Canterbury Structural Group Forum: The Earthquake and what do we do next

6.00pm – 8.00pm, Wednesday 22nd September 2010
Lecture Theatre E11, College of Engineering, University of Canterbury

This Canterbury Technical Clearinghouse meeting summary was prepared by Bruce Deam on behalf of the NZ Society for Earthquake Engineering (NZSEE). This final version (4 Oct 2010) contains corrections from most of the contributors. Click on contributor's names to send them an email message.

This joint Canterbury Structural Group and NZSEE forum was introduced and chaired by the Canterbury Structural Group chair, John Snook (John Snook Consulting). A show of hands survey indicated that 12 of the 75 attendees were involved in research collection, 20 in the Civil Defence safety surveys, 40 in post-earthquake building assessment, 3 in rebuilding design and 2 were conducting business as usual.

Andy Buchanan (University of Canterbury) gave an overviw of the activities of the science clearing house meetings during the previous fortnight and some of the investigation groups that have been active since the earthquake, including: Structural questions are now just coming to light: Greg MacRae (University of Canterbury) listed 12 InvestigationGroups that have been active following the earthquake and are now preparing reports: Henri Gavin (Duke University) outlined how he is aggregating data about damage. He distributed survey forms for those presnt to complete during the meeting (or e-mail to him later) to quantify damage to various classes of buildings.

John Berrill (Canterbuty Seismic Instruments) outlined their development of a scheme similar to the BORP scheme in San Francisco that provides an instrument that that records building movement and displays a red, yellow or green classification after an earthquake. A more advanced version provides a display for the building manager that indicates where to look for damage to avoid intrusive examinations. Both provide accelerograms for later analysis. He asked whether this is worthwhile and received a small number of positive indications.

Bruce Deam (University of Canterbury and NZSEE) presented an overview of the ground motion during the mainshock (2.7 MB PPT that has been tidied up since the presentation). John Berril contributed some useful references on seismic hazard in Christchurch and distribution of ground motion intensity in Christchurch after the meeting.

Greg MacRae (University of Canterbury) presented a list of decisions that need to be made following the earthquake (see below and DecisionQuestions) and identified (by show of hands) how many regarded each question as important.

John Hare (Holmes Consulting Group) presented a set of questions and answers prepared by David Hopkins to brief a group of Government Ministers on Tue 21st. Commentary and discussion about the specific items included:
  1. Between 1/3 and full code motion – infrastructure probably protected by the short duration
  2. No comment s
  3. NZS 3604 and NZS 4229 need provisions for classifying good ground
  4. Liquefaction – This is the subject of the NZ Geotech Society, Canterbury Branch Forum
  5. Building performance – This needs qualification: From CCC and own observations, all taller structures require a detailed inspection rather than relying on external tagging
John then presented a list of his own observations
  1. Interaction of gravity and seismic frames: Some gravity systems were damaged because they moved.
  2. Loss of diaphragm integrity identified
  3. Floor beam elongation has been identified
  4. Older in-situ floor slabs seem to have performed better than precast ones. Many shrinkage cracks have opened further.
  5. Stairs and other Non-structural interference has had bad effect on stairs and stairwells
  6. Pin details have often not behaved as pins – e.g. Canterbury centre
  7. High shears were observed in shearwall structures, particularly in higher storeys
  8. Long period structures need additional attention
  9. Masonry – well maintained lime mortar has performed well.
  10. Standard of building reviews has been questionable
  11. Everyone should stop using mesh in floors
  12. Massive stone masonry seems to have performed better than that with smaller bricks

Open Discussions

Geotech implications: Misko Cubrinovski reported that 20 experts in liquefaction and lateral spreading were studying area. They will produce a detailed report in 10 days. Swedish weight sounding and other tests are ongoing (for 2 weeks). They are mapping lateral spreading as case histories to identify settlements. There is good information sharing with government agencies. There are soft soils and liquefaction in well known areas but they don’t expect much improvement (densification) before another event. The worst areas are near spreading (up to 4 m max and 2 m in some residential areas). Soils are now prone to settlement. The evidence suggests moderate to strong ground shaking. An integrated effort is required to address issues, but need to wait for a systematic solution.

Insurance: Nothing to contribute.

Demolition: Only one decision was reported. That was in consultation with many other stakeholders (which is difficult when it was a life-safety matter).

Building performance: – This will be quantified by Henri Gavin’s survey

DBH document for exempt work: They have a detailed Guide to building work that does not require a building consent. There is (24 Sep) a link to a short guide document on the right of the DBH home page.

Order in Council: The brief Canterbury Earthquake (Local Government Act 2002) Order 2010

Land surveys: These indicate Christchurch has moved about 70 mm toward the mountains and has dropped about 30 mm. CCC are resurveying their benchmarks. LINZ have resurveyed and once the post-seismic deformation has stabilised, they will carry out more extensive surveys of the geodetic network (... more).

Work on buildings designed by others: This needs to be in accordance with normal ethical practice.

Construction resources: It was suggested (does anyone want to take credit for this?) that the bottleneck will probably be other parties such as TA’s rather than resources. The Press indicated that about 2500 houses will need reconstruction as uninhabitable (c.f. 17000 new starts NZ wide per annum) and about the same number are not waterproof.

Sharing advice: Reports on existing buildings. What disclaimers are required? Need detailed investigations. Focus to date has been on eliminating hazards. Need conditions of engagement with clear briefs about what needs to be done. Site inspections (pre-purchase) need geotech input.

Shoring: Des Bull (Holmes Consulting Group) has observed many examples of very poor shoring. He noted that shoring systems need to: Additional design aspects (contributed after the meeting) include:

Greg MacRae's Questions

After a brief second review of the DecisionQuestions it was suggested that these be put on a blog site for people to contribute to. The first three need to be addressed by the Geotech group and we need to identify champions (probably from amongst the contributors) for each of those and the others.

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