Further Q&A from earthquake engineers

Following from last month’s SMC Q&A with earthquake engineers, we have several more earthquake related questions answered by a panel of expert engineers.

The engineers include John Hare, president of the Structural Engineering Society, Win Clark, executive officer at the New Zealand Society for Earthquake Engineering, Mark Batchelar a consultant engineer and earthquake engineering expert David Hopkins.

This Q&A was put together in association with the Institution of Professional Engineers of New Zealand (IPENZ). Feel free to use the comments below in your reports.

Is there any generic information which members of the public should keep in mind in terms of non-ductive or inflexible and brittle columns in other buildings elsewhere in the country? For instance, is it possible that councils and other building authorities may ask for remedial work such as  wrapping column elements in fibre reinforced polymers (FRP)?

The Department of Building and Housing (DBH ) is reviewing buildings built between 1982 and 1995 which have “gravity” columns.  These are the columns that could have been detailed to be non-ductile.  Before 1970, design for ductility was not considered. From 1970 to 1982 a provisional standard required ductile columns with no exceptions.  The 1982 concrete Standard introduced an option for non-ductile detailing of gravity columns in certain circumstances. From 1995 this option was no longer available.  Thus pre-1970 buildings (possibly pre-1976) would not be ductile (eg PGC) but after that ductility would be expected for most buildings with the exception noted above.  Note that the existence of non-ductile columns does not necessarily mean that they would fail in earthquake – it depends on how the seismic resisting elements restrict the likely movement of the columns.

The New Buildings Standards (NBS) code set by the updated Building Act of 2004 essentially said buildings deemed to be less than 33 percent of the NBS code were considered not safe, while engineers recommended buildings be at least 67 percent of code. What advice or indication does IPENZ have that where tenants in commercial buildings are now demanding compliance higher than 67 percent of NBS, a significant portion of the buildings requiring the most work were built in the last 25 years, including some high-rise buildings?

The 2004 legislation targeted 33% or less to capture the worst of buildings.  It was expected  that the legislation would raise awareness of the risk represented by the 34 to 100% buildings as well as leading to strengthening of the 33% and less buildings.  The legislation has been of limited success, though much has been done to identify earthquake-prone buildings and some strengthening has taken place.  What needs to be corrected is the impression that a 34% building is safe in a design earthquake – it is not.  The DBH is reviewing earthquake-prone building legislation and policies and the Royal Commission has signalled that changes should be considered.

The question for NZ inc. is “how do we manage the risk of building failure in an earthquake on a national basis without excess economic cost ?”. i.e. “what can we afford as a nation to invest in our building stock to reduce the effect of earthquakes to an acceptable level, and what is an acceptable level of risk ?”.

The corollary question is “what level of earthquake resistance should we aim for so that the cost of reinstatement following a major earthquake event does not impose an unacceptable financial burden on the region or the nation as a whole ?”

It is agreed that 33%NBS is too low, but can we afford, in the short term, to bring the nations building stock up to 100%, or 67% ? or can we afford not to ?

To answer these questions more information is required on what is the risk to life in out regional and metropolitan centres, what would be the cost/time frame to reduce the risk to an acceptable level, and who will pay. How ever the economics are structured, the public will end up paying through a higher price for goods and services that operate out of these less risky but higher cost buildings. This margin may be quite acceptable to the general public; requires more information to answer these questions, but what will it cost to gain this information, how accurate will it be, and how long will it take to provide?.