Pilaster minimum reinforcement

[tmalik3156]

Good day all.
Sketch below is a pilaster of a wall. It's cross section is made of a steel section, and a reinforced concrete cover (C-shaped). The void where the steel is placed is grouted so that there is a composite action between the steel and the concrete.

A pilaster does not carry any significant vertical load, so it's not treated as a column. But it carries a significant horizontal wind load. The steel section is designed to carry that horizontal force alone. However, composite action reduces the horizontal deflection, so the concrete can not be treated merely as a cover.

What minimum reinforcement (shown in green) should we place in the concrete cover? Some people say just use 0.2% for temperature and shrinkage. But I think 0.2% is for slabs, not for a pilaster.

Thank you for your comments / suggestions.

 

[EngDM]

For concrete columns you can go as low as 0.005 * Ag (in accordance with CSA) provided you reduce your Mr and Pr accordingly. Since the Mr is largely from the steel, you might be able to do this here.

 

[Greenalleycat]

I don't agree with the composite assumption. Composite action relies on vertical shear transfer between the steel and concrete interface...this is slick and there will be very little friction. You need welded studs or similar to be making that assumption.


I would be placing 4/ 10 or 12mm bars (1 each corner) and stirruping at say 300-400crs to provide a basic level of confinement
This isn't really based on numbers - just what feels right to me

 

[tmalik3156]

@Greenalleycat Thank you for pointing out the composite action mechanism. Indeed, without some connectors (like Nelson studs) between steel and concrete, we can not rely on a composite action by friction only. That's why, the W shape is designed to carry the force and moment alone. However, W shape is very flexible, and deflects a lot if it all by itself. So, we are providing the grout and concrete confinement to reduce the deflection.

 

[Greenalleycat]

@

@Greenalleycat Thank you for pointing out the composite action mechanism. Indeed, without some connectors (like Nelson studs) between steel and concrete, we can not rely on a composite action by friction only. That's why, the W shape is designed to carry the force and moment alone. However, W shape is very flexible, and deflects a lot if it all by itself. So, we are providing the grout and concrete confinement to reduce the deflection.

Your logic is inconsistent.
Currently, it sounds like you're wanting to avoid designing the concrete as a column but also want to rely on it as a structural element - you can't have your cake and eat it too

There are two ways that the concrete encasement can stiffen the W steel beam

1/ By composite action - this would require welded studs or similar, and you'd need to design the concrete surrounding to take this load
You've said that you aren't doing that

2/ By load distribution proportional to stiffness
In this case, you have a steel column and a separate concrete column
You therefore need to design (aka reinforce) the column to take the proportional load
So, this is what will set your reinforcement design, not some arbitrary shrinkage amount

 

[KootK]

For encased beam elements, which this is, AISC allows one to assume fully composite action with the compression side concrete without any positive shear connection such as studs etc. Since this is considered valid for strength purposes, I would certainly consider it at least as valid for deflection.

This paper discusses this as well as recommendations for the rebar in the encasing concrete: Paper When last I checked, this method of design still existed in the AISC manual and specification.

 

[Greenalleycat]

For encased beam elements, which this is, AISC allows one to assume fully composite action with the compression side concrete without any positive shear connection such as studs etc. Since this is considered valid for strength purposes, I would certainly consider it at least as valid for deflection.

This paper discusses this as well as recommendations for the rebar in the encasing concrete: Paper When last I checked, this method of design still existed in the AISC manual and specification.

View attachment 14540

Interesting article, thanks for that

 

[KootK]

The reversible nature OP"s load does give me pause. I can't really think of a good reason not to use some kind of nominal shear connection.

The only reason I know od this alternate procedure is because I was forced to spreadsheetize it in grad school. I remember thinking it was pretty crazy at the time. I have seen it in the wild in historic structures though.

 

[jhnblgr]

Agreed that shear studs aren’t required but due to the nature of the construction joints to the wall units the design of t&s steel would be difficult to install without something to attach to. Real simple solution is to use weldable rebar.