what is different between diagonal shear and shear friction?

[sirithorn333]

please explain what is different between diagonal shear and shear friction. I have to consider both or not. Now I would like to design retaining wall with lift join.

 

[JedClampett]

Diagonal shear is the traditional theory for carrying shear through a concrete member. It's been observed that shear failures normally occur at a 45 degree plane, hence the diagonal description. This failure plane is useful in designing shear reinforcing, because all the reinforcing that crosses the diagonal parallel to the shear force can be used to resist the shear.
Shear friction only applies at a construction joint. It relies on the roughness of the two adjacent pours providing a surface that has a frictional resistance. The bars connect those two surfaces together. So to fail in friction, the bars must yield. A common error is to try to apply shear friction at other locations than at construction joints. I've seen good structural designers make this mistake. By code, you can't apply shear friction at anyplace other than CJ's although there's a kind of pseudo shear friction at corbels and dapped beams.
For a retaining wall, shear friction would only apply at the wall to slab joint. Right above that, you have diagonal shear.

 

[hokie66]

Agree with Jed in general. "Diagonal shear" is really just a term for the tension that exists in areas where the shear is high. If failure occurs, it is due to tension, so we use shear reinforcement across the planes of tension failure.

For friction to work reliably across a joint, the two sides of the joint must be clamped together. In the shear friction concept, the clamping force is provided by reinforcement acting in tension.

 

[ukengineer58]

Not sure if it's being implied that shear friction occurs at a wall kicker, I have never heard of that. This is just shear or diagonal shear as described. Shear friction occurs or needs to be checked where you have two section trying to or that you are relying on acting as one in bending Another term is shear flow. For instance if you cast a section of slab then cast another over. The shear flow is resisted by the friction between the section and any reinforcement that crosses the interface. For instance look up omni deck slabs. Same theory for two steel section bolted together.

 

[hokie66]

No, shear friction and shear flow are different things. Shear friction depends on reinforcement to provide a clamping force. Personally, I don't normally use it, but rather prefer to rely on what I consider more reliable means. Your mention of ensuring composite action between layers of concrete, and composite action of steel sections, as well as concrete/steel composite sections are good examples of shear flow.

 

[msquared48]

The theory of Shear Friction was originally developed by one of the partners of ABAM Engineers in Tacoma, Washington, back in the late 60's. Had him for a prof in my Prestress Concrete course and he was definitely a big proponet of it (for obvious reasons). Can't think of his name offhand (I think he was the "B" in ABAM), but I think I am almost as old now as he was then.

At the time, ABAM did a lot of prestress and precast design if I remember correctly, and the prestress yard they helped create still exists in Tacoma and is known as Concrete Technology Corporation.

Bob Mast was the "M".

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[msquared48]

It was "Birkeland".

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com