Concrete Between Steel Beams 1

[VBI]

I am investigating a 1-story structure that has 3 bays with (3) W12x22 beams runing 250" long 6'O.C.. 12" thick concrete is located BETWEEN the beams. When I analyze the steel, it is insufficient to support the concrete as a dead load + live loads and I believe that the concrete is actually working as a beam (self sufficient) in the long direction.

Can anybody offer any advice as to how I would make this determination? I am trying to show that additional dead load could be hung from the underside of the steel beams but it is not possible if the beams are supporting the dead load of the concrete placed between them.

THANK YOU for any help that could be provided...

 

[DaveAtkins]

If the concrete is reinforced, I think it would be much stiffer than the three beams, and therefore, would carry most of the load. To be sure, check the ratio between the EI of the three steel beams and the EI of the two concrete slabs. Then, make sure the beams and the concrete slabs can carry the load assigned to each.

DaveAtkins

 

[graybeach]

Do you know which way the reinforcement spans in the concrete? Do you know if the concrete was shored when it was poured?

 

[VBI]

I am sorry but I don't know the direction of reinforcement. Should I analyze this slab as an minimally reinforced 20.83'(250") slab?

Thanks again

 

[DaveAtkins]

I guess you could assume temperature and shrinkage steel in the long direction. Is there support (like a continuous concrete or CMU wall with adequate footing) for each end of each concrete slab?

DaveAtkins

 

[VBI]

There is a rubble foundation wall that the concrete and the beam sits on.

 

[msquared48]

You mentioned that this is a one story structure. At what level is this concrete and steel beam platform - a mezzanine?

Do you know anything about the local history of the building - what it was used for? Any record or skinny of why this construction type was used? Machinery support?

Any indication of burn marks / discoloration on the exposed side of the steel beams that might indicate rebar being welded to the beam?

Mike McCann
McCann Engineering

 

[miecz]

I'm having trouble picturing this. Is the 12" slab flush with the top and bottom of the 12" beams? How do you know the beams are W12x22?

 

[msquared48]

That's what I assumed from the post is that the slab isflush with the top and bottom of the steel section.

In that the side of the two outside beams can be observed and measured, I would have to assume the dimensions measured fit that of a 12X22.

Mike McCann
McCann Engineering

 

[VBI]

Yes exactly. The structure is a messanine.The concrete is flush with the top an bottom of the beam and can be observed (measured) at each end.

I guess the question boils down to are the 2 elements(concrete and steel) working alone or in composition to some extent. Or that the steel is "doing all the work" which is what I am trying to prove is not the case.

Thanks all for your interest and response.

 

[miecz]

With no more information, I think you can invoke IBC Section 3403.2, and safely add 5% of the existing load without calculation. Question is: 5% of what? I would take the minimum of 5% of the beam capacity, and 5% of the existing dead load. If your additional load exceeds that, then you need to find out how the slab is reinforced.

 

[graybeach]

Unless you know that the slab forms were shored, I think you have to assume that the beams carry the full dead load of the concrete. For additional loads, I think you could assume the concrete in compression contributes to the capacity of the system.

 

[VBI]

When you say shored, what exactly are you referring to? That is, I can presume they were formed from the underside and shored during concrete placement? How else could they have been poured in place unless you mean something different.

 

[FoMoCoMoFo]

would measuring the deflection in the beams provide any clue as to what the concrete is doing and how it was placed ?

 

[graybeach]

VB1 - If the forms for the concrete were supported on the beams, and no temporary falsework was placed under the beams or forms during pouring and curing, the beams carry 100% of the concrete dead load.

 

[haynewp]

Did you measure the floor deflection?

 

[graybeach]

I should also add that if there was shoring, the concrete dead load would be carried by the steel and concrete compositely - depending on if the details can achieve strain compatibility.

 

[VBI]

I did not realize that about shoring the concrete as opposed to using the bottom flanges as supports for the forms. Thank you.

No deflection measurements were taken. I guess I could work backward if I measured deflection and determine how much load can be applied to the steel beam.

 

[graybeach]

I think the deflection would only tell if the structure acts compositely for live load.

 

[haynewp]

If the steel beam deflection under self weight of the floor only was say 3/4 inch (which I doubt it was designed this way), I would say the beam is carrying the slab self weight. Even if the live load is shared between the slab and beam, the additional live load deflection may overstress beam and cause a plastic hinge at midspan if the beam is actually carrying the slab weight. Do you have any pictures of the mezzanine?