Following every major earthquake, tornado, tsunami, hurricane, or typhoon there is a chorus of why did this or that structure fail. A lot of these collapses had a predictable outcome. Over the years, we have learned a lot about seismic ground motions, but society can put pressures on engineers to follow practices that are not appropriate for the state of the practice today. |
It took a huge effort for the Midwest to start to plan, design, and construct for seismic. It is interesting to compare the Midwest response to the response in Utah to the Wasatch Fault. In Utah, they have torn down and reconstructed schools and bridges and rehabilitated government buildings without the stark and painful lessons of body bags.
Regards, Harold Sprague
Date: Tue, 31 May 2011 09:05:27 -0700
Subject: Re: New Madrid
I was hoping for some magic bullet the whole weekend... Unfortunately, in my case, anything short of seismic isolation is not going to work, and even that does not look like a viable option.
It appears that the current installation was designed without any seismic considerations, that will definitely add to the frustration of the client ("there's nothing wrong with it, it's still standing, right?").
Thanks a lot - as always.
V. St,eve Gordin SE
SGE Consulting Structural Engineers
On Tue, May 31, 2011 at 8:52 AM, Harold Sprague <spraguehope(--nospam--at)hotmail.com>
I will try this again.
I have worked on several industrial projects in that area and participated in the code development work that dramatically changed the ground motions. Rehabilitation projects are a challenge because the increase in demand is enormous. This is especially true in some areas of the New Madrid Seismic Zone (NMSZ) that precluded seismic design in some municipal building codes until relatively recently.
The huge difference in seismic ground shaking is in part what led to the NEHRP provisions becoming the bench mark for seismic codes and pointed up the differences in West Coast probabilities and Eastern North American probabilities for ground shaking. The ripple effect helped facilitate the merger of the 3 model building codes.
There are many ways to address the problem, but it is necessary to educate your client before he is shocked.
I have used buckling restrained braces (BRB's) in the New Madrid area to reduce demand on the structure and on the foundations. The BRB has been an effective solution. They will get pretty beat up in a seismic event, but they will limit demand to the structural system.
If the client has any thoughts about higher performance standards, it is a formidable task because of the seismic demand. You may also want to look at the vertical ground motions as opposed to the 20% of the SDS as implied by the code.
What the future may hold for interplate earthquakes is to use a time dependent probability model as opposed to the current practice of a time independent probability. But that is not the case today. The potential of the NMSZ is enormous and recurs about every 400 years with multiple large earthquakes. The 1811 & 1812 earthquakes were actually a series of almost 2,000 earthquakes. Three of them were in the moment magnitude range of 7 or 8. They started in December of 1811 and did not let up until about May of 1812. They caused damage in Charleston, SC and were felt in Boston, MA. The 400 year recurrence interval is what has been shown in trenching studies.
Correction - it was 500,000 lbs.
On Sat, May 28, 2011 at 6:03 PM, V. Steve Gordin <sgordin(--nospam--at)sgeconsulting.com>
I need to do some analysis for an industrial outfit right by New Madrid, MO. Of course, I am familiar with the seismic history of, and the code requirements for, the area. However, I was not quite ready for the LRFD seismic coefficient of 1.13 and the corresponding pullout forces of 500 lbs per baseplate for a moderate-size (not the heaviest) equipment. The currently existing (not-so-old) installation at the same site does not appear to meet anything close to the criteria dictated by the IBC. Sure enough, before 2000 IBC, it was Seismic Zone 3; after that, it became about 2-4 times worse that the former Zone 4 - or anything on the West Coast.
Can somebody working in the NMSZ share the experience on how do they do practical structural design utilizing existing structures there?