To: SEAOC Listservice <seaint(--nospam--at)seaint.org>
Subject: Re: ASD vs LRFD
From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
Date: Tue, 15 Jun 1999 18:46:55 -0400
I've held my tongue on this thread until the teeth marks are deep and painful
and I've got to let go.
I will use ASD and it's not because I'm an old f**t. I will try to
rationally explain my reasons.
1. *Unless* the dead load *and* other loads are applied in the same manner
and at each and every location, the different load factors can give very bad
results. In other words, wherever the dead load is distributed, the live
load, wind load, seismic load, etc., *must* also be distributed at the same
locations. Wherever there is a concentrated live load, wind load, seismic
load, there also has to be a concentrated dead load at those locations.
Otherwise, your points of maximum moment, inflection, etc., will be at
locations different than where service load maximum moments and points of
inflection are located. If these shift/change, the factor of safety
decreases. If points of inflection change from where they are located under
service loads and you use the factored load inflection points to terminate
cover plates, reinforcing bars, etc., you may not have an adequate section
under service loads. (LRFD does not say how far past the point where it is
no longer needed that a cover plate/larger flange section must extend.)
2. In ASD, the factor of safety is with respect to first yielding; in LRFD
the factor of safety is with respect to a fully plastic (yielded) section.
Therefore with ASD, you have an additional safety margin between first
yielding and when the section becomes fully plastic. The LRFD fudge factor
for material understrength is ridiculous. What I am interested in is what
the safety factor is to failure (and I will define failure later), regardless
of whether it is caused by overloads or by material understrength or a
combination of both.
3. The structure has to be analyzed under (unfactored) service loads to
determine if it complies with serviceability (deflection, vibration, etc.)
requirements. If serviceability requirements control the design, then
strength is irrelevant, ergo, LRFD is not necessary. If a structure does not
meet the serviceability requirements, then it has FAILED, just as much as if
the members fractured under load. (Failure is when a structure can not
function as originally intended.)
4. If, under service loads, yielding takes place, and service load
conditions were not investigated under LRFD, then deformation is going to
occur and grow.
5. Probability is for gamblers and abstract mathematicians. It is based
on, "You win some, you lose some." If the probability is such that you win
more than you lose (a la the "house" at Las Vegas), that is fine for
gamblers, but I, for one, don't want to lose any! While gamblers gamble with
money, I don't want to gamble with lives. 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! The glass industry at one time produced charts that were based on
probability. (Probability of failure: 8 lights per 1000, or a reliability of
99.2 percent!) So many lights failed that the glass industry revised their
charts so they were based on engineering principles with a factor of safety
instead of probability.
Yes, I use USD for concrete design, but I use it much more carefully and with
more skepticism than I did 40 years ago. I have found that whenever someone
tells you something is "better than sliced bread" or is the "cutting edge,"
then you had better look at it real carefully -- cutting edges are usually
sharp and can injure badly. What works in the lab is not something that will
work in the field. In the lab, only one aspect is investigated; the real
test occurs in the field and inadequacies may not be found for 50 years or so.
A. Roger Turk, P.E.(Structural)