Achieving Composite Behaviour 1

[GalileoG]

I have a grade beam with dowels sticking out on the top surface of grade beam and are spaced every 48 inches. The slab on grade will be poured over the grade beams and will set on the dowels. How can I determine whether or not I will have composite action in such a way that my rectangular stress block would be on the slab on grade as opposed to the grade beam. Would the calcs/equations be identical to that of steel composite beams with concrete deck?

Also, the slab on grade has rebar at mid-depth. When the grade beam spans continuously above a pile, would my grade slab not crack quite a bit at that location? Do I not have to add extra rebar on the parameter of the slab on grade which is where my grade beams are?

Thanks all.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[BAretired]

A dowel spacing of 48" is likely inadequate to develop composite action between beam and slab. If stirrups extend up from the beam into the slab and the top surface of beam is roughened, composite action would develop. As you have described it, the slab may develop some composite action, but probably not enough to rely upon. I think you should treat the two as separate elements.

BA

 

[GalileoG]

Thanks BAretired,

I didn't think 48" c/c would achieve composite action either. But I'm curious as to how one can determine the spacing of dowels required in order to achieve composite action.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[BAretired]

If you provide stirrups which satisfy the code for a Tee beam and you provide a roughened surface at the top of the beam stem, you will achieve composite action. Otherwise, you will not.

BA

 

[oneintheeye]

i had a similar problem recently, I found the answer in BS8110 cl 5.4.7. Don't know if you have access, this basically gives a method how to determine the horizontal shear at the interface and how to design for it. Eurocode has something similar albeit more theoretical than 8110. It takes account the surface frictiona and the dowels/links etc. I realise your probably state side but you may get the answer. Any probs I might be able to post the relevent details here.
It does not behave like two steel beams as the distribution of shear is not through a uniform material.

 

[Lion06]

There is a composite section in ACI. Have you looked at that?

 

[Ltwine]

The prudent way to look into this:

If you need flexural strength from both, then do as BAretired mentioned - provide stirrups and top bars and treat the complete section as a beam. Now the slab is fixed on the support beam, and the combined section is L-shape edge beam.

If you simply wanted to prevent separation of the two, then the dowells will do. However, the moment capacity along esge of slab is difficult to evaluate/determine, thus, it (the slab) should be treated as pinned on the support beam.

 

[oneintheeye]

apply the max vertical shear at the support as a horizontal shear at the interface and design the dowels for that. That is conservative.

 

[OCI]

ACI318 sec 11.7 provides a method to check the shear strength at this interface.

 

[hokie66]

A lightly reinforced slab on grade is not a structural element, and I wouldn't try to make it work as such. When you tie the slab to the grade beam, the slab will develop shrinkage cracks across the grade beam, thus reducing the slab's ability to act in compression.

 

[JAE]

Hey all - note that the "dowels" coming up from below must be fully developed per ACI shear friction provisions (11.7).

What that means is, a short little dowel isn't going to cut it. For #4 bars in 3000 psi concrete, you'd need 22" of development length. For a hook, you'd need a hook length of 8". With 3/4" cover you'd essentially need to hook the bars and use a 9" thick slab.

You cannot use As(req'd)/As(provided) to reduce this per ACI 11.7. So composite action technically isn't possible without the thick slab and the development length established.

Factually, there will be some composite action developed, but it could not be "calculated" per ACI or shown to work without specific testing.

 

[OCI]

Good call JAE.

 

[abusementpark]

Why can't you simply treat your dowels like shear studs in a composite steel section?

 

[BAretired]

Shear studs can only function because they have a head which prevents slippage. See the comment by JAE above.

BA

 

[asixth]

You want to concrete of the grade beam to have a rough finish as well to further help composite action. The Australian concrete code has been previously revised so now the horizontal shear checks are based on stresses and not forces which makes the calculations much easier to follow. I will scan and post.

 

[GalileoG]

Thank you all.

asixth, I would be interested to see how the Australian concrete code deals with this. Eagerly waiting for the scan.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[asixth]

Australian provisions attached (all units are SI). As JAE mentioned above, the dowel/shear reinforced needs to be anchored to develop it's full strength at the shear plane.

 

[asixth]

You could do something like this if you are trying to engaged a topping to behave compositely.