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Saturday, October 8, 2011

Earthquake Royal Commission - Review of the report 'The Performance of Unreinforced Masonry Buildings in the 2010/2011 Canterbury Earthquake Swarm'

From the Royal Commission's website:
The Commission has received the following advice from Fred Turner, structural engineer, from Sacramento, California in the USA. The paper reviews the technical report 'The Performance of Unreinforced Masonry Buildings in the 2010/2011 Canterbury Earthquake Swarm' by Associate Professor Jason Ingham from Auckland University and Professor Michael Griffith from Adelaide University.
The review contains eight pages of comments and recommendations, which can be downloaded here.

Information on the initial report to the Earthquake Royal Commission - The Performance of Unreinforced Masonry Buildings in the 2010-2011 Canterbury Earthquake Swarm, can be found here.

The following is a short, and not necessarily representative, sample of the comments made by Fred Turner.

Page 2. Limitations of Retrofits:
The recommendations should acknowledge, as evidenced from past retrofit performance, that it is neither practical nor feasible to state conclusively that the public can be effectively protected from “all” falling hazards and that “strengthened URM buildings will survive severe earthquake ground motions.”
 The reason for proposing these clarifications is that the public should be made aware of the practical limitations of seismic retrofits, considering the margins of safety from collapse and parts of buildings falling, particularly in light of the large known variability and uncertainty of ground motions, as well as variations and uncertainty in the quality of building materials, the states of repair, and the integrity of connections between building components.
Retrofits that represent best practices may not always guarantee that all masonry units will remain in place, nor that URM buildings will always avoid costprohibitive repairs or demolitions after experiencing severe ground motions.
Page 3. Two additional recommendations warrant consideration:
(Recommendation 1)
Adequate staffing and retrofit training within building regulation enforcement agencies should be implemented to ensure that: 1) retrofit designs are thoroughly checked for compliance with regulations before construction permits are issued; 2) retrofit construction is thoroughly inspected to ensure strict compliance with approved plans and; 3) damaged buildings are effectively assessed, placarded, barricaded and stabilized after future earthquakes.
Page 4. Comments on Section 1 Introduction and Background
Section 1.3.1 should include a discussion of New Zealand’s policies and practices for bracing URM parapets. Estimates of the rate of compliance with parapet policies and judgments about their effectiveness should be included.
Section 1.4 should include a brief summary of ground motions in Lincoln and Lyttleton for the September and February earthquakes. In Lincoln, ground motions were substantially higher than in the Christchurch CBD in the Darfield earthquake. And Lincoln provided several examples of the performance of retrofitted and unretrofitted URM buildings in more severe, longer-duration ground motions than in the CBD. In Lyttleton, ground motions and soil conditions are considerably different than Christchurch CBD. While this is somewhat beyond the scope of work, including comparative discussions about Lincoln and Lyttleton will emphasize that the public should grasp that performance will vary considerably based on locations, soil and rock conditions, differences in ground motions, as well as building earthquake resistance.
Page 5. Comments on Section 2.5 Seismic Vulnerability of the New Zealand URM Building Stock
Consider adding a paragraph discussing the fact that performance of URM buildings in one earthquake is not necessarily indicative of future performance in other earthquakes.
... So owners of undamaged or slightly damaged URM buildings may misconstrue their sense of security since similar performance in future earthquakes is not necessarily assured by past performance. Even slightly damaged URM buildings are considerably more vulnerable in future earthquakes and aftershocks since their strength and stiffness tend to degrade with each additional cycle of significant motion. Therefore, owners of slightly damaged or undamaged URM buildings will likely still need to evaluate, repair and retrofit their buildings to ensure reliable performance in future earthquakes.
Page 6. Comments on Section 3 Observed Performance
Describe approximately how many URM buildings were retrofitted or partially retrofitted prior to September 2010 and how their range of performance compared with the range of performance of unretrofitted URM buildings nearby.
Page 7. Concluding comments
In closing, I find that Professors Jason Ingham and Michael Griffith have produced a remarkably good report. They are making excellent progress toward meeting the Royal Commission’s scope of work.

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