$10 million earthquake strengthening on Wellington building complete

The external rings are like "putting a plaster-cast on an arm that is not broken" but act as an extra precaution. Photo / Supplied

Earthquake strengthening on a prime Wellington waterfront office block has been described as world-leading.

The $10 million strengthening work has been completed on the building at 36 Customhouse Quay, revealing exterior steel supports that have not been used before in the capital.

The building, owned by Cornerstone, has been going through strengthening and refurbishment for two years.

Cornerstone chief executive Andrew Cotterell said the addition of external steel ring brackets around the columns at the four corners on the upper levels of the building was world-leading and unique.

"You wouldn't see a single other building in Wellington that has rings around the outside of the columns," Cotterell said. "It's highly visible and innovative."

The steel rings and steel arms coming off them have been used to add strength and resilience to the connection between the beams supporting each floor and the external corner columns to remove any uncertainty about the post-earthquake capacity of reinforcing steel rods internally connecting the beams and columns.

"Like a paperclip, the reinforcing steel rods can only bend and flex a number of times before they will become over-stretched and snap, and we wanted to be certain about the resilience of those connections after the Kaikōura Earthquake," Cotterell said.

"Having investigated it, there remained some uncertainty around whether the steel at the corner beam and column connections had stretched to its capacity. We wanted to eliminate any doubt so we worked with our engineers, Holmes Consulting, and a peer reviewer to come up with a unique solution.

"You could argue it is overkill, like putting a plaster-cast on an arm that is not broken, but we decided to do it because we care about our buildings and our tenants.

"No one has ever queried these beam-to-column connections before because you can't see the reinforcing steel rods inside, so there is no way of knowing how much capacity is left, but if it has been over-stretched the risk is that the reinforcing steel rods could snap in another big quake."

Cornerstone was already well underway with a seismic strengthening programme before the earthquake.

The installation of all of the external steel beam-to-column brackets was completed in November and final work is now being done on a car-parking floor and exterior awnings as part of the original strengthening programme.

The owners expect all work to be complete and the final scaffolding to be down in the first quarter of this year.

The 1980s construction is 13 floors of commercial office space and one floor of residential apartments on the top of the building.