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]

-----Original Message-----
From: T. Eric Gillham [mailto:gk2(--nospam--at)kuentos.guam.net]
Sent: Tuesday, July 28, 1998 4:07 PM
To: seaoc list
Subject: Fw: LRFD, ASD, and USD




----------
> 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.

[Bill Allen]
Why calculate a capacity using a method that is nowhere near the strain
state the member is actually expected to undergo? Those who blindly adopting
USD and ignoring service conditions (deflections, crack control) will
eventually develop into very poor engineers (IMO).


> 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.

[Bill Allen]
As I pointed out above, strength of a member is not the only design issue.



>

Regards,
Bill Allen