NZ building steel 'second to none'

By SIMON COLLINS science reporter

A veteran structural engineer says New Zealand's reinforcing steel is still "second to none in the world" despite concerns about a new grade of steel.

Esli Forrest, who has 50 years' design experience and is now technical editor of the Structural Engineering Society's journal, said the problems being reported with the new 500E-grade steel were just "normal teething troubles" as engineers learned how to use it.

"They relate to the usage of the steel in construction and on the building site," the Manurewa engineer said.

"The steel produced in this country for use in seismic design is second to none in the world.

"In a major earthquake attack, I would rather be in a tall building designed and constructed in New Zealand, with our construction industry, our advanced research and design and our known materials performance, than in any other country in the world."

Last week, the Building Industry Authority said it had documented four or five cases where the new 500E steel had broken after welding.

On Thursday, the authority said builders should stop re-bending or welding the 500E steel on site.

On Friday, it wrote to all city and district councils asking them to give special attention to the use of 500E steel when considering building consents and inspecting buildings.

The 500E grade is made from scrap steel by Otahuhu-based Pacific Steel, part of Fletcher Building. It has quickly become the country's predominant grade of reinforcing steel, especially in tall concrete buildings.

Mr Forrest said the real concern should not be the local steel but imported steels, mainly from Asia, which now made up 15 per cent of the reinforcing market.

"They have not got the measured and known strength properties designed for seismic risk," he said.

The 500E grade is unique to New Zealand, designed both for our high earthquake risk and for a design philosophy that aims to let some building elements fail so that they absorb some of the force of a big earthquake, protecting people inside the building.

In contrast, Mr Forrest said, the massive force of the 1994 Los Angeles earthquake literally shook people to death inside buildings because the structures were designed to withstand the force rather than absorbing it.

The general manager of Steel and Tube Reinforcing and head of the industry group Reinforcing NZ, Roger O'Neill, said 500E steel was introduced to keep reinforced concrete construction competitive with steel beams.

The New Zealand standard specified the strength required in each beam and joint.

Upgrading the steel's strength from 430 to 500 megapascals (equivalent to about 31 to 36 tons per square inch) allowed builders to use less steel. However, research published in the latest Structural Engineering Society journal shows that 500E steel requires a higher "overstrength factor", or extra safety margin, of 35 per cent for columns and 40 per cent for beams, compared with 25 per cent for the previous 430 grade.

This is because the extra strength also makes the 500E more brittle.

Said Mr Forrest: "I believe it will still be more economic, even with the bigger overstrength factor."