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Re: LRFD, ASD, and USD[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: LRFD, ASD, and USD
- From: "Brian K. Smith" <smitheng(--nospam--at)dos.net>
- Date: Fri, 31 Jul 1998 05:59:07 -0500
MJSLAYSMAN(--nospam--at)aol.com wrote: ...We still need to remember what they mean in terms of servicability. If a floor is "bouncy" or "doesn't feel right" customers or users will not go there.... As I said in my original post, I checked the floor system for deflection based on AISC's Steel Design Guide Series 11 - "Floor Vibrations Due to Human Activity." With the large number of responses to this topic, I guess I need clarify myself a little: I am young (29), I was taught LRFD, I have always used LRFD, and I will always use LRFD. I really do not care what anyone else is using; however, I get tired of going to seminars and short courses and hear some of the older engineers down talk LRFD, simply because it is new(er) and they don't want to change. In the commercial and industrial work that I have done, the dead load to live load ratio is high enough that LRFD produces economical designs. I assume however that if an engineer is overly conservative, the LRFD method may be no better than ASD. In fact, based on conversations I have had with a software developer for the "pre-engineered" building market, the dead/live ratio is so low in these buildings that ASD actually produces more economical designs than LRFD. Now, I do not work in a high seismic area (yet) so I want to limit my next question to pure gravity considerations; but, what's the big deal? Why does everyone shriek when they hear the term limit states design. In ASD you are limiting the stress in the extreme fiber of a beam/column to what, 60-70% of its yield stress. Why? Is there not reserve capacity in the member after the extreme fiber reaches its yield stress? In LRFD you factor the loads based upon their relative probability of occurance as a single load as well as multiple loads. After increasing the load, you limit the maximum moment to 90% of the plastic moment capacity of the section. True you have to check deflection, vibration, etc., but you have to do this with any design methodology. Additionally, if you are using composites or continuous members, many times the member strength controls.
- Re: LRFD, ASD, and USD
- From: MJSLAYSMAN
- Re: LRFD, ASD, and USD
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