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RE: RFP Reinforcing

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Peter Griem and Tom Hunt touched on problems with FRP reinforcing bars which 
would extend to any other reinforcing material with a true linear 
stress-strain curve as well.  Failure will be sudden with either the concrete 
crushing or the reinforcing failing without yielding.

When I was an actual student, it was explained that we, as structural 
engineers, want our structures to deflect like a sway-back mule before they 
collapse.  Give a visual indication that disaster is approaching.  The worst 
thing that we can experience is a sudden explosive (boom!) failure without 
any visible indication of impending failure.  We want the public to be afraid 
to use a structure when failure is imminent.

It would seem to me that with FRP (and similar reinforcing materials with a 
true linear stress-strain curve) that the maximum stress in the reinforcing 
should be limited to not more than 50 percent of ultimate, possibly even as 
low as 25 percent.  It would also seem to me that USD should not be allowed 
with these materials as there is no yielding and, as such, you cannot 
"under-reinforce" them (provide less than balanced reinforcing) so that the 
reinforcing yields before the concrete crushes.  There also should be a 
*minimum* large, visible and apparent deflection required when the calculated 
stresses in either the concrete or the reinforcing reach half-way between 
the permitted design stress and ultimate.

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

Peter Griem wrote:

. > I just used GFRP on a mat foundation under some equipment that was
. > particularly sensitive to magnetic fields.  The problem we ran into was 
. > that since since the mod of E is lower you wind up using a lot more bar 
. > that what you may be used to seeing. Thomas alluded to some design 
. > differences - one is that it is suggested that over-reinforcing the member
. > actually leads to a more "ductile" member than conventionally using a 
. > minimum amount of reinf (idea is that you would see concrete crushing 
. > before failure. I had a 14" thick mat with the equivalent of #8 bars at 
. > about 6" on center each way, top and bottom.

. >  The contractor had a fit about it until he realized they were so quick 
. > and easy to install. They were able to vibrate without too much problem.  
. > Another reason for so much reinforcing is to control crack widths - 
. > especially in overhead conditions.
. > If I had to do it over again - I'd look into the different types of fibers
. > listed in ACI 440.  You should talk to Hughes Brothers - they manufacture 
. > GFRP rods and are extremely helpful. They have a consultant that may be 
. > able to look at a lot of things for you.  Look at
. >  for some 
. > good information - it looks like they updated it within the last couple of
. > months.

. > -----Original Message-----
. > From: THunt(--nospam--at) [mailto:THunt(--nospam--at)]
. > Sent: Tuesday, June 17, 2003 1:41 PM
. > To: seaint(--nospam--at)
. > Subject: Re: RFP Reinforcing

. > Charles, 

. > I am guessing you mean "FRP" reinforcing (i.e. plastic rebar).  If this 
. > is the case then you can find detailed information in ACI 440R-96 
. > (Reapproved 2002) and ACI 440.1R-01. 

. > On the design side, the issues are that the strengths are very high but 
. > with little ductility. The stress-strain curves are very straight then, 
. > boom, it breaks. This requires a different design proceed than we are 
. > normally use to with steel reinforcements.  The other design issue is 
. > that these products normally have a very low modulus of elasticity 
. > compared to steel so deflection can be a real problem for elevated 
. > structures. The coefficient of thermal expansion between FRP and concrete 
. > can be another issue. 

. > On the installation side, the big problem is that this stuff can not be 
. > field bent.  Typically you have straight bars and put them together with
. > pre-manufactured "L" or corner bars.  This can make certain applications 
. > very congested. 

. > Thomas Hunt, S.E. 
. > ABS Consulting

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