Friday, February 10, 2012

Earthquake engineering is not an exact science

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AFTER THE REPORT ON the collapse of the CTV building, everyone now wants to hang the builder and designers.

“Someone is incompetent!” Is the cry. “Someone must be to blame!” “There's criminal negligence going on there somewhere…."

No. Not necessarily. None of that follows necessarily from the report.

It seems on the face of it that people are not so much blaming people for not being competent, but for not being omniscient.

Because I think the problem is not one of negligence but one of the nature of knowledge.

COMPARING THE POOR STATE of Christchurch’s heritage buildings after the earthquake with most of its modern buildings is enough to tell you that earthquake engineering has improved rapidly over the last century. That knowledge has increased and will continue to increase.

It is a heroic tale. From a position of almost complete ignorance one-hundred years ago, engineers acquired increasing understanding and ingenuity in protecting buildings and the people in them--with new and revolutionary systems introduced in recent decades such as K-Braced Frames, Base Isolation and Ductile Design—saving literally millions of lives around the world, and hundreds of thousands in Christchurch.

Where just seventy years ago in First World countries like ours people still died en masse in earthquakes like the Napier disaster, today the earth can shake well beyond what even modern building were designed to handle—as it did in Christchurch on February 22nd—and ninety-nine per cent are still able to survive heroically and allow people to get out safely.

That we are talking about just two that didn’t (this one and Pyne Gould) is a tragedy on a massive scale. Let’s not downplay that. But that we are talking about just one that didn’t is a testament to the engineering in all the buildings that did. The engineers responsible used all the the knowledge acquired in recent years to design them; knowledge that will increase in future years.  But as the knowledge continues to increase, some of the methods used today will also shown to be wrong and less than adequate by engineers fifty years from now (as they undoubtedly will).

That  will not make today’s engineers negligent. They will simply be revealed as less than omniscient.

Just like every other human being.

SO EARTHQUAKE ENGINEERING IS still an inexact science, with new understanding emerging  after every earthquake that helps engineers understand more for the next one. After this one, for example, we’ve learned that the ground can move in very different ways than buildings have been designed for. It’s not necessarily a matter of criminal negligence, then—it’s more the nature of knowledge and how it improves, is tested and expands.

Reading summaries of the report with that in mind, when you boil it down it seems that in the early eighties we knew less about designing buildings to resist earthquakes than we do now.  Which is nothing to blame anyone for. And (perhaps) that too little was done to upgrade buildings like CTV’s that were designed before the modern era of seismic design.  Which is where any blame, if it’s deserved, probably lies.

This building for example was designed with its bracing walls disposed asymmetrically. But, for whatever reason, the importance of symmetrical bracing was less well understood then.

The building’s floors appear to have “pancaked,” which is what happens when columns collapse and one “soft storey” after another collapses on the one beneath. But back in the early eighties, engineering wisdom was still dictating that beams be designed stronger than columns—a situation eventually recognised as causing columns to fail before beams, leading inexorably to the pancake problem.

The columns are described as “brittle”—which is say they were not ductile—on which were imposed extra loadings from the increased twisting of the building. But the building was designed before the importance of Ductile Design was fully understood. 

As the engineers responsible for designing CTV, Alan Reay Consultants, said in a prepared statement yesterday:

We need to remember that the [design methods] of the day, when the building was designed and constructed, were not intended to withstand the magnitude and type of earthquake ... experienced on February 22.

They’re quite right.

Things have changed since then—but to call the engineers of the time negligent because they knew less than they we do now, and will know in the future, is to blame human beings for not being omniscient.

Which is not anything you can blame engineers for.

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7 Comments:

Anonymous Anonymous said...

This post should be published in the Christchurch Press.

2/10/2012 10:00:00 pm  
Blogger Michael said...

Port-au-Prince was leveled on 12th January 2010, with a building failure rate of approx 90%. That only a few buildings totally collapsed shows that the earthquake engineering worked.

2/11/2012 04:01:00 pm  
Blogger PC said...

@Michael: The logic of your argument escapes me.

2/12/2012 11:07:00 am  
Anonymous Dolf said...

I think michaels point was that, compared to port-au-prince, Christchurch had excellent earthquake engineering.

2/13/2012 09:28:00 am  
Blogger Manukau Mum said...

You appear to have missed the part of the report which says that the CTV building was not designed or built up to the code applying at the time!

2/14/2012 10:47:00 pm  
Blogger PC said...

@Manukau Mum: No, it appears to have been designed accordiong to the code which the 1984 code had only just replaced--so it it highly likely, I'd suggest, that it was either "grandparented," or designed under the earlier code.

Unfortunately, neither report nore media reports expains the situation fully.

2/15/2012 09:34:00 am  
Blogger Manukau Mum said...

The panel of expert engineers said in their report that the CTV building was not built to the standards of the time: http://www.stuff.co.nz/the-press/news/christchurch-earthquake-2011/6395045/CTV-building-report-very-thorough.
Even the executive summary of the report is a massive read.Can't claim I understand it all, but there seems to be a problem with the design.

2/15/2012 08:03:00 pm  

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