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Fw: LRFD, ASD, and USD

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> From: Bill Allen, S.E. <Bill(--nospam--at)AllenDesigns.com>
> To: gk2(--nospam--at)kuentos.guam.net
> Subject: RE: LRFD, ASD, and USD
> Date: Tuesday, July 28, 1998 6:56 PM
> 
> Yes, most of the time, temp. steel governs (in the foundations I usually
> work with).
> 
> If the design moment (of your example) is only say, 80 ft-k (factored, or
a
> number low enough where the section doesn't crack), then why check a
state
> of stress that does not occur?
> 

The way I see it, when computing the ultimate strength of a reinforced
member, you aren't checking it for a state of stress other than the
ultimate moment.  The moment that is being calculated is independent of the
state of stress imposed on the member by the loads--> it is only a function
of the member size, reinforcement, and concrete/steel strengths.  If the
state of stress imposed by the loads is much lower than the ultimate
strength of the member, then the member is overdesigned for strength.  THAT
was my point.  Your original post implied that since the strain in the
concrete was less than .002, then USD should not have been used --> untrue,
IMHO.  The moment calculated IS for .003, NOT for the level of strain
manifested by the factored loads, for reasons I stated above.


> Also, fortunately in your example, crack control is satisfied, but this
is
> something that is often overlooked especially with lower ratios of d/dc.
I'm
> sure you are very watchful of crack control in an environment such as
Guam.
> Tough on (exposed) reinforcement.
> 
> Your problem (or most foundations for that matter) did not address
> deflections. Designing shallow sections in USD have a tendency to be
soft.

My discussion was restricted to the strength of the member.


> 
> I hope I did not strike a nerve when I made a statement of young
engineers.
> I am just going by my experience of the 8-10 or so that have worked for
me.
> Most engineers that I am familiar with grab a formula, check Phi*Mn and,
if
> it is greater than Mu, call it a day. There is much more to it than that.
> BTW, I would have been more impressed if you had walked through the
moment
> curvature and strain states by your own hand.

No offense taken.  Regarding my making a stronger impression --> I'm a bit
more concerned with impressing my boss with the money making work I am
doing, rather than the time I spend participating on this server.


> 
> Keep in mind that the concrete industry first came out with USD in order
> that they become more competitive with steel. There was nothing "high
tech"
> about it.

???


> 
> O.K., now I can go back to bed.
> 
> Bill
>