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Re: Why LRFD vs ASD (was: We're Not Getting Older, We're Getting DUMBER)[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Why LRFD vs ASD (was: We're Not Getting Older, We're Getting DUMBER)
- From: ad026(--nospam--at)hwcn.org (Paul Ransom)
- Date: Thu, 26 Jan 2006 12:02:22 -0500
> From: Christopher Wright <chrisw(--nospam--at)skypoint.com> > > I'm sure there are other reasons but I can't think of them at the > > moment. > I confess, I'm not convinced. Calculation of live loading is > independent of the methodology used to determine response to the loads. > some situations--seismic response being the best example I can think > of--but I've never (and still haven't) seen a persuasive argument for > the LRFD approach as a replacement for working stress methods. If there > were holes in the ASD approach they need to be fixed, and if the > approach itself is clearly invalid then replace the whole thing--no 1) more consistent "factor of safety" in ALL design checks (e.g. ASD is not consistent for beam vs. connection). 2) consistent "factor of safety" for a range of load conditions (LRFD was calibrated to ASD to produce a similar member for a very narrow range of live:dead). At extremes, ASD does not really produce a consistent "factor of safety". 3) consistent "factor of safety" regardless of materials (e.g. mill section, cold-formed, aluminum, concrete, A36 vs. A992 - see more below) 4) easier to apply in various construction economic situations (e.g. adjust load factors to suit socially acceptable reduced FoS to reduce constructed cost of materials). 5) cross-border consistency (I think that AISC ASD is the single major steel standard still based on ASD). Many international engineering offices are using an LRFD variant regularly, anyway (sounds like US customary vs. SI). ASD vs. LRFD is trivial where "service" loading controls (e.g. vibration, fatigue, simple deflection, etc.). For these applications, you can call the analysis calculations ASD but it is still covered in LRFD as a limit condition (e.g. load factors = 1). Other locations are using AISC ASD or derivations of earlier ASD/WSD (e.g. India is based on British WSD, as Vish noted) and will update according to economic demand (e.g. when the market in a country decides that it is more expensive to maintain ASD independently rather than borrowing another country's LRFD). I suspect that code and material compliance is more important than type of code in some of these situations. There are no holes in the ASD concept. This is reflected by the fact that AISC is releasing the combined 13th edition. The real debate, in the US, is that the AISC ASD 9th has not been maintained and no longer represents an appropriate level of due diligence in design. A designer SHOULD be doing more than the ASD 9th minimum. However, the "law" is interpreted to permit a designer to meet ASD 9th minimum, in some cases, to the detriment of public safety. I'm certain that Charlie or Scott could comment on the assumptions and compromises that go into developing a conveniently useable steel design standard. Some of those in ASD 9th may no longer be valid with the material changes (e.g. from Tube to HSS and from A36 to A992) that we have seen for other economic reasons. In this sense, ASD 9th may be broken for some applications. The cost of maintaining a dual track is not insignificant for either the standard writers, structure owners (generally) or government bureaucracy financers (e.g. tax payers). Regards Paul -- R. Paul Ransom, P. Eng. Civil/Structural/Project/International Burlington, Ontario, Canada <mailto:ado26(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html> ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. 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: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
- From: S. Gordin
- Re: Why LRFD vs ASD (was: We're Not Getting Older, We're Getting DUMBER)
- From: Christopher Wright
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