Shear Transfer across Vertical Construction Joint

[asixth]

Hi

When transferring a large shear force across a vertical construction joint, is the ACI shear friction method an appropriate shear model for design.

I have compared different scenarios using the shear model for beams and have found that a typical situation (0.25% flexural reo and Asv/s=4.52) I can get a shear capacity of 1.7N/mm^2 (roughly 0.04*f’c).

When using the shear friction model, if I load the cross-section full of reo so that I have 1% reinforcement crossing the shear plane I can get a shear capacity of 3.0N/mm^2. It does not seem intuitive to me that I can get more shear capacity thru dowel action than by a monolithic section.

Also, is the shear friction method only applicable if the dowels are located in the compression zone of a suspended slab?

This has all come about from a terrible detail where I am attempting to transfer a large shear force across a vertical construction joint in a suspended slab (at least for now, I have a feeling that I will be relocating the construction joint).

All input will be appreciated.

Thanks

 

[hokie66]

You will get different opinions on this, but I would never use shear friction for a vertical construction joint. And I don't see why if would be applicable at all with the dowels in the compression zone, as the shear friction concept is that the steel provides a clamping force, while in the compression zone, you already have clamping due to flexure. When I use dowels (big ones) to transfer load by bearing, I put them at mid-depth, because that lessens the chance of breakout.

 

[rowingengineer]

Agree with hokie. I don't really like dowels for "large" shear transfer either, would prefer a slip bearing connection for "large" shear forces, but this statement would all depend one what is considered large.

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"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[asixth]

Hi guys,

Do you guys spend your saturday's keeping up with eng-tips?

It's large, V*/(b*D) is 2.3N/mm^2 if it where a monolithic section. I'm not going to run the gautlet and just re-locate the joint. They won't like it but I don't care.

Thanks for the help.

 

[asixth]

450t if you want to put a number to it!! It's got me scared.

 

[asixth]

I remember reading something that RAPT wrote a while ago regarding bottom dowels in punching shear reinforcement that were shown to help delay the onset of punching. I thought that maybe the dowels being thru the compression zone may help with the shear design.

 

[dik]

For that sort of load... I'd be looking at putting a real bearing connector in place...

Dik