Engineers go outside square for solutions

By Bob Dey

Some of the most imposing new structures in New Zealand would not have been built the way they were if engineers had been forced to stay "within the square".

These buildings include Sky Tower and Sky City in the heart of Auckland, the WestpacTrust Stadium in Christchurch and the airport extensions in both Wellington and Auckland.

A key ingredient in the changes has been how to deal with fire regulations. Richard Brand, an associate at Auckland-based engineering firm Beca Carter Hollings & Ferner and head of fire engineering for the firm, says most of Beca's fire-related work now involves design categorised as alternative solutions.

Brand is one of four finalists in the Building Industry Authority's alternative solutions category of the New Zealand Institute of Building James Hardie awards for excellence in the building professions, which acknowledge the individuals where other awards focus on the project.

Beca won the fire engineering contract for the 137-unit Oxford City apartment building on Mount St, developed by Charta Group and with most of its strata-titled units managed by Allfields.

The developer wanted a steel structure to meet its tight construction programme, but normal fire proofing would require an insulating blanket to stop the steel from heating too quickly in the event of a fire. That can be done with non-combustible boards, sprayed-on insulation or with a paint which bubbles or fluffs in a fire to create the blanket.

Rather than picking up the standard prescription, Brand says Beca's engineers looked at factors such as the load-carrying capacity and whether the steel could still function as a structural membrane when hot.

He says every apartment is like a cell, with fire-resistance between neighbouring units, and the amount of fuel and ventilation governing the amount of fire.

The computer-calculated time-temperature curve showed the steel would not reach the same temperature as the fire because of a lag in any blaze reaching the steel.

"Unprotected steel is being used more because the major steel manufacturers have been doing more research into how it performs with elevated temperatures. For a building such as the Oxford, it means they could build it much more quickly.

"If it required applied fire proofing [the normal treatment], it probably have been much more expensive." The result is a fully steel-framed building, with bare steel throughout.

Two of the other alternative solutions finalists are from Canterbury and the fourth a Central Otago couple, part-time log house builders Jim and Suzanne Benck.

The Bencks poured 11m-high concrete pillars, allowing a semi-rigid structure to withstand wind and uplift pressures, without using conventional trusses. That meant they could install extensive exterior windows for unobstructed views.

At the 70-bed St Nicolas Hospital for the elderly in Christchurch, consulting engineer Brady Cosgrove worked closely with fire protection contractors Benefis Carbonic Systems to introduce a number of smoke detector, alarm and sprinkler features during renovation.

At the Bainswood rest home in Rangiora, consulting engineer Carol Caldwell also proposed a more cost-effective fire safety solution to the normal raft of fire separations, instead using automatic sprinklers and a smoke detection system.

The existing award categories are for construction under and over $5 million, residential work and innovation. Seventy entries were cut back to 29 finalists, with the winners to be announced on July 23.