Placing a concrete beam under existing suspended slab

[Schneidly]

I have a question in another less active part of the forum.

http://www.eng-tips.com/viewthread.cfm?qid=274095

I was hoping to get some input on how to detail a pair of reinforcement beams for a 3ft x3ft saw cut opening in a 12" thick existing suspended slab. The existing slab has a 16ft span. The beam below will also span 16ft. The construction is required to be concrete.

How is the beam "poured" if it is cast directly under an existing slab?

What is the smallest practical size for a beam under these conditions?

If required, how often do core holes through the existing slab need to be? How big should they be?

Please see the other part of the forum for my sketch.

Thanks.

 

[msquared48]

I would suggest using a steel beam instead and afterset bolting to the concrete structure at each end. After in place, use a non-shrink grout between the top flange and the existing slab. Could also use vertical aftersets to stabalize the top flange to control the beam size.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[csd72]

If you do as msquared48 has suggested you can then concrete encase the beams in a non shrink mix. At least that way when it is not properly compacted then it is an aesthetic rather than a structural problem.

Just a few word of warning - be careful of how this is going to effect the moment in the adjacent spans and also about inducing a negative moment in regions where there may be no top steel.

here is a good reference:

http://www.cement.org/buildings/Openings in Two Way Slabs.pdf

 

[dhengr]

Are you going to try to tie the existing slab to the new conc. bms. with doweled stirrups of some sort? If a stainless stl. bm. would work, without stirrups, then why not a prestressed conc. bm.? Into pockets in the conc. walls, jack the precast bm. up to slab and grout the pockets.

 

[Schneidly]

Thank you csd72. I will look into that. Do I reinforce the non shink grout? The beam will be 12" deep, I have another 2" of available depth for grout.

Dhegr-How big of pockets would be required for a precast beam?

 

[hokie66]

Schneidly,
The existing slab, scaling the long dimension from your drawing as 24', behaves as a two way slab. I would be concerned about the new reinforcing beams changing this behavior and placing too much load over the 9' opening in the exterior wall. Do you know how that wall opening is spanned?

The slab is lightly reinforced even for the original design loading if all 4 walls are simple supports, leading me to think that it must be continuous to the right and bottom of your plan, in which case the moments at the point of your opening may be low enough to allow the opening without reinforcement.

 

[Schneidly]

Hokie66

You are 100% correct. The floor does continue to the right and bottom of the plan. I was going to check the 9 ft opening. The wall extends above the slab and the top of the opening is 3ft below the top of the existing slab. I have calculations for the building and there are none for the slab. But it does just works if you consider the back span. The back span is 11ft wall face to wall face.

The up down dimension is 23'-9" inside/inside.

You say I can get this to work considering the back span. How? My calculations showed it just worked under orignal design loads.

 

[hokie66]

I just said you MAY be able to get it to work. Don't just consider this as a one-way slab, as the other direction assists. Using bending moment coefficients for rectangular slabs supported on four sides, aspect ratio 1.5, with two adjacent edges discontinuous, the midspan positive bending moment in the short direction would be .058 wl^2. Could be worth inputting the whole thing into a concrete slab analysis program.

 

[hokie66]

Another thought...if you can't justify the slab by analysis, this would seem an ideal application for reinforcement with carbon fibre strips. As your only problem is lack of flexural reinforcement, this method should work.

Does the whole slab need to be upgraded to 300 psf, not just the strip with the hole? If so, the top reinforcement across the walls may not be adequate either.

 

[csd72]

Another option is to reinforce the region around the opening with carbon fibres.

 

[Schneidly]

I am modeling the slab in SAFE. I have not used the software before. It might take sometime.I will see if the slab will work without reinforcing.

Where do I find information on carbon fibres? Are they subject to corrosion? I am not using a W-section because of corrosive environment.

 

[rowingengineer]

Sika is the local people around here; they produce carbodur and other products worth a viewing. Carbon fibres are not subject to the same corrosion mechanisms as steel (mostly sunlight is the problem, but a few other minor issues), so worth a viewing on that front as well.

An expert is a man who has made all the mistakes which can be made in a very narrow field