Earthquake lessons for engineers
Last week’s North Canterburycentred earthquake has left many Wellington building owners scratching their heads over how their building performed under pressure.
Auckland consultant engineer David Hopkins said it’s hugely important to not just repair the damage but understand why it occurred. For him, the quake highlighted the importance of good engineering.
‘‘We often hear people demanding building consents on time or complaining about the consent fees.
‘‘We never hear people demanding rigour to check the structural design and detailing or the quality during construction. They often want the cheapest structural engineer and the cheapest structural solutions.’’
As they assessed the state of buildings, Hopkins said engineers would be taking a number of matters into account. ❚ The event was largely under code. The quake, and the buildings’ response, seem to have been generally less than the maximum for building code levels, but it may be comparable with code levels for some buildings. ❚ The response of each building is different. Variation can be huge, depending on the site, foundations and structural details. ❚ It takes time. With damaged buildings, it is important that engineers understand what has happened to decide on safety, stability and options for repair. Survey measurement to check for movement is critical, Hopkins said, and that can be reassuring enough to allow closer inspection and remedial work. ❚ Glass in an earthquake. Since the 1980s, glazing systems generally have special details to allow for earthquake movement. These systems should not be susceptible to damage – apart from isolated panes. The buildings that still have glazing from the 1950s, 60s and 70s are much more susceptible to damage. ❚ What went wrong and why? Hopkins said if there was damage, there are three main possibilities. The first was that the shaking and the building’s response was more than allowed for in the building code, for example, the load on a column was higher than expected.
The second was the design or detailing was not up to code. such as not enough support length for a precast concrete floor unit.
The third was that the construction was not built to design and/or specification. This could pose questions about material quality or supervision, such as low concrete strength, or inaccurate steel placement.
Modern Wellington buildings were no more vulnerable than those in other parts of the country, he said. ‘‘Arguably they are less so. Our building code takes account of the varying seismicity around the country.’’
This means buildings in each region have about the same chance of experiencing a designlevel earthquake. ‘‘After 1976 Wellington buildings have been designed to about three times the ground-shaking intensity used in Auckland or Dunedin and about double that used in Christchurch, prior to 2011.’’
"We never hear people demanding rigour to check the structural design and detailing or the quality during construction." David Hopkins, engineer