Return to index: [Subject] [Thread] [Date] [Author]


[Subject Prev][Subject Next][Thread Prev][Thread Next]
Bill Polhemus wrote:
> > -----Original Message-----
> > From: Laurence B. Oeth III [mailto:viacalx(--nospam--at)]
> > Sent: Wednesday, June 16, 1999 10:19 PM
> > To: seaint(--nospam--at)
> > Subject: ASD v. LRFD
> >
> >
> > We now need to run a strength analysis and deflection
> > analysis.  Perhaps
> > the Owner will save a few cents at our cost for more work (as usual).
> So you NEVER thought it important to check deflections before?
> > Concerning conrete design, ASD vs. USD:  I design many water retaining
> > structures, wherein crack control is the key, not strength.
> Then "crack control" would be the governing limit state in such a design.
> The fact is that it is the older ASD-style design codes that tend to
> concentrate on strength as the primary concern. The newer codes, while based
> on LRFD theory for strength, also go a step farther and get the engineer
> thinking about the governing "limit states" in his design.
> This is a "good thing," but some would have you believe it just means "more
> work."
> Tell me, how is taking care that you correctly identify the governing design
> criterion a "bad thing"?
RE CHECKING DEFLECTIONS:  Deflections were output/calculated directly
using the same forces as used for stress/strength calculations.  No need
to run unfactored or differently factored loads (barring special
conditions, engr'g judgement, & etc.).

RE LIMIT STATE = CRACKING:  Sure that's the limit state, just as with
ASD in other instances the allowable stress (or deflection) is the limit
state.  It's almost humorous looking at the ACI 350 load factors for
water retaining structures.  Factoring loads into "ultimate" states the
structure never sees, to model SERVICE LEVEL stresses & cracking.

THINKING ABOUT LIMIT STATES:  While not using the term "limit states," I
would propose that historically all competent, practicing engineers
understand and have used the concept of "limit states."  For example,
how about fatigue considerations with "limit states" (allowable stress
levels) adjusted according to # of stress reversals?

We engineers are not monks, intent on wielding increasingly inscrutable
formulii just for the sake of...of what?  We are builders of REALITY,
turning desires and needs into REAL functioning projects.  Our time is
limited, and there are other "limit states" which call for our
attention.  Cost is a limit state.  Constructability is a limit state. 
Environmental and societal factors are limit states.  Successful
projects and their engineer of record have always had to deal with limit
states and we must try to avoid overkill in one area of our
responsibility displacing more important considerations elsewhere.

I appreciate the great work/research on loading and its probabilities; 
excellant work on stability, seismic, and on an on, that has occurred
concurrent with the trend toward LRFD.  But it all could have been
applied to ASD philosphy, and LRFD has yet to prove that the increased
complexity yields significant benefits either in constructed cost,
improved servicability, or ease of design!

Having said that, I feel better, even though I know by the end of my
career we'll all be using LRFD.  But that isn't a good justifcation.