# Re: ASD vs. LRFD

• To: seaint(--nospam--at)seaint.org
• Subject: Re: ASD vs. LRFD
• From: Paul Crocker <PaulC(--nospam--at)ckcps.com>
• Date: Thu, 17 Jun 1999 12:15:43 -0700
```> (RT) The LRFD provisions have *nothing* to do with "probability of exceedance."
> (RT) They probability that they deal with is demand (based on total load
> (RT) irregardless of load factors) vs. capacity and that, except for a
> theoretical
> (RT) 3 percent overlap, the capacity is greater than the demand.  The loads,
>  (RT) particularly wind and seismic loads, whether they are used in ASD or LRFD,
> can (RT) be determined by "probability of exceedance (or non-exceedance)."

"Probability that capacity is greater than demand" versus "Probability that demand
exceeds capacity"?  I begin to feel like I'm on the wrong side of a glass is half
empty versus half full debate.

Thanks to Michael Valley for his reference to the beta factors.  3% seemed too way
high for gravity load cases, glad to see something in print to back that up.

(RT) Why not use the same factor of safety (load factor) as the material
(RT) recognizes only the total load applied to it doesn't care whether it is
(RT) composed of dead, live, wind, or seismic loads or a combination of them.

(RT) This sounds rather logical, until one realizes that the material does not
(RT) know, or care, whether it is being loaded with dead or live loads, only what
(RT) the total load is that is being applied.

The problem with this logic is that the designer has only a foggy idea what loads
that a member may see over its lifetime.  I would wholeheartedly agree with you if
building loading was completely precise and knowable.  Unfortunately, it isn't.
If you are designing a W12x30 to sit in a vacuum and carry no load aside from
itself, you can be quite happy designing for a bit over 30 lb/ft, because the load
is established with a high degree of certainty (LRFD factor of 1.2).  If you let a
marching band march across it, you have much less idea what load the beam is
subjected to.  You would probably put 100psf on the top area, which is probably
sufficient...  (LRFD factor of 1.6).  What load is the beam really feeling?  I can
assure you that whatever design approach you use (LRFD, ASD, or whatever), the
beam is *not* seeing exactly the load you designed it for, the inflection points
are *not* where you think they are, and the material is *not* as strong as you
think it is.  The service load is not somehow the "correct" load, it is just a
says it's prudent to be more careful (hence the higher load factor) with the
poorly established live load than with the well established dead load.  If you
don't agree with that principle, then I guess there is just a philosophical
difference that will not be resolved by any amount of discussion.  As I said at
the top, in an idealized world where a 30lb/ft beam weighs exactly 30.00lb/ft and
a 100psf LL means exactly 100.00psf on each and every square foot, I would agree
with your approach.  In a real building however, I'll take the LRFD load factors.

Paul Crocker

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