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RE: tornado alley[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: tornado alley
- From: "Dave Adams" <davea(--nospam--at)laneengineers.com>
- Date: Mon, 9 Jun 2008 10:52:23 -0700
It is difficult to directly assign a performance level objective to the current codes because they are not performance-based. However, take a look at ASCE 7, Section 1.4 (“General Structural Integrity”), where it states that the intent of this loading standard is to provide sufficient continuity, redundancy, and ductility to “resist loads without collapse.” Additionally, the commentary section of SEAOC’s “Blue Book” states that structures designed and built according to the requirements and prescriptions of the code should be able to “resist collapse” when subjected to a major earthquake (from the 1999 edition). “Collapse Prevention” is commonly identified as the lowest acceptable level of performance for any structure, and since the current codes specify minimum regulations only, then this must be the target level of performance. I believe, however, that in reality we are closer to a “Life Safety” level of performance … IMHO.
Dave K. Adams, S.E., P.Eng.
Lane Engineers, Inc.
Keep in mind that the code is NOT intended to prevent EVERY natural event from destroying a house. The code is meant as a life safety level for protections from is deemed by society as some "acceptable" level of effort. In more technical terms, all the natural event loading covered by the codes have some assumed average return period. For wind, if I recall properly, that is a 50 year return period. That means that on average, every 50 years there should be a wind event that exceeds the code level wind event. Thus, if you take your example of Oregon, if those are the only two events have happened in the past 50 or so years, then the code level of 90 mph seems to be right on target.
The point is that there is a difference between the code level intent and what I might deem to be a good idea. Code is to protect life. To me, the lack of straps in for the vast majority of the country still can achieve that goal. And unless your uncle was in the shed at some point during that severe storm, straps were not needed for life safety on that shed. Of course, there is NOTHING that says one cannot go beyond the code. The reality is that no too many people want to go beyond the code and spend money on stuff that they don't see and don't understand (at least until they house is in ruins around them). It is an unfortunately part of human nature.
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- From: Scott Maxwell
- RE: tornado alley
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