Can I count on shear-friction capacity of the reinforcement? 3

[RobertEIT]

I have checked an existing concrete beam, Its shear capacity is not enough, and there is no shear reinforcement(i.e. no tie rebar), But its flexural capacity is over kill and it has an extra layer of longitudinal rebar, Can I take this extra layer of rebar as shear-friction rebar and caculate its shear-friction capacity to compensate the lack portion of the shear capacity?

Thanks a lot.

 

[Lion06]

Probably not. The problem is that the critical shear location is away from the critical moment location (for simply supported beams), and at the critical shear location that rebar is most likely not developed. The bars need to be fully developed to count on it yielding and getting the clamping force associated with the shear friction model.

 

[RobertEIT]

Ok, I need to mention that the development length is long enough from the critical shear section (rebar extended long enough pass the beam supporting point. So the development length is not a problem.

I raise this issue here is becauseI haven't seen any example of using this longitudinal reinforcement in shear capacity caculation for beams. But base on my engineering knowledge, these longitudinal rebar should has contributions to beam shear resistance and also ACI318 does talk about shear-friction in Chapter 11.7.4. What I don't understand is it seems no people used this shear-friction in beam design? Am I wrong?

 

[JAE]

I believe that any reinforcing you use (to have a Vs in your shear capacity) must have some incline to them...see ACI 318, section 11.5.1.2 for the definition - this requires a 30 degree minimum incline.

See also 11.5.6.4 and 11.5.6.5.

 

[miecz]

I would say, no. Commentary R11.7 says that the shear friction provisions "provide design methods for conditions where shear transfer should be considered: an interface between concretes cast at different times, an interface between concrete and steel, reinforcement details for precast concrete structures, and other situations where it is considered appropriate to investigate shear transfer across a given plane."

The shear friction provisions are based on testing of the specific conditions mentioned. It is not appropriate to apply the provisions to situations other than those for which the testing was performed. Specifically, it is not appropriate, in my opinion, to apply shear friction to slabs and beams, except at the interface with another concrete element.

 

[lkjh345]

My 2 cents:

I think you can use the shear friction capacity, but it is NOT additive to the shear capacity of the concrete beam. It is an either or situation. You can use either the straight shear capacity of the beam OR you can use the shear capacity via the shear friction method, but they do not add togther.

Shear frictition is really a clamping force to generate a high enough friction to keep two planes of material from sliding past each other. This implies, in my mind, that there already is a plane of interface bewteen two surrfaces, which, in your case, would mean that the concrete has already cracked due to shear failure, and you are computing whether there is enough steel to keep the now two seperate surfaces in close enough contact so as to not slide past each other.

JMHO, Concrete shear topics are definately not my strong suit.

 

[UcfSE]

With no stirrups in the beam, how will the shear friction bars help with respect to the diagonal tension in the beam due to the shear?

 

[theonlynamenottaken]

The simple answer is NO. Others here have touched on the reasons:

1) There is no "forced" shear plane, it is an inclined failure plane = diagonal tension.

2) Flexural reinforcement cannot serve as shear-friction reinforcement in the way described. All the flexural reinforcement will provide is "dowel action" - which is included in ACI's Vc.

Shear friction relies on aggregate interlock, with the clamping force provided by the reinforcement. The two faces along the failure plane essentially have to "slip" a tiny bit to really engage. By the time your stirrup-less beams "slips", it has an inclined diagonal tension crack = failure.

 

[jt12]

miecz: I think any place where the concrete is loaded to a point where a crack may form should be checked for shear-friction regardless if poured at different times.
I've always considered shear-friction to be a secondary check to the normal shear capacity calculation. I've never seen that it can be used as a substitute, but I don't know for sure.
Perhaps if you were able to get a strut and tie model to work, you could justify it.
Like several have stated, you're crack will be diagonal, so you could possibly only use a portion of the shear friction tension bars for this purpose (see 11.7.4.2)

 

[JAE]

You cannot use shear friction in the case of a beam with a diagonal shear crack.

Here's why:

If you look at section 11.7.4, there is equation 11-26 which provides you the shear friction capacity for a bar inclined to the shear plane.

In the case of a beam, the shear plane is about a 45 degree diagonal. But if you look at Figure R11.7.4, it indicates that the shear direction across the crack is such that it drives tension into the reinforcing bars. For a beam, longitudinal reinforcing is oriented OPPOSITE to the bars shown in the figure. Thus, the value of [α]f is 135 degrees, not 45 degrees.

(see the attached drawing)

So from Equation 11-26:

Vn = Avf(fy)([μ]sin(135) + cos(135))
= Avf(fy)(0)
= 0 kips

Now if the reinforcing bars are rotated 90 degrees (i.e. vertical stirrups), then equation 11-26 is:

Vn = Avf(fy)([μ]sin(45) + cos(45))
= Avf(fy)(1.41)

 

[RonRoberts]

The case of an assumed failure plane that would result in compression in the reinforcing is discussed on pages 253-254 in Reinforced Concrete Design By S Unnikrishna Pillai, et al (see attached).

 

[miecz]

jt12

The code says "concrete poured at different times." I didn't make that up. Section 11.7 references two research papers that support the provisions. If I wanted to try to stretch the application of the code, I would get a hold of the research papers and see if my condition was tested. I'll bet it's not.

 

[JAE]

RonRoberts - yes...your upload better describes why the use of the long. bars with shear friction doesn't work.

 

[jt12]

Good post RonRoberts & JAE. That makes sense. Miecz: they have a coef of friction value for concrete cast monolithically that is part of the shear friction equation. You need to design for shear friction as a failure mode anyplace that a crack may occur, regardless if it's in the same or different pours. i.e. from the code commentary: "...other situations where it is considered appropriate to investigate shear transfer across a given plane."

 

[RobertEIT]

Please Note there is one bug in this theory:

"For a simply supported beam, if at left support the inclined stirrup is 45deg, then at right support, the inclined stirrup becomes 135deg because the crack line is 90deg for left support vs. right support." How do you guys explain it?

 

[jt12]

you incline your stirrups perpendicular to the crack, not the same inclined direction over the entire beam

 

[miecz]

jt12-

You need to design for shear friction as a failure mode anyplace that a crack may occur

I disagree. I only check for shear friction under certain circumstances, i.e., where the shear force is carried by shear friction. For most cases, where the shear force is carried by the concrete, or by stirrups, I don't check shear friction.

"...other situations where it is considered appropriate to investigate shear transfer across a given plane."

This key word here is "appropriate". Unless the condition fits one of the cited conditions, i.e., concrete cast at different times, appropriateness must be determined. Well, how does one determine appropriateness? Is the condition at all similar to the cited conditions? Doesn't sound like it to me. The only way then to determine if it's appropriate to use shear friction would be to obtain the cited references 11.35 and 11.36, and see if any tests were performed on this type of situation. I would be very surprised if beams or slabs.were tested for shear friction away from the supports.

 

[jt12]

miecz: `11.7.1 Provisions of 11.7 are to be applied where it is appropriate to consider shear transfer across a given plane, such as: AN EXISTING OR POTENTIAL CRACK, ...'
They even give you a 1.4*lambda value for mu in concrete cast monolithically. Doesn't that tell you at some point it may be appropriate to look at shear friction in a monolithic pour?

 

[miecz]

jt12-

I agree that it is appropriate to investigate/justify a monolithically poured connection that has a geometry similar to those cited in R11.7 or tested in references 11.35 and 11.36. None others.

The phrase "a potential crack" does not give you a blank check to apply shear friction to any condition you please. If it were appropriate to investigate/justify shear at any potential crack in any monolithic or non-monolithic pour, then R11.7 would not bother to give specific examples of where it may be used.

 

[mhijazi]

I am working a 40'-deep underground rectangular concrete tank. Due to soil and Hydrostatic pressure loading, the wall thickness is becoming over 5 feet near the bottom. Any ideas to reduce the wall thickness..?
Can I use additional vertical reinforcement to help withstand the shear...?