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RE: ASD vLRFD

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I'll respond here, as this is where phase 2 of this discussion started.

At this point, I design with LRFD because it is faster for _me_.  Usually, I
determine my governing gravity load combination, then design all beams,
girders, columns, et cetara, for this load.  Standard Building Code requires
that all spread footings be designed so that dead load stresses are all equal,
so I'm screwed when I get to the foundations anyway.  Even if UBC becomes law
here in Arkansas, standard practice will still govern this criteria.

Generally speaking, the deflection criteria tend to govern large parts of my
design, but this is a symptom of 50 ksi steel or thin structural systems in
most of my work.  Beam equations are a bit easier is LRFD, though I haven't
done much regular plate-girder work.  Bending/Axial capacity is simpler, also,
by reducing the calcualted stresses required.  For roof designs, I get much
better performance from LRFD because of where the beam curves break, and I
don't end up in the big equations until braced length is relatively enormous.

Checking vibration sucks.  Whether you use ASD or LRFD seems entirely beside
the point after checking several beams.  We should all consider putting Murray
et al to the rack (bless his pointed head!).  In all seriousness, AISC's design
guide, prepared with Murray's (et al) help, is a godsend.  I just wish the
checks weren't so difficult.  When this is revised, somebody needs to take a
close look at reducing the design steps to a few shorter equations by use of
tables.

At this point, I've moved past the ASTM A992 spec issues and am trying to
decide whether to spec the new (to me) anchor rod ASTM instead of A36/A307.

Of course, all these comments are regarding hand calculations, since when
computers enter the picture, most of the issues are input errors, program bugs,
and modeling fallacies.

Your mileage may vary,

-Keith Fix, PE
-Little rock, Arkansas


--- HAWNENG(--nospam--at)aol.com wrote:
> Having not posted anything to the list server in many months, I could not 
> help but "weigh-in" on this discussion (pun intended).  Our office uses 
> nothing but "allowable stress design" and has since 1981.  We find that many 
> times, our design is governed by deflection, not stress.   We check 
> deflection at two stages, immediately after the mud is poured and then for 
> total deflection.  For "mud load" deflection, we try to limit the deflection 
> to 1/2".  Often times, rather than camber a steel member, we will increase 
> the member size by a few pounds to limit the deflection.  In the discussion 
> to date, I haven't read how deflection and floor vibrations play a role in 
> sizing the steel members.  We also design and detail all our steel 
> connections on the structural drawings then review the shop drawings to help 
> ensure the contractor is interpreting the drawings as intended by our design.
> 
> Not that this is necessarily the best way to design, but ASD gets my vote.  
> Also, when it comes to the foundation design, you have to track two sets of 
> different load factors.  Yes, we do design concrete by ultimate stress 
> design.  We are definitely middle-aged engineers, but not real old-timers, 
> yet.
> 
> Greg Showerman, S.E.
> Modesto, California
>   
> 
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