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Shear-friction across an interface with permanent ACTIVE compressive force via PT bars

Shear-friction across an interface with permanent ACTIVE compressive force via PT bars

Shear-friction across an interface with permanent ACTIVE compressive force via PT bars

(OP)
Shear-friction ‘rides’ again, but with a bit of a twist that from my forum search has not been specifically discussed!

As per ACI-318-11, section 11.6.5 on shear-friction states:



For f’c = 4,000 psi, the maximum value of Vn is 0.2f’cAc (or 800 Ac). So basically it is stating that a maximum of 800 psi for 4,000 psi concrete.

If there is a permanent applied compressive stress across the interface, then 11.6.7 permits this value to be added to the rebar passing through the shear plane too:



And clause 11.6.6 states:




So in the case of NO rebar passing through the interface, then it therefore implies that the maximum applied P/A is 800 psi from the permanent applied compressive force, AND if high-strength PT bars pass through the interface (that actively applies the permanent compressive force) the maximum stress in the PT bars must only be 60 ksi (or 40% of their UTS, assuming 150 ksi PT bar)?

Appears to heavily penalize the use of ACTIVE prestress across an interface.

Am I missing something or interpreting something incorrectly?

RE: Shear-friction across an interface with permanent ACTIVE compressive force via PT bars

Quote (Ingenuity)

So in the case of NO rebar passing through the interface, then it therefore implies that the maximum applied P/A is 800 psi

It's Vn/A that is limited to 800 psi, not P/A.

Quote (ingenuity)

AND if high-strength PT bars pass through the interface (that actively applies the permanent compressive force) the maximum stress in the PT bars must only be 60 ksi (or 40% of their UTS, assuming 150 ksi PT bar)?

I interpret the 60 ksi limit to only apply to passive shear friction reinforcement. I don't believe that it should reduce the clamping attributed to prestressing.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Shear-friction across an interface with permanent ACTIVE compressive force via PT bars

(OP)
KootK,

Thanks for the reply.

I want to agree with you re 800 psi BUT...11.6.7 does state that the "permanent net compression across the shear plane shall be additive to Avffy", and if Avf is zero (no passive grade 60 rebar), that means max permanent net compression across the shear plane is therefore limited 800 psi, which is a round-about way of limiting the P/A if that is all you have across the shear plane.

Commentary R11.6.7 has a reference 11.44 that I shall read up over the weekend:



Reference 11.44 is paper by Mattock and Hawkins entitled SHEAR TRANSFER IN REINFORCED CONCRETE - RECENT RESEARCH here: Link

I too interpret the 60 ksi limit to passive shear friction, but the code is silent when you only have permanent net compression and zero passive rebar.

RE: Shear-friction across an interface with permanent ACTIVE compressive force via PT bars

This illustrates one of my misgivings about shear friction in general. I don't know why the clamping force would be related to yield strength, as all steel has pretty much the same elasticity, and will keep cracks together the same amount with a given force, until yield occurs.

I think active prestressing is a lot more reliable than some nebulous clamping action by bars that are stretching.

RE: Shear-friction across an interface with permanent ACTIVE compressive force via PT bars

For the aggregate on each size of the shear plane to separate far enough to allow the two sections to displace across the shear plane, the bar would first have to yield.

RE: Shear-friction across an interface with permanent ACTIVE compressive force via PT bars

We're talking about unbonded PT here, right? If it's bonded, that would change my answers a bit.

Quote (Ingenuity)

that means max permanent net compression across the shear plane is therefore limited 800 psi

I disagree. It is not the compressive stress on the shear plane that is limited to 800 psi. Rather, it is the shear stress on the shear plane that is limited to 800 psi.

While the code isn't explicit in its treatment of what should be done when there is no passive reinforcing, my (our) understanding of shear friction fundamentals leaves me with little doubt as to what ought to be done in practice. The permanent net compression should replace the AsFy term in the SF capacity formula and that capacity ought not exceed 800 psi or f'c/5.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Shear-friction across an interface with permanent ACTIVE compressive force via PT bars

(OP)

Quote (KootK)

I disagree. It is not the compressive stress on the shear plane that is limited to 800 psi. Rather, it is the shear stress on the shear plane that is limited to 800 psi.

Thanks KooktK, I agree. A star for you! Could not see the forest for the trees. Obviously should be shear stress.

The PT would be UNbonded during construction, then grouted prior to full-service. The PT is high-tensile 150 ksi bar - short lengths.

RE: Shear-friction across an interface with permanent ACTIVE compressive force via PT bars

Phew! I was starting to feel as though I was being a bit combative.

The grouted PT raises an interesting case. I would submit that one could take as the clamping force:

1) The permanent compression due to pre-stressing and.
2) Whatever additional axial force would be required to actually yield the pre-stressing (TLHS's point I think).

I'm assuming that we're talking about something like Dywidag bars that would be developed either side of your joint.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Shear-friction across an interface with permanent ACTIVE compressive force via PT bars

(OP)

Quote (KootK)

Phew! I was starting to feel as though I was being a bit combative.

No. Eng-Tip-free long weekend for me for this past Memorial Day weekend, hence my delayed replay.

I did read the paper (referenced above) by Mattock and Hawkins entitled SHEAR TRANSFER IN REINFORCED CONCRETE - RECENT RESEARCH and they did a bunch of testing with varying normal forces across the shear plane, varying up to 3920 psi! They also discuss the 800Ac limit and how PCI goes beyond that limit using different expressions (considering both friction and cohesion separately etc).

Yes, the PT bars are Dywidag, Williams, etc or equal. This is a pair of new stiffened steel brackets supported off an existing concrete wall with through connected PT bar that stress the brackets (loads applied are vertical gravity D+L only) to the wall using shear-friction. Steel interface is sand blast, concrete face roughened, and paste epoxy used in the interface to 'add' to the capacity. No external AXIAL loads applied to the connection, only moment and shear through the stressed interface.

RE: Shear-friction across an interface with permanent ACTIVE compressive force via PT bars

I didn't really have much of a point. Just threw that out there as a response to hokie.

I'm always tempted to pull out the Canadian shear friction formula in these conversations, just to see how people respond to the cohesion term.

RE: Shear-friction across an interface with permanent ACTIVE compressive force via PT bars

I used to feel that way about cohesion too. That is, until I found out that it's actually baked into the cake in the ACI provisions that don't pay any direct homage to it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Shear-friction across an interface with permanent ACTIVE compressive force via PT bars

I actually like the cohesion bit. I just expect it might result in some excited discussion. The only bit that irritates me is how it gets divided out by the area of reinforcement so that it can get multiplied by the area of reinforcement again. I get why they did it that way, but it hides the mechanism a bit.

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