Interface Shear for Concrete Cast at Different Times

[KC__]

Hi everyone,

I'm stuck at calculating the design shear resistance for concrete interface cast at different times. From Eurocode 2 Clause 6.2.5(1) Equation 6.25, there is contribution from the 'external normal force across the interface'.

My senior opined that this value is equivalent to the horizontal shear force due to Bending Moment, but I can't seem to find any elaboration from the Eurocode/book on how to calculate it if that's the case. Hope anyone could share where do I get this value. Thanks!

 

[Lomarandil]

I got into a major disagreement with some French engineers a few years ago about this.

Their position was that because Eurocode prescribes that force needs to be "external", that internal forces like vector component reactions or weight of concrete cannot be used as the "clamping force". It would need to be something like external post-tensioning.

I don't know how true that statement is for Eurocode, but I know that US practice does allow internal force components to be used for shear friction.

 

[lexpatrie]

That doesn't sound right. The ACI talks about a normal force as well but it's intended to be a downward gravity load from the weight of a structural wall, i.e. uniform compression across the surface, if you ask me.

If you wanted to go that route with compression force from a flexural load, the depth of the element isn't the full depth it'd be the part in compression and I've not seen an example calculation that does it that way.

Euro code may vary, however, sometimes they are a bit ahead of ACI from my impression (talking about punching shere, here)

 

[VoyageofDiscovery]

Do you have a sketch of your scenario? e.g If it is a wall scenario across a wall horizontal construction joint, then that "external force" can be the weight of the wall and other supported Dead Loads. If it is for precast bridge girder elements, this could be post-tensioning forces, etc..

 

[KootK]

I'm going to assume that we're talking about a vertical joint rather than a horizontal one (topping etc).

My senior opined that this value is equivalent to the horizontal shear force due to Bending Moment, but I can't seem to find any elaboration from the Eurocode/book on how to calculate it if that's the case.

I don't think that it's equivalent to the horizontal shear force but, rather, the compression generated by the flexural compression block.

It's important to recognize, however, that the flexural compression block only exists because, in most cases, there is flexural tension steel elsewhere in the cross section that facilitates the development of the flexural compression block.

So, functionally, your flexural tension reinforcing is able to be used, concurrently, as shear friction reinforcement. You cannot, however, count the flexural compression in addition to the flexural tension reinforcement that facilitates it. That would be double dipping.

 

[KC__]

Thanks all for the reply.

In my case, this is a transfer slab with horizontal construction joint, so I would say that the normal force will mostly be the self weight of the full-section transfer slab. That means most of the time the external force would be in compression, and the only tension force I could think of would be prestressed structure.

If this is the case, then Moment actually has less effect towards the horizontal shear stress on the interface. Basically it only affects the lever arm.

 

[lexpatrie]

This needs a drawing. What you are describing currently makes zero sense to me. Normal forces in a slab don't develop from the weight of the slab, it goes into bending and shear.

Or is this post tensioned?

I'm also having a great deal of difficulty visualizing how a prestressed structure has a cold joint in it, as a slab.

If this is a transfer slab you should not be playing games with the standard....

None of this makes sense to me, yet.

 

[cam_b]

We use a similar approach in Australia, I haven't looked at it for a long time, but I thought it's just a shear friction formula where this part of the formula is accounting for the component of shear resistance provided by multiplying the permanent pressure acting on the horizontal C.J. (probably just the self weight of the upper slab pour and the SDL) by the coefficient of friction along the joint.

You could always cross check your results against another code like the Canadian code to ensure you get similar answers. Especially if its a large transfer slab.

I have seen horizontal construction joints in deep transfer slabs where there are issues with the formwork supporting the wet weight of concrete. Especially if sections of formwork are multiple levels high. What is the reason for the horizontal C J. On this slab?

 

[KootK]

I typically ignore the dead weight of the upper portion of the slab for the shear friction and use a bunch of hairpin bars to connect to connect the two pour layers. Obviously, contractors would prefer to not have to do that.

Looking at the upper poor self weight in more detail:

1) You've got the weight acting as clamping force prior to removing the formwork for sure.

2) Under a transferred column, you've surely got some great clamping force.

3) Over a supporting column, you've probably got pretty good clamping force.

4) Away from concentrated loads, it's tough to know just what is going on. The upper pour will posses its own stiffness which will reduce the compression that it imposes on the joint. It's even possible that there could wind up being tension on the joint.

My gut feel is that [2] & [3] probably make things work out naturally most of the time. It's just tough to prove robustly.

 

[lexpatrie]

Cam I think the contractor just decided to put a joint there.

 

[cam_b]

KootK, if the beam is designed correctly and has adequate longitudinal shear capacity then the top and bottom section will behave as one section. So i don't understand how the stiffness of the top section reduces the load on the C.J. without slippage along the joint. Perhaps i am missing something, would you mind elaborating on this?

But i do agree with having reinforcement crossing the construction joint plane. I would also intentionally roughen the joint surface as well.

 

[KootK]

@cam_b: sure. It's really the same phenomenon that we recently discussed here in some detail: Link

 

[cam_b]

KootK, thanks, i see what you're saying. Although for a suspended transfer slab, I assume the bottom pour would be supporting the wet weight of concrete from the top pour rather than making the formwork support both. So, i would think that changes things a little.