Need a book? Engineering books recommendations...

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

Fw: LRFD, ASD, and USD

[Subject Prev][Subject Next][Thread Prev][Thread Next]

> ----------
> > From: Bill Allen, S.E. <Bill(--nospam--at)>
> > To: gk2(--nospam--at)
> > 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
> > work with).
> >
> > If the design moment (of your example) is only say, 80 ft-k (factored,
> 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
> state of stress imposed on the member by the loads--> it is only a
> 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. 
> 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 -->
> 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
> 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
> 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

I guess I am not making myself clear enough for you, Bill.  My point, and
only point, was that the assumption of .003 for concrete strain in an RC
member is valid, in my opinion, when a person is calculating the ultimate
strength of said member using USD.  This fact is pretty much independent
from the factored loads that will be applied to the member, since the
ultimate strength of the member is a function of the member properties. 
For the example beam I used, with a nominal flexural strength of 165k-ft
(ult), that beam will have an Mn=165k-ft whether you put 10k-ft or 165k-ft
of moment in it.  Furthermore, that Mn will occur when the concrete strain
is somewhere around .003.

If it is the case that the factored loads would manifest a moment well
Mn, then the member is overdesigned for strength.  Steps could certainly be

taken to modify the element such that it was closer to the mark.

As to whether USD should be used for the 10k-ft case --> it IS VALID,
because Mn is being calculated.  If it is overdesigned, as seems to be the
case for your example, then by using USD you can find out that this is
indeed what you have.  The fact that there is only 10k-ft of moment on the
beam does not change the fact that its Mn=165k-ft. 

As I also stated, deflections and cracking ARE important.  However, I am
discussing ONLY strength.  If you disagree with the above point on flexural
strength calculations using USD, so be it, we disagree.  

T. Eric Gillham PE