Does Flexural Reinforcement Count for Shear Friction 1

[mhijazi]

Can you count the flexural reinforcement across a wall construction joint in calculating the shear friction reinforcement needed to transfer the shear into the footing..? ACI 318 is not clear on that..!

 

[miecz]

For a previous discussion on this topic, see thread507-122089.

 

[mhijazi]

meicz: Thank you so much for the referal. I read it all, and I concluded that no additional steel should be provided if the flexural steel is adequate to transfer the shear force using the shear friction formula.

I have never done anything more than calling for "intentionally roughened surface" when designing retaining walls. The question is do you count both tension steel and compression steel or only the tension steel for shear friction.

 

[DaveAtkins]

Flexural steel can be used for shear friction. The reason for this is that when the flexural steel is in tension, there is an equal and opposite compressive stress block which creates the friction.

DaveAtkins

 

[miecz]

Chapter 14 of PCA Notes on ACI 318-02 sheds some light on this.

Section 11.7.8 requires that the shear-friction reinforcement be "appropriately placed" along the shear plane. Where no moment acts on the shear plane, uniform distribution of the bars is proper. Where a moment exists, the reinforcement should be distributed in the flexural tension zone.

All of the examples and figures show the shear-friction reinforcement distributed, i.e., not concentrated on the tension face. As such, it does not apper to be proper to use the tension reinforcement as the shear friction reinforcement.

 

[JAE]

You also might want to read this thread - asking if you can count on longitudinal reinforcing to replace stirrups. This isn't exactly about shear friction, but it does enter into the discussion that overlaps the concepts in this thread:

thread507-226141

 

[kslee1000]

In general, shear friction bars shall be uniformly distributed, or placed close to where the potential crack is to initiate. The flexural bars in the flecural compression zone usually can be counted on to transfer the shear force, the bars in the tension zone (As REQUIRED for moment), which are already in tension, should not be used as shear friction reinf. (shear causes add'l tension in the bar) unless the bars are excessive.

 

[DaveAtkins]

As I said earlier, flexural tension steel CAN be used to resist shear friction.

Quoting ACI R11.6.7, "When moment acts on a shear plane, the flexural tension stresses and flexural compression stresses are in equilibrium. There is no change in the resultant compression Avf*fy acting across the shear plane and the shear-transfer strength is not changed. It is therefore not necessary to provide additional reinforcement to resist the flexural tension stresses..."

I don't know how it can be more clear.

DaveAtkins

 

[miecz]

Dave

It is therefore not necessary to provide additional reinforcement to resist the flexural tension stresses...

This assumes that the designer first provides the shear friction reinforcement and distributes it according to 11.7.8, and then check to see if additional reinforcement is needed for moment.

You'r doing it the other way around, by first providing the flexural reinforcement and then checking shear friction. It's not the same. The difference is that the shear friction reinforcement must be distribututed along the shear plane according to 11.7.8 (see PCA notes, chapter 14), not concentrated at the tension face.

The classic application for shear friction is a corbel. The A_h shown in Fig. R11.9.2 and required by 11.9.4 shows the required shear friction distribution.

 

[DaveAtkins]

I agree--the Code states the reinforcing must be distributed to both faces. But this post is about whether or not the tension reinforcing can serve double duty, and be used for flexure and shear friction, and as I stated before, it can.

DaveAtkins

 

[mhijazi]

I tend to agree with Dave. If you calculate your steel for moment, you would find it is more than adequate for shear friction. The compression steel is really a tension steel for opposite loading conditions and I would not count on it. I believe we should add steel only if the flexural tension steel is less than the shear steel required for the section.
Thank you all for the valuable comments.