From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
Date: Wed, 16 Jun 1999 19:32:49 -0400
Paul Crocker wrote:
. > >That LRFD is set up for a 97 percent "reliability" means that there is a
. > >chance of 3 percent of the members failing: that is 3 out of every 100
. > >members! I don't like those odds!
. > I have never heard it quoted anywhere near 3% for failure before. How
. > did you arrive at this value?
"Reliability" is the compliment of "failure" or "error" and is a euphemism
coined to make it sound better than to report that the analysis "... results
in an x percent failure or error." People would much rather hear how correct
or safe something is than hear how wrong or unsafe it is and 97 percent
reliability sounds much better than saying 3 out of 100 will fail. (Even a
reliability of 60 percent sounds safe, when it really isn't.)
. > I would rather accept a 1% probability of failure than a factor of safety
. > of 3 in a system with a 50% chance of exceeding the design load by a
. > factor of 4. A "factor of safety" is only as good as the odds that it
. > will be exceeded.
Now, you're throwing a whole bunch of other stuff into the bowl. If for
every two loadings, one is going to exceed the design load, then the design
load is inadequate. A design load should be such that it is *never* expected
to be exceeded. The Factor of Safety is to take into consideration that
there *may be* occasional, unanticipated overloads, or that the material
strength is less than what was assumed in design and/or specified. Look at
the design live load for classrooms, 40 psf. That would require a 160 pound
person at 2 feet on center. While I am adamantly opposed to using people to
perform load tests, sometimes it is the only way to get valid information.
In a test by the Milwaukee Board of Education, a room (hopefully on the
ground floor) normally intended for 48 pupils was crowded with 258 pupils,
filling all seats double and all aisles and open space. The resulting live
load, including furniture, was 41.7 psf. [Ref. Gaylord & Gaylord, "Design of
Steel Structures," 1957] Does this justify designing a school for a live
load of 8 or 10 or 16 psf? Would you do it and justify it by saying that
"... there is little *probability* that the load will be exceeded."
Exits are required to be designed for a live load of 100 psf. That would
require 200 pound people spaced every 1.4 feet on center. (Ever try to
space 200 pound people 1.4 feet on center?) What is the probability that
this will be exceeded? Zilch! What is the recurrence interval? Once! Is
there a possibility for an occasional unanticipated overload? Yes! But
because we cannot anticipate the overload and design for it, we provide a
"Factor of Safety."
BTW, the only structures that I know of that are designed for *actual* loads
are highway bridges, and that is because truck weights are controlled (or
supposed to be controlled) by law.
A. Roger Turk, P.E.(Structural)