Shear capacity of dowelled connection across expansion joint

[sybie99]

Hi Folks

I have a suspended slab and need to transfer significant shear forces accross a mpovement joint. By UK codes the shear capacity of a deformed reinforcement bar = 0.6 x 0.9 x Area x fy. Now I can calculate the area of steel required to transfer the forces from one side of the suspended slab to the other side of the joint. But if the bars are loaded to near their shear capacity I would think that the bar will spall off the edge of the concrete (250mm slab).

How does one calculate the capacity of the dowelled connection? I would think edge distance would play a major role (distance from dowel bar to slab edge) and that the capacity of the shear connection will be governed by the concrete's capacity to absorb the shear stresses from the dowel bar and not by the shear capacity of the dowels.

I have scanned the forum and cannot find anything on this.

Thanks

 

[JedClampett]

I wouldn't trust that detail for any significant shear forces. Those dowels, which I don't use, are meant for alignment. For exactly the reason (spalling) you mention, they're poor in transferring forces. Plus you would have bending forces in them.
I'm assuming something has changed if you need to transfer shear across an expansion joint. If that's the case, you need to modify the design.

 

[a2mfk]

Maybe consider a continuous embed plate or angle at each slab edge with headed studs into the concrete, and then slip connections between the two at X distance on center that will transfer shear parallel to the joint but allow for the joint to move perp.

 

[ishvaaag]

Or use specialties like Ancon, it has free software.

http://www.ancon.co.uk/products/shear-load-connectors

 

[hokie66]

I assume you mean transferring gravity shear forces rather than lateral. If that is the case, I agree with Jed. Although the proprietary connectors such as the Ancon ones which ishvaaag linked to are used in places, they have shortcomings. When you say "significant shear forces", I would restrict this kind of connection to "minor shear forces".

One problem, especially in residential buildings, is the noise these connectors make when they move. Sounds like a rifle shot when the built up friction releases.

 

[rscassar]

The capacity of the steel is 0.6*fsy*As and you would probably need to apply ACI-318 appendix D to calculate the allowable shear force for the concrete. I would also use a proprietry ancon product for an expansion joint.

 

[ishvaaag]

The problem with an open joint with dowels passing through is that it needs a compresive strut difficult to produce in the required magnitude if significant without some kind of device.

So you use some device with an open joint and add if required geometrical restraining components to it.

At least a shear friction mechanism is not directly of use, since the formation of the compressive strut from ample contact is lacking.

You can get compressive strut but for the limited area of the projection of the rebar, or as hokie66 implies, the limited area of the proprietary connection, hence seen ashe says "minor forces".

For big forces you need to devise a detail for the proper wanted transfer.

However not as "minor" forces in proprietary devices, can go tens of tons, more than enough for lots of common building tasks, surely not for bridge works, concentrated forces.

 

[keenaseng]

I have used Ancon shear connectors on jobs - They can be handy especially for PT slabs. But they are expensive and require specific reinforcement detailing around them.

I'm not sure if the OP question was answered however - Is there a calculation that can be done to determine concrete failure of dowel joints that incorporates edge distance? I don't think so... Ramset, Hilti etc give capacities for bolts with defined edge distances, but I think alot of this is based on testing..

 

[kieran1]

Could you provide a sketch of your proposal.

Kieran

 

[sybie99]

Kieren

I think I will go for a proprietry product like Ancon, the detail would have been a simple dowel rebar to the middle of the slab depth with sleeve to one side allowing sliding.

I know HILTI have software that calculate the capacity of the dowel but also not sure on what they have based their calc