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RE: Shear Friction

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The precast guys gave us an article that explained where the shear-friction equations came from.  I can't seem to find it at the moment, but it had and interesting point.  If you demand is low, (say 1-2 sqrt(f`c) ), then the concrete helps as much as the steel and you get very high capacities per bar.  In other words, at the low end you don't need much steel as ACI-318 will tell you.  The equations in ACI 318 are for when you have high shear stress (say 8-10 sqrt(f`c) ).  At this point, only the steel is effective and you get next to no help from the concrete (it has already failed).  This is the case modeled in ACI.  I will have a look and see if I can find the article later today.  So if you are in the low end, I wouldn't be that worried about the steel development.  I would take the reduction like you mentioned and let it be.
Jake Watson, P.E.
Salt Lake City, UT
-----Original Message-----
From: M. David Finley, P.E. [mailto:pec(--nospam--at)]
Sent: Thursday, November 14, 2002 1:24 PM
To: seaint(--nospam--at)
Subject: Shear Friction

ACI 38-02 11.7.8 reads
"Shear-friction reinforcement shall be appro-
priately placed along the shear plan and shall be
anchored to develop the specified yield strength on
both sides by embedment, hooks, or welding to spec-
cial devices."
Normally, development lengths can be reducd by the ratio of the area of steel required divided by the area of steel provided, but the phrase "specified yield strength" appears to imply that the full development length must be provided, regardless of much extra reinforcing is provided.  The commentary even mentions "full-tensile anchorage"
Normally I would take the conservative approach and provide the full development length, but for this particular detail, I'm a few inches short of being able to provide the full development length for hooked bars and increasing the length creates a lot of other problems.  If I reduce the development length based upon the area of steel provided, I have more than 3x the shear friction capacity required.
Any comments?
M. David Finley, P.E.
2086 SW Main Boulevard - Suite 111
Lake City, FL  32025