Motovated > News > 22nd February 2011 Quake Synopsis

The Canterbury Quake: Round 2

I was going to write an article in this month's newsletter outlining how good analysis can go a long way to eliminating failures in the field. But, after last week, one thing that really hit home is that there's only so much you can do! Whilst many of the buildings that collapsed in our city were older and maybe didn't comply to modern building codes, that certainly wasn't the case across the board. It hit home for me the reality of designing to an appropriate standard and how sometimes, even with the best laid plans, things can still turn to custard!

One of the biggest issues in analysing something for potential failures is making absolutely sure that you understand what the potential failure modes really are, and then perform sufficient simulation to suit those modes. Wherever possible, we like to use existing standards and engineer to those standards, however, there are times when failure modes can occur that are well outside of even these.

As I understand it, the Earthquake the 22nd Feb 2011 measured up to 2.2 g at one location (220% of the acceleration due to gravity at Heathcote Valley Primary School, scarily enough) with the shaking exceeding 1.8 g (180% g) in the city centre. This exceeds the design strength of most buildings. 124% g is regarded as "Extreme" shaking (higher than "Violent") (see image opposite).

According to one seismic engineer, the force of the quake was "statistically unlikely" to occur more than once in 1000 years, with a PGA (Peak Ground Acceleration) greater than many modern buildings were designed to withstand. By comparison, the Sept 2010 quake - in which damage was predominately to pre-1970s buildings - exerted 65% of the design loading on buildings. For a great explanation of the difference in earthquake intensity compared to the "big one" of six months ago, follow this link: http://en.wikipedia.org/wiki/2011_Christchurch_earthquake

I understand that this would have flattened most cities around the world and it was only NZ's strict building codes that meant more of the city didn't disintegrate. The interesting point for me here is that design is always a compromise: strength vs. cost vs. likelihood of failure, and sometimes there's very little that can be done to completely eliminate all risk even with the best intentions.

At Motovated Design & Analysis we certainly hope to add that extra piece of mind when it comes to the uncertainty of design, but at the end of the day there are sometimes forces and events way beyond our control, and beyond the scope of even the best design standards.

Kia Kaha Christchurch!

Jonathan Prince
Business Development Manager
Cell: 021 582 199
jonathan.prince@motovated.co.nz

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