Does this Constitute Composite action?

[jschwey]

Hi All,

The situation I am working with is looking at a partially encased steel beam in concrete. The beams have web stiffeners at points where connections are being made for the large loads that are applied, and are periodically placed along the beam length. There is rebar running through the beams as well with a portion of the steel beam exposed. There are no Headed studs used in the current design, ( I am newer to the company and am tasked with looking at r&d in these beams.)

The main question I am battling in my head here is if this beam set up constitutes a composite beam action or not? I am familiar with typical composite floor design, but this situation is a bit more then I have been exposed to before. Just looking for thoughts and if there is something I am missing when looking through the codes that would point to something.

 

[dik]

Can you attach a sketch showing the elevation and x-section? The 'roughness' can be designed for composite action, but, my preference is to use headed studs. They are manufactured for this purpose, and tend to attach the concrete to the beam and keep it from 'spalling' off.

Dik

 

[jschwey]

That was what my thoughts are is to make the beam act compositely by adding headed studs to the webs of the beams or to the flange.

jschwey

 

[BridgeSmith]

If your sketch is roughly to scale, I would question the necessity for the steel beam at all. Why not just get the capacity using longitudinal reinforcing bars? With the beam buried that deep in the concrete, the gain due to composite action is fairly small. You'd do far better with a shorter beam, since it would bring the top flange further away from the compression zone, increasing the moment of inertia. The most efficient composite sections have the steel beam completely outside the concrete.

 

[jschwey]

The sketch is not to scale, due to the nature of the R&D project, the amount of concrete cover and size of beam are being looked at as a whole. I agree that the beam should have sufficient cover over the top flange to increase the moment of inertia.

The beams are loaded by a hanging load with large loads in all X/Y/Z directions, so the main reasoning to having the beams is the strength of material to then transfer the loading through a roof into the walls. the bigger question is the current design uses web stiffeners at attachment points around the bolt location, and no headed studs for composite action.

My thoughts are the beam in this situation is meant to be designed to carry the load and then transferred into the concrete roof which is then designed around the force from the beams and the forces of nature/self weight. This to me does not make a composite beam action considering there is not a stud, but I have not seen anything other than typical composite floor design in my past positions.

 

[BridgeSmith]

There are alot of unknowns with regard to composite action with that configuration. First there's the transfer of stress from the steel to the concrete by the rebar through the girder. Presumably the hole in the girder will be larger than the rebar by a bit, which may allow some relative movement before engaging (even if concrete gets into the annular space around the bars, is it strong enough?). Then there's the question mark regarding the transfer through the concrete from the location of the rebar to the compression zone towards the top. Honestly, a short, stocky beam with shear studs on the top flange, with only the studs embedded in the concrete, would add more to the capacity and simplify the construction (no holes for rebar needed). Those automatically end-welded studs are very common and very reliable. They also come at a very low cost; likely cheaper than the labor to put holes in the web and get the rebar placed through them. We do it for 4 or 5 bars at our integral abutments, and the contractors complain about having to just do those few.

 

[jschwey]

Thank you for the thoughts HotRod. I honestly would agree with you, I have a hard time saying that this is actually acting compositely with what I have to work with. Some of the higher ups are saying we have to include the concrete into the design, which to me means this needs to be looked at compositely.

In that I have some ideas, but looking at one of the configurations that has been done in the past (similar to the sketch) almost looks like the steel beam can take the load directly. Then the concrete is encasing the beam to hold the beam in place and to transfer the load to the walls and ultimately to the foundation.

Working with unknowns is never fun, and hate to make an assumption on anything that could jeopardize safety. If anyone has any more thoughts on this would love to keep hearing them, and discussing further.

 

[Agent666]

Yes you can get composite it's very similar to the slimflor system out of the UK which uses asymmetric beams, they achieve composite action by embedding the top of the section within a slab with no studs. It's been tested extensively with design procedures developed, so read up on it as a start to understand how you might achieve similar levels of composite action.

 

[BridgeSmith]

From the slimflor brochure linked: "Slimflor beams can be noncomposite
or composite with the inclusion of a shear stud..."

The beams are fully encased in concrete, but not considered a composite system unless they have shear studs.

It seems that the concrete could be reinforced sufficiently to be the load carrying member, with a small beam attached or embedded as the attachment point/hanger.

 

[271828]

See Section I3.3 of the AISC Specification.

The tricky part is that the shear transfer between the slab and beam is not ductile as with a series of shear studs.

 

[Agent666]

The beams are fully encased in concrete, but not considered a composite system unless they have shear studs.

Its coming back to me (probably should have read what I linked to), those rolled asymmetric beams (ASB's) used to have a pattern rolled into them that gave you a dependable bond along the beam when they were encased, without studs you used to be able to take a triangular distribution of shear bond over the shear span with 0.6N/mm^2 at the center of the beam.

So yeah, with a smooth beam you probably don't get any dependable bond for ultimate conditions.

Also check the UK SCI design guides P175 & P342 as it might give some advice on whether the bars passing through the section can be relied on. But flicking through P342 it does mention that for ultimate composite action cannot be relied on, but it also notes that it seems reasonable for serviceability that some limited composite action can be relied on provided there is a certain level of reinforcement passing through the section even with the top flange virtually out of the concrete (check section 4.3).
P342

 

[BridgeSmith]

"...have a pattern rolled into them that gave you a dependable bond along the beam when they were encased,"

If the beam is notched or otherwise deformed so that sufficient interlock is provided with the concrete, it could be considered composite...I don't believe it would ever be more cost-effective than adding studs, but if there's no room for studs, as in some of the slimflor systems, it's an option.