Need a book? Engineering books recommendations...

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

Re: LRFD, ASD, and USD

[Subject Prev][Subject Next][Thread Prev][Thread Next]
MJSLAYSMAN(--nospam--at) 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.