Placing a concrete beam under existing suspended slab
Placing a concrete beam under existing suspended slab
(OP)
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.
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.






RE: Placing a concrete beam under existing suspended slab
Mike McCann
MMC Engineering
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Motivation: Don't ask
RE: Placing a concrete beam under existing suspended slab
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://ww
RE: Placing a concrete beam under existing suspended slab
RE: Placing a concrete beam under existing suspended slab
Dhegr-How big of pockets would be required for a precast beam?
RE: Placing a concrete beam under existing suspended slab
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.
RE: Placing a concrete beam under existing suspended slab
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.
RE: Placing a concrete beam under existing suspended slab
RE: Placing a concrete beam under existing suspended slab
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.
RE: Placing a concrete beam under existing suspended slab
RE: Placing a concrete beam under existing suspended slab
Where do I find information on carbon fibres? Are they subject to corrosion? I am not using a W-section because of corrosive environment.
RE: Placing a concrete beam under existing suspended slab
An expert is a man who has made all the mistakes which can be made in a very narrow field