O.K., I've got to ask.
With all this bru-ha-ha about LRFD, what's so special about it?
Are structures going to be safer if designed with LRFD?
Are structures going to be more economical (while performing as well) if
designed with LRFD?
Research is based on LRFD? Then where did the 12/23 factor come from in the
Personally, I see a point about checking two stress levels. One being a
stress level that is encountered on a regular basis and the structure should
be designed to have acceptable deflections under this load condition and is
able to withstand fatigue issues, etc. Let's call that "Service Level". The
other is a maximum credible (or minimum INcredible) load condition checking
against collapse. Let's call that "Strength" or "Ultimate" Level. IMO, LRFD
is for the latter but it doesn't make sense to use this method if the loads
are distributed in an elastic method and not by plastic hinging or using
limit state analysis.
One gentleman used the arguement that LRFD (or USD) made sense for
non-homogeneous materials such as reinforced concrete. I don't agree at low
strain states. You can do a transformed section analysis that s/b every bit
as "accurate" as the Whitney stress block. I believe most footings are
within a strain state such that it is behaving elastically anyway. At least
the ones I design are that way, even with factored loads.
The reason I will be reluctant to move towards LRFD (unless coerced) is that
I will have a difficult time justifying the effort.
There would be one other reason to use LRFD and that is from a business
point of view. If I had an office where all the engineers working for me
learned LRFD in school, it would be easier (read: cheaper) for my office
(and me) to adopt LRFD than to teach them ASD (AMHIK).
Just my two cents.
Bill Allen, S.E. (CA #2607)
Laguna Niguel, CA