Quake impact on design to be outlined

Scientists and engineers will release on Wednesday details of what the earthquake damage to Christchurch buildings means for building future building designs.

The information paper - jointly compiled by the national science academy, the Royal Society, the Institution of Professional Engineers (Ipenz), the Structural Engineering Society the Geotechnical Society and the Society for Earthquake Engineering - is expected to provide answers to critical questions about buildings and structures.

Engineers have been studying damage to commercial buildings, homes and other structures to work out what types of structural design, building strengthening techniques and construction methods led to the most resilient behaviour, and the causes of failures, including buildings which had suffered loadings greater than those for which they had been designed.

The New Zealand government is later expected to make a decision on whether the design levels in the current building code represent the risk appetite of the New Zealand public and balance that against building and occupancy costs.

A scientific briefing on the paper will be provided by an associate professor of civil engineering, at Auckland University, Dr Charles Clifton; the principal engineering adviser to Christchurch's earthquake recovery authority (CERA , Holmes Consulting Group director John Hare; the Beca engineering firm's technical director for earthquake engineering, Richard Sharpe; and a Canterbury University associate professor of civil engineering, Dr Gregory MacRae.

Ipenz has already said that the Christchurch, buildings that failed were generally older, and that no loss of life occurred in any building constructed since 1991.

Reasons for differences in resilience among buildings of a similar age could include the type of building design or structural concept, whether or not there were improvements such as increased tie-backs, or the orientation of the building to the direction of the shaking, the extent to which minimum load and detailed design requirements were exceeded by the design engineer, the underlying ground conditions; how well the builder implemented the designer's plans.

Another factor could be whether there was any building to pound against another, as is thought to have happened when a multi-storey carpark damaged the Grand Chancellor Hotel high-rise.

Many buildings which performed quite adequately and suffered no loss of strength in September's original magnitude 7.1 earthquake were severely damaged in the magnitude 6.3 aftershock in February.

Previously, a senior researcher at GNS Science, Dr Hamish Campbell, said Christchurch was "inevitably going to be brought to its knees" by an earthquake because of the nature of the ground on which it stood.

Christchurch was built on a plain comprising gravels, silts and peats, so it was "very unconsolidated". It was also low-lying but had a high water table.

The focus for rebuilding should be on the nature of the ground and the foundations, rather than the building materials, he said.

Separately, Ipenz said liquefaction effectively causes some soil types to lose their strength, become fluid, and "slump" toward low points though lateral spreading. This could cause building damage, but structures with piles built through liquefiable soils could still perform satisfactorily.

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