Need a book? Engineering books recommendations...

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

ASD v LRFD significant results?

[Subject Prev][Subject Next][Thread Prev][Thread Next]
On 25 Jan 2006 at 15:11, Eric Ober wrote:

> Chris,
Note that often the same
> framing members/system results, in large part because LRFD has been
> calibrated to ASD.


> 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

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at:
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*   Questions to seaint-ad(--nospam--at) Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: 
******* ****** ****** ****** ******* ****** ****** ********