Shear-friction design method or Appendix D? Both?

[jeffhed]

I have a question on when to use the shear-friction method or Appendix D. One particular instance I can think of would be for a suspended concrete slab on top of a concrete basement wall. It seems to me like shear friction could be used, but there are no reductions for edge distances, etc. ACI 318-08 11.1.6 states that it can be applied at "an interface between two concretes cast at different times". So my questions are:

1. Is the shear friction method acceptable for both in plane and out of plane forces on our hypothetical basement wall? If I try to make this work with appendix D I will get much tighter dowel spacing than with the shear-friction method.

2. If the incorrect dowels are installed and I have to come back and epoxy additional dowels then Appendix D would have to be used, right?

Any input is appreciated. I am just trying to define when one method can be used versus another.

 

[weab]

I'm having trouble picturing your concern. Can you provide a sketch?

 

[jeffhed]

I am just trying to discern when the shear-friction method is appropriate. Being in seismic design category D area, I have always just used appendix D. So I figured a suspended slab to concrete wall attachment would be appropriate for the shear friction method for in plane loads. But for out of plane loads is it appropriate as well? This is hypothetical, just trying to gain an understanding of the shear-friction design method and its application.

 

[RobertHale]

My understanding of shear friction reinforcement is that the reinforcing is necessary only to keep the concrete surfaces in contact and the surfaces is actually where the shear is transferred (the reason the surface roughness affects the capacity). Since this reinforcing is only encountering tension is is governed by chap 12 if it is cast in place. I think you are correct though if if is post installed. The adhesive connection would need to be designed for tension only via App. D. I think it is important to note that this is adversely impacted by tension across the joint, and ACI goes on to state that you need additional reinforcing for tension across the face (11.7.7). Hope this helps and doesn't serve to muddle the subject further.

 

[jeffhed]

RobertHale,
Thanks for the response. So then for this situation it sounds like shear friction would be ok. If we did have to add dowels after the fact and epoxy per appendix D, how would you arrive at the appropriate tension force to design for? Is there a way you could back calculate it from the friction factor?

 

[RobertHale]

The code specifies that the reinforcing needs to develop fy on each side of the joint (11.7.8). It doesn't give the option to try and calculate a tension on the reinforcing.

 

[zaxen]

Guys 11.6.7 commentary tells you -

"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 demonstrated experimentally."

Lets say your
Required Avf = 0.14 in^2
Required Ast = 0.16 in^2
Required Asmin = 0.31in^2 T & B(in say a 2' deep slab cold joint)

Now you're providing a # 5 for temperature where you could've gotten by with a # 4 for shear and moment. I would match the dowels with # 5 reinforcing for ease of construction.

Would i interpret 11.6.8 to mean i need to develop this # 5? - No
The bar that needs to be developed for fy is the bar that's providing the clamping force Avf Fy. Since bars only come discrete sizes, in order to ensure that my Avf develops Fy, i am going to develop the bar for T = Avf x fy... Not a (#5)0.31 x fy

11.6.8 applies for Avf only unless your Ast requirements are significantly higher than your Avf in which case you may want to develop the differential (Ast - Avf) per Chapter 12

 

[TXStructural]

ACI 318 12.2.5 allows a reduction of ld by As(reqd)/As(prov) (which is what suggest), but this applies only to flexural members. This reduction is specifically excluded for cases where full development of fy is required.

ACI 318 11.6.8 requires that shear friction reinforcement be anchored for fy, so the 12.2.5 reduction for ld does not apply. The commentary in R11.6.8 is very clear about "full tensile anchorage on both sides".
--
In order for the assembly to remain ductile, you must fully develop the bars used, not just the amount of load expected. If ductility is not a concern and the appropriate factors are used, you may not need to fully develop the bar, but be aware that the behavior will not be the same as a fully developed bar of smaller diameter.

 

[jeffhed]

Zaxen,
I havent thought about it the way you stated it. So your saying if my avf required was half of Ast required then I would only end up developing 1/2fy?