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Re: Shear Friction Design Method: ACI 11.7.7 & 11.7.8

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Here is my interpretation of the wordings...Just my opinion.

On 8/15/06, Wontae Kim <kimwontae(--nospam--at)email.com> wrote:

I need some explanations about shear friction design method,

 which I have been using for precast concrete shear wall design for a while .

 

Here are my specific questions:

 

1) [R11.7.7] "...(second paragraph) When moment acts on a shear plane, flexural tensile 'stresses' and

flexural compression 'stresses' are in equilibrium..."

Isn't 'stress' equilibrium  confused with 'force' equilibrium?

 

       You are right. It should read force - rather than stress. Stresses are never in equilibrium.
 

2)[R11.7.7] "...(continue) There is no change in the resultant 'compression Avf fy' acting across the shear plane and the shear-transfer strength is not changed..."

What does this phrase mean?

     In shear friction (look at page 172 R11.7.3 for description) the clamping occurs when the cracks slip

 
relative to each other such that the reinforcement goes into yield and clamps the two surfaces together. This
is probably the meaning of the 'compression Avf fy' . Maybe the paper referred to in that section will give a better explanation.
 

3)[R11.7.7] "...It is therefore not necessary to provide additional reinforcement to resist the flexural tension stresses, unless the required tension reinforcement exceeds the amount of shear-transfer reinforcement provided in the flexural tension zone..."

Doesn't this mean that shear reinforcement Avf under flexural tension can be used to resist shear force?

 YES. In the form of the shear friction concept-like a dowel action.

([R11.7.8] "..If a moment acts across the shear plane, it is desirable to distribute the shear-transfer reinforcement primarily in the flexural tension zone. Since the shear-friction reinforcement acts in tension, ..." Very confusing?

 
I agree it is confusing. The language (or the intent of it) in this part is not clear. But I understand it
as --- the crack will cause the two sides to slip causing the rebars go in tension and clamps the concrete
pieces together.

Avf should be defined clearly.   According to ACI definition of Avf, Avf is all reinforcements across a shear plane which  include reinforcements in the flexural tension and compression zones in addition to shear-transfer reinforcement.

Isn't Avf of 11.7.7 contradictory with that of R11.7.8: how can I place shear-transfer reinforcement in the flexural tension zone if 11.7.7 is correct?

I believe that 11.7.7 says any reinforcement in the flexural tension zone can not be considered  as shear-friction reinforcement: [11.7.7], "Net tension across shear plane shall be resisted by 'additional' reinforcement.")

 Net tension - I believe is if the member is in tension.

4)[R11.7.7] "...if a resultant compressive force acts across a shear plane, the shear-transfer strength is a function of the sum of the 'resultant compressive force' and the force Avf fy in the shear-friction reinforcement...."

Does 'resultant compressive force' consider both concrete compressive strength 0.85f'c ab and compressive strength of reinforcements under flexural compression as in AASHTO?

 
    The  resultant compressive force probably is the axial forces that act on a column. From the wordings it appears that they would have testing with pure moment and moment+axial force combination ( have not read the paper..so I could be wrong)

(What does 'net' from 'permanent net compression across shear plane [11.7.7]' mean?)

 

I am looking forward to your comments.

 

Thanks!


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