From: Neil Glaser <NGlaser(--nospam--at)pkainc.com>
Date: Thu, 03 Feb 2000 08:35:07 -0600
LRFD would typically yield lighter beam sizes only when the dead loads are
very large. This is due to the load factors (1.2 DL & 1.6 LL); the higher
the dead loads are the more beneficial the design becomes because of the
lower load factor. I think that you will see the same or similar beam sizes
on the second floor since the live load is even higher than the roof.
LRFD was incorrectly promoted as yielding a less expensive design. I
believe that most typical situations will result in similar designs using
ASD or LRFD. LRFD is simply a different design approach.
By the way, I learned LRFD in college and ASD on the job. Over the past
eight years I have only used LRFD on one project, simply because the owner
required it. I prefer using ASD and so do all of the engineers that I have
been associated with (at least those in private practice).
"Ritter, Mike" wrote:
> My intent here is not to raise anybody's blood pressure. However,
> yesterday I had a couple of minutes and thought I'd try a quick
> comparison between ASD and LRFD. I have a large 2-story structure
> (210'x270'). Bay sizes are 21'x27', 10 each direction. I optimized a
> typical bay of roof framing using ASD, and then tried LRFD. Know what?
> I got the exact same member sizes! I guess that might not be a surprise
> to some of you, but it was to me.
> The whole thing took me a little longer than ASD (not that its hard or
> difficult, just different). It didn't feel right and I certainly didn't
> save any money for the client.
> Question: When is it that LRFD would produce smaller beam sizes? Maybe
> I'll try the same test on the 2nd floor framing, since I have much
> higher live loads than on the roof. I'll report my findings if they
> favor LRFD.
> Michael Ritter, PE