Demolishing/re-pouring portion of existing suspended concrete slab

[GalileoG]

I have an existing 12” thick suspended reinforced concrete slab. For functional reasons, I am required to step the top of the slab down by almost 22”. The step is 8’x8’ in plan. The tricky part is that there is a 22x48” concrete beam that runs right at the center of where the step would be. The idea initially was to demolish the portion of the existing slab where the step is required, epoxy into remaining existing slab at the perimeter and re-pour a thickened slab. However, with the concrete beam that runs through the step, I am completely stomped on how to deal with that. Any ideas?

 

[JAE]

Is this a two-way flat slab? One way slab? Is the beam part of the overall slab system which supports the slab? Perhaps a sketch.

 

[GalileoG]

JAE,

It is a one way slab system. The beam that runs through ‘the drop’ supports the one-way slab that runs perpendicular to it. Hope this helps.

 

[msquared48]

If you modify the beam section by cutting the slab, essentially cutting the top of the beam away, you will have to reinforce the beam to a haunched condition.

With the beam depth problem, how will this affect the headroom below?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[GalileoG]

There are no concerns with headroom below. What exactly do you mean by ‘a haunched condition’? How do I ensure continuity between the existing and the new beam?

 

[hokie66]

With no headroom problems, I suggest you should install a new beam at the lower level to span the full distance between columns. Maybe a steel beam would work best. Stepping a beam is hard enough in new construction without trying to do it as a retrofit.

 

[GalileoG]

Thanks Hookie,

Since the depression is less than the depth of the beam, I was thinking of not removing the existing concrete beam entirely, but to just add a new steel beam below it that would be epoxy-anchored to the columns at each end (carefully cutting through rebar in the columns.) Any reason why fasteners would be required between the new steel beam and the concrete beam? As long as I design the steel beam for non-composite action and unbraced for the whole span, I can just have the top of the steel beam flush with the underside of the concrete beam without any sort of fasteners. Do you agree?

 

[hokie66]

Basically, yes. But I would still want some nominal fastening, and I would want to use wedges to preload the new steel beam, and then grout the gap.