows uncertainty remains over how walls, elevated floors and foundations of the 50m high tower will perform in a quake.

"The foundations do not appear to have sufficient capacity to support demands from the superstructure at the rocking point modelled," it said.

This indicates that the tower, used to ring the bells for national commemorations of Anzac Day but shut since February, could remain closed a lot longer still.

"We recommend the DSA be extended to incorporate the full performance of the tower," the Holmes Consulting review said.

It also recommended potential weakness be assessed as part of early design strengthening, and treating the superstructure and foundation system together rather than in isolation.

The isolated or patchwork approach run by the ministry for a decade has included not assessing the bell frames properly before trying to fix them, launching strengthening in 2012 without a design report, and not identifying major life-safety hazards till last year.

RNZ revealed in June the ministry went ahead with seismic work, and reopening the Carillon in 2018, despite warnings.

The peer review makes nine recommendations to undertake key investigations such as ensuring the bells don't fall into the foyer, gauging how stable the soil is, and ascertaining how the tower will rock about in a quake.

It would be risky if the ministry now tried to strengthen the tower without the investigations, the report said. "For a complex building like the Carillon Tower, proceeding to the strengthening scheme without completing the DSA (nor closing out open comments in this review) adds risk and should be communicated clearly to the client [ministry] with respect to impact on potential strengthening costs."

The ministry warned in July that bells or "other material might fall on someone in the Carillon foyer".

The peer review expressed scepticism the bells could fall but also asked Dunning Thornton: "Is it possible to explore strengthening measures that prevent the bells from falling into the foyer?"

Dunning Thornton responded that a four-tonne bell was directly above the foyer opening, and smaller than the opening size, and even the 25kg bells rated as a "significant life safety hazard". It was worth trying to strengthen the foyer floor, it said.

The review comes two years after the completion of $3m of strengthening work that closed the tower and adjoining Hall of Memories for long periods, and $300,000 spent fixing the frames that hold the 70 tonnes of bells.

Despite this, how the tower will stand up in a quake remains uncertain.

"The loadpath is unclear," Holmes Consulting said, referring to "inadequate" reinforcement at the corners. This, and many other unresolved engineering problems, needed to be assessed in the early design phase of strengthening, it said.

Dunning Thornton said such vital questions as how walls and diaphragms would perform "have not been checked in full detail".

Holmes Consulting responded by repeatedly stressing the need to understand "the building as a whole".

Some observers have expressed scepticism to RNZ that the tower is really as weak as reports make out.

However Dunning Thornton, in its response to the peer review, rejected the idea that calculations could be redone in a way that significantly altered the low NBS rating.

Dunning Thornton has been working on a "rough order of costs" to now fully fix the monument, but that is not disclosed in the review.

The ministry promised it would be "working hard to meet" the May 2022 strengthening deadline set in earthquake-prone building legislation. It did not address a question from RNZ on whether it may seek an extension.

The ministry's deputy chief executive of delivery, Tamsin Evans, said upgrade work from now on would cover outstanding issues raised by both the DSA and the peer review.

"This work involves contracting in engineering expertise to design the necessary strengthening - a step which will be taken as soon as possible.

"Once we have a work programme in place, we'll have a better understanding of the costs involved and the timeframe to carry out the work.

"We are looking forward to getting this work underway so the National War Memorial can reopen and continue to be the place of solace, remembrance and reflection."

RNZ