Did Hilti Make a Mistake? Or Do I Not Understand Shear Friction 4

[CDLD]

Good afternoon!

I've been looking through one of Hilti's guides (Post-Installed Reinforcing Bar Guide) and have found that they are using the bars in the compression zone as shear friction reinforcement.

It is my understanding that when a moment acts across a joint, the bars that are in tension are used as shear friction reinforcement.

I have attached a worked example from Hilti showing the bars in the compression zone being used as shear friction reinforcement, which in my opinion is wrong.

Additionally, on Hilti's Profis software package they have an explanation of their "default assumptions" for shear friction which show that they only count on bars in the compression zone for shear-transfer when moment acts across a joint.

I have a hard time believing Hilti would be wrong on this, but at the same time feel I am correct.
Am I misunderstanding shear friction or did Hilti make a mistake?

Thanks for the replies and apologies for the long post.

 

[WesternJeb]

Hilti has followed Eurocode instead of ACI on a few items, I wonder if this might be one of those items. I am not familiar enough with Eurocode to know though.

There have been threads on this before. thread507-227505

 

[CDLD]

It seems that the consensus from the thread you posted is that the tension reinforcement acts as the shear friction reinforcement, which is in line with the ACI commentary and my original post.

Does not make any sense that Hilti would only count on the reinforcing in the compression zone, it's backwards.

I posted this question to Hilti a few months ago and this is the response I got:

"Hi Colin

The decision of whether the whole area of the member’s cross section is resisting shear or only the upper part is left to the engineer. It would be a case by case decision and in the example you shared the calculations were based on the reinforcement to be considered in upper and bottom part.

Please see this article for more information, and this article for the Hilti method."

 

[la belle vie]

I agree with Hilti. Shear friction means that shear across a joint causes the concrete surfaces to move relative to each other, and surface roughness forces the surfaces apart putting the bars in tension. You generally don’t count the bars that are already in tension from flexure.

 

[WesternJeb]

What code do you practice, la belle vie? Eurocode or ACI?

 

[hokie66]

Hilti is wrong. But then that is understandable, because none of us really understand shear friction theory. Other than with some testing, it has never been shown to be logical.

 

[CDLD]

You generally don’t count the bars that are already in tension from flexure.

ACI indicates that the bars can do double duty. What code are you using?

 

[DaveAtkins]

My understanding has always been that all of the reinforcing bars help with shear friction, in both the compression zone and the tension zone. And I have never added shear friction reinforcement on top of flexural reinforcement.

DaveAtkins

 

[CDLD]

Dave, I would agree that ACI allows for engineering judgement to be made when selecting which bars contribute towards shear friction reinforcement.

That being said, I would be cautious counting on bars in the compression zone given that the commentary suggest placing bars primarily in the tension zone when moment acts on the joint.

Hilti relying solely on bars in the compression zone and neglecting bars in the tension zone is flat out wrong (probably would wind up being conservative in most cases though).

 

[NorthCivil]

Once in my younger years (~2013) I made up a mathcad spreadsheet to verify and calculate all the background calcs that hilti does to evaluate their anchor bolts. was about 15 pages long for a single anchor.

since then, I've relied entirely upon their black box.

If something comes out of the hilti calculator that doesnt pass the smell test, its usually human operator error (bad inputs)

 

[Prestressed Guy]

My understanding is that the Profis software is not doing a shear friction calc. The shear is based on dowel bearing so it is appropriate for the shear to be all in the compression side.
That is how they analyze the post-installed and headed anchors.

 

[hokie66]

What Prestressed Guy says makes some sense. The shear friction formulas which were developed were an empirical attempt to replicate testing results. The shear was recognized to be a combination of friction and dowel action. So friction created due to clamping of the tension bars, and dowel action achieved by the compression bars. I have never seen an attempt to apportion the two.

 

[Enhineyero]

Logically speaking any long steel bar placed at a shear plane will resist shear via dowel action, doesn't matter if it is tension or compression zone. In real life the two planes will not separate until all bars crossing the shear plane fails.

 

[hokie66]

That is true unless the dowel bar is too close to the edge where it is bearing. Thus, nearer the compression face takes away the possibility of breakout. My preference for dowel bars is to use big ones, centrally located.

 

[CDLD]

@ Hokie and @Prestressed Guy, Hilti is using shear friction provisions, not dowel action.
I am referencing the software package used for concrete anchored to concrete (extensions).
If you look in the photo I attached above, Hilti references CSA 11.5.1 which is the shear friction section.

I understand Hilti has software for concrete breakout capacity (dowel action), but that is not really the point of this discussion. Also, I don't believe they have any software that combines shear friction and dowel action (although the empirical shear friction equations probably have dowel action built in to some extent).

 

[hokie66]

That's what I meant. Shear friction theory consists of a combination of clamping and dowel action. But it is empirical.

 

[CDLD]

Even if the formula considers in part the contributions of dowel action, Hilti should be using bars in the tension zone which is providing the clamping force.

 

[la belle vie]

With shear friction, no dowel action should be considered. The shear resistance is equal to the clamping force times a coefficient of friction, which depends on whether the shear surface was cast monolithically, separate but intentionally roughened, etc. At ultimate, the clamping force is the yield stress of the bars time the area. I think the reason Hilti defaults to neglecting the bars in the tension zone is because it doesn't know if the bars there are already going to yield as part of the ultimate moment resistance.

 

[hokie66]

This thread is a good example of what I said above, that the black magic of shear friction theory has many different understandings.

 

[CDLD]

I think the reason Hilti defaults to neglecting the bars in the tension zone is because it doesn't know if the bars there are already going to yield as part of the ultimate moment resistance.

La belle vie, posting this quote from ACI commentary again in case you missed it. Tension bars can be fully utilized for flexure and still be counted on as shear friction reinforcement.