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RE: ASD v LRFD significant results?

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There is another basic problem with LRFD that plastic design overcomes. That
is, in LRFD we still do our analysis based on elastic behavior of the
structure, specifically in indeterminate structures. Then we take those
values and multiply them by load factors and design them as though they will
yield at the locations of maximum moment, for example, that were determined
by an elastic analysis. The actual locations of maximum actions changes when
the first hinge is formed. This is something that the ACI tried to
incorporate (a little at least) with moment redistribution.

Nevertheless, without a cheap (relatively speaking) piece of software to do
the shake down analysis we are left with what we have, an elastic analysis.

Mark E. Deardorff, Structural Engineer
Burkett & Wong
San Diego, CA

> -----Original Message-----
> From: Lloyd Pack [mailto:packman90(--nospam--at)qwest.net]
> Sent: Thursday, January 26, 2006 11:23 AM
> To: seaint(--nospam--at)seaint.org
> Subject: ASD v LRFD significant results?
> 
> On 25 Jan 2006 at 15:11, Eric Ober wrote:
> 
> > Chris,
> >
> <snip>
> Note that often the same
> > framing members/system results, in large part because LRFD has been
> > calibrated to ASD.
> 
> <snip>
> 
> > Eric R. Ober, PE, SE
> > Associate
> > Holbert Apple Associates
> > Olney, Maryland
> 
> This is what has bothered me about the discussion of ASD v. LRFD.
> If you end up with the same framing members or system, then what
> has been gained by a more rigorous analysis through LRFD?  It would
> seem that unless you get a change in design, then there is only more
> time spent in calculations and checks with maybe a corresponding
> ease of mind when done.
> 
> This question will be asked too, about the difference between ASD 9th
> edition specs and the 13th edition specs.  Is there a change in the
> resulting
> members, connections, stiffners, etc.?  If there is not a change, then
> what
> is the benefit of taking more time to run through more calculations?  We
> can get wrapped up in over-analyzing things to the extent that we've gone
> over our design budget by more than double, but haven't added anything
> significant to the design for all that time spent.
> 
> I was taught ASD in school, and therefore, may be biased in my
> feelings about ASD.  And I've learned some of the LRFD
> as I've tried to better understand why it's "better".  In school we
> were told that the benefit of LRFD was only evident when many
> repetitions of members would allow the difference to translate into
> significant
> savings to the client.
> 
> Another offsetting thing that I've run into with steel designs, is
> the condition where the builder or owner go to the steel supplier
> and are told that they've got a stockpile of a certain shape that
> the supplier will sell at a discount, in order to move the stock.
> I get a call asking it this change is acceptable and usually it is
> an acceptable substitution, many suppliers are good at their jobs,
> and know their product.  Normally this shape would cost
> more than the designed shape, but the discount offered makes
> it a better deal.  So, where would the benefit of that 5-10% savings
> through LRFD methodology be after the substitution? Supplied
> shapes can have a greater influence on choice of member sizes
> than the calculations.
> 
> Those of you who like the LRFD method, have you noticed
> a difference in the results of your designs in LRFD v ASD?
> Are you getting larger members? Are you getting more bolts
> in a connection? Are you getting more or thicker stiffners?
> How have you seen the resulting shapes and connections from
> LRFD change versus that same design using ASD?
> 
> Take Care,
> Lloyd Pack
> 
> 
> 
> 
> 
> 
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