Babbage Pioneers: Christchurch Innovations.

Babbage engineers have brought a raft of pioneering innovations to the new BNZ Centre in Christchurch.

In a development described as “inspirational” and taking an “enormous amount of bravery” the structural design of the buildings include K-brace frames that allow warped central link-beam sections to be replaced after an earthquake; foundation work has been designed to overcome liquefaction; an anchoring system has been used for the first time in New Zealand; and the steel frame has been exposed for occupants and visitors to see.

Matt Harris, Christchurch Business Manager and Chartered Professional Engineer, says many concepts on the design have evolved from the industry lessons learned from the devastating 2010-2011 Canterbury earthquake events. The five-story BNZ building on Cashel Street in the central city was opened by then-Prime Minister John Key on December 9 (see video link here Password: BNZ) as stage two of a project for Lichfield Holdings Ltd.

The adjacent six-story Stage-1, across a central courtyard and bordering Hereford Street, is occupied by government departments on three floors and has car parking on the first floor, while Stage-2 has the BNZ on the top floor, with the other floors occupied by ACC, a legal practice, and a first-floor car park, gym and offices, while the ground floors of both stages are retail. Babbage Consultants were the structural engineers for both stages of the project and geotechnical engineers for a peer review of the ground improvement. The team worked with architects Sheppard and Rout and the contractor Leighs Construction to produce the first new complex in Christchurch’s central retail precinct.

The structure of this building consists primarily of a steel frame supporting lightweight roofs and walls, and suspended concrete floors. Matt said. The steel structural frame is used to minimise the weight of the building and as a result the seismic lateral loads are reduced; but behind that design is further innovation; while buildings have traditionally been designed to protect life during an earthquake, they were often so damaged they had to be replaced. What the Babbage wanted this time was a building that was recoverable - and quickly. “A lot of work was done to review buildings damaged in the 2010-11 quakes and we found two key issues that would have to be addressed in future builds,” Matt said.

“We found a lot of irreparably damaged beams in concrete-frame buildings, and we found that liquefaction had caused a number of buildings to tilt - including on this site - making them too expensive or risky to repair.“We applied those lessons to these new buildings.”

The steel K-brace frames (also called eccentric braced frames) around the perimeter and within the building footprint have been designed with an “active link” that will deform and absorb energy under extreme seismic loading. The links have bolted plates at each end between the diagonal braces so if they become permanently deformed they can be replaced, significantly reducing the time taken to re-occupy the building after a large earthquake.

“At the top floor the seismic mass of the roof is relatively small, so instead of using the K-braces for stability, Babbage reduced the cost by using the inherent strength of the columns, which cantilever from the lower floors,” he said. The foundation work also required special consideration.
“To minimise the risk of differential settlement from the liquefiable soils across the site, we had to either drive 25-metre deep piles or create a 10m thick crust of improved ground,” Babbage looked at deep-soil mixing - mixing concrete into the soil - but that was relatively expensive. The team then looked at vibrating timber poles into the ground and densifying the soil between them, but in a site trial the contractor’s equipment couldn’t get the poles through the thin upper layer of gravel without destroying them.

“The design settled upon a Canadian system called rammed aggregate piers - RAPs - which Golder Associates had brought to New Zealand under licence. This involved driving a steel mandrel into the ground then dropping aggregate into its base and using the mandrel to ram the aggregate into a wider pier so the surrounding soil became so compacted it was too dense to liquefy.” The resulting 600 millimetre-diameter RAPS were constructed to a depth of 8-10m on a 1.8m grid to provide a non-liquefiable crust across the whole building platform.

Because the building was now so light the team needed to consider how to stop uplift at the K-brace frames during an earthquake, this was resolved by anchoring the frames to the RAPs. “Babbage are the first in New Zealand to use Tension-RAPs to anchor a building, and while standard RAPs have already been used in Christchurch, this is the first time they’ve been used on a large development.” Matt said.

The Babbage team also persuaded the client and architect that the building’s steel frame should be exposed to give tenants and visitors confidence the building was strong.

The result is a signature building described by the contractor’s managing director Anthony Leighs as “inspirational”.He told the opening ceremony: “It took an enormous amount of bravery … it’s one of the first significant projects in the core of the CBD.”

Babbage Pioneers: Christchurch Innovations