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RE: Post yielded beam performance

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Roger,
I'm glad you brought this point to my attention because it made me look 
into it and I found one graphic reference that indicates a 20% elongation 
at ultimate and another that indicates 20% at failure.  Can you give me a 
specific reference that clarifies it?  ASTM A36 and A6 gave me nothing.
Thanx
JDC

-----Original Message-----
From:	Roger Turk [SMTP:73527.1356(--nospam--at)compuserve.com]
Sent:	Wednesday, November 18, 1998 11:08 AM
To:	seaint(--nospam--at)seaint.org
Subject:	RE: Post yielded beam performance

Jerry D. Coombs wrote:

. > Be careful here.  The elongation to failure is in the 20% to 24% range. 

. >  But the ultimate strength is reached in the 12% to 16% strain range.
. >  Beyond that point, the slope of the stress-strain curve is negative. 
 As
. > loading proceeds up the new curve to a point that exceeds the new 
maximum
. > point on the curve, major failure can proceed very rapidly because all 
of
. > the strain hardening has been used.
. >
. > JDC
. >

In my original post, which Jerry Coombs refers to, I have assumed that the
beam has been unloaded.  When steel has been stressed beyond the elastic
limit and has been unloaded, the unloading stress-strain curve will 
parallel
the original stress-strain curve between zero stress and the elastic limit, 

with the only resulting effect being that there is a residual elongation.
Reloading the steel will have the stress-strain curve retracing the 
unloading
curve up to the elastic limit, and then tracing along the stress-strain 
curve
for steel which had not been unloaded.

If required elongation is in the 20% to 24% range, the ultimate strength 
has
to be at a strain greater than this amount.  For A36 steel, strain at yield 

is 36,000/29,000,000 = .0012, or .12%, considerably less than the 20%
elongation required.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona