Return to index: [Subject] [Thread] [Date] [Author]
Re: ASD vs. LRFD
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: seaint(--nospam--at)seaint.org
- Subject: Re: ASD vs. LRFD
- From: Paul Crocker <PaulC(--nospam--at)ckcps.com>
- Date: Wed, 16 Jun 1999 16:29:51 -0700
Roger Turk wrote: > ASD is not based on "empirical" or "arbitrary" stress limits. ASD is based > on a factor of safety with respect to first yield. LRFD is based on a factor > of safety with respect to a fully plastic (yielded) section. I'm not sure that this shows that ASD stress limits are not arbitrary, because while ASD attaches a specific factor of safety to first yield, the choice of the factor of safety was probably arbitrary or empirical. How were the factors of safety chosen? Why use 1.67 on the gross section and 2.00 on the effective section as in your example. What rationale does the ASD system have for these choices? If it is based on testing, then it is empirical. If it was chosen based on the judgment of practicing engineers or academic, then it is arbitrary. Really, empirical and arbitrary are NOT bad words. Read the ACI Chapter 11 shear calculations (particularly for corbels) and those two words may come to mind, but I still believe that the ACI is a fine and informative code. LRFD attempts to create systems where you can say that there is an X% probability of exceedance, and designs the system on that basis. You quite rightly pointed out that you can go back through the LRFD calcs and determine what the corresponding ASD factor of safety would have to be to get the same results. But, the fact remains that the LRFD calcs ideally lead you to a system with a known (or estimated) probability of exceedance rather than a factor of safety with an unknown probability of being exceeded. This creates a certain uniformity of logic in design, since probability of exceedance is a central principal in calculating code wind pressure, seismic forces, rainfall amount, and probably many other things that I've never worked with. Paul Crocker
- References:
- Re: ASD vs. LRFD
- From: Roger Turk
- Re: ASD vs. LRFD
- Prev by Subject: RE: ASD vs. LRFD
- Next by Subject: RE: ASD vs. LRFD
- Previous by thread: Re: ASD vs. LRFD
- Next by thread: Re: ASD vs. LRFD
- About this archive
- Messages sorted by: [Subject][Thread][Author][Date]