shear friction versus allowable stress 1

[wildehond]

I am working in South Africa and struggling to reconcile the difference in shear force capacity that I get when i run 2 different calculation streams. Our SA code doesn't have a section on shear friction, but I have a copy of the shear friction section from an old ACI code which, with minor readjustment of the formula, gives Vu=phi*As*fy*mu. Phi, I understand, is the material factor of safety, As is the area of steel across the section, fy= yield stress of the reinforcing and mu=is a coefficient of friction for various conditions across the assumed shear plane.

If I turn my example into a "real life" one.
Square pile 400mmx400mm
4No 16mm diam longitudinal bars
6mm diam shear links at 150mm pitch
fy= 450MPa for longitudinal bars
fyv=250MPa for the shear links

SHEAR STRESS model:
I calculate vc=0.36MPa (based on our SA code)
if I add the contribution from the links, then v=0.56MPa and Vu=79.5kN (allowing for effective depth to be d-45mm)

SHEAR FRICTION MODEL:
phi=0.85
Asv=4*201=804mm^2
fy=450MPa
mu=1.4 (for monolithically cast concrete)

Vu=0.85*803*450*1.4=430kN

Surely the difference can't be this big. Please can someone point out the flaw.

 

[slickdeals]

Shear friction has to have a defined shear plane. You cannot count on flexural reinforcing in the beam towards shear capacity based on shear friction. There have been discussions in this forum on the exact same topic.

It’s no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don’t know which half - LORD KELVIN

 

[wildehond]

Hi Slickdeals
I did search the formum before posting my question, but continued with my post as I wasn't 100% clear. The one thread seem to conclude that flexural reinforcing could be used as shear friction reinforcing. Would this mean, hypothetically, that if there was a cold joint, that I could use shear friction as a check across this face but not for the rest? Please could you help me by directing me to the post that clears it up. Thanks in advance

 

[hokie66]

I don't think you will get clarity on shear-friction in this forum, at least not a unanimous opinion. The ACI Code does, at least the way I read it, allow the flexural reinforcement to be used for the required clamping force. The shear friction "concept" seems to be based almost entirely on testing rather than on accepted theory. I think in the case given by wildehond, the traditional diagonal tension failure mode still has to be considered, thus the shear-friction result is irrelevant. May be just me, but I still consider the concept of shear-friction to be a kind of black magic which I don't want to depend on.

 

[slickdeals]

Would this mean, hypothetically, that if there was a cold joint, that I could use shear friction as a check across this face but not for the rest

There has to be a defined shear plane. It is not to be used for diagonal tension.

It’s no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don’t know which half - LORD KELVIN

 

[asixth]

I was thinking the same thing not so long ago.

thread507-322740

 

[slickdeals]

@asixth
According to ACI, it would apply in your situation because you have created a shear plane by putting a vertical construction joint. I am not a bridge engineer: but if you think of segmental bridges, you always have vertical joints. And when you have shear walls in any building, they almost always have a horizontal joint across which shear has to be transferred.

It’s no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don’t know which half - LORD KELVIN

 

[asixth]

Slipformed or jumpformed you do. Would you say that the key then constitutes a roughened surface and the mu factor of 1.0 can be applied instead of the 0.6 for a smooth formed finished?

I think the shear friction model compared with the beam shear model is still a relevent one. As wildehond has pointed out, in many examples the flexural reinforcement when considered as a dowel alone generally has more shear capacity based on 0.6*fy*A.

 

[dcarr82775]

Shear Friction reinforcing (at least for the roughened surface condition) is based on the tensile strength of the steel so you can't double use the tension strength of the flexural steel unless you provide enough steel for the sum of the tow conditions.

 

[hokie66]

dcarr,
Your interpretation is not in accordance with the Code Commentary. In Paragraph R11.6.7, it says "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, unless the required flexural tension reinforcement exceeds the amount of shear-transfer reinforcement provided in the flexural tension zone. This has been demonstated experimentally."

I don't want to be seen as arguing for less reinforcement or supporting the concept of shear-friction in general, but that is what your code says.

 

[asixth]

I am fairly green when it comes to construction practice, for a formed surface which is rather smooth, how much time and effort is required to roughen the surface by removing the laitance and expose the coarse aggregate. I am looking at a surface area of approximatley 8'x2'?

 

[slickdeals]

There was a good article in this month's Concrete International that addresses the exact same issue.

It’s no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don’t know which half - LORD KELVIN

 

[asixth]

Cheers mate. I have to renew my membership to the ACI, I always found there Q'n'A and detailers corner useful.