Reinforcing existing suspended floor for new equipment

[GalileoG]

I have an existing suspended 12” reinforced concrete ground floor slab (basement below) in an industrial building. This slab will need to support new equipment that will apply a uniform pressure on the floor exceeding the original floor design load of 250psf. These are large equipment with weights exceeding 50,000 lbs.

I am evaluating several options for reinforcing the existing floor. One option is to build a “drop panel” under the existing ground floor with a plan size to match the new equipment. The “drop panel” would be a 12” thick slab supported on steel beams that are supported on steel posts. The steel posts would be supported by micropiles at basement level.

The slab would be tight against the existing slab. There would be ties connecting the existing slab to the new slab and the surface between the two slabs would be roughened and a bonding agent would be applied. The effects of stress redistribution on the existing slab and girders would be considered.

Is this a practical approach to reinforcing the existing floor? What are some constructability issues that may arise? Any alternatives?

 

[Teguci]

Make sure you get a cost for complete demo and replacement. This seems like a "start over" project. External post tensioning would be an elegant solution for the right circumstance, but you said industrial building, so I think you should stay away from elegant. For 350 psf and greater I think I'd lean towards steel beams and plate and leave the concrete out of it.

 

[GalileoG]

Equipment pressure exceeds 350 psf, how would you treat the surface between the steel plate and the existing concrete slab?

 

[jayrod12]

What about just providing steel beams and posts under the current slab to increase it's capacity?

 

[andriver]

I second jayrods comment, transfer the load down assuming the basement does not have equipment in the way.

 

[Teguci]

If possible, I would not address the underside of an existing concrete slab. Overhead welding is bad enough. Having someone operate a 15 lb pneumatic hammer overhead is fairly cruel and appropriately cost prohibitive. Use the 12" slab for 1-way shear and to carry it's own weight. For the rest of the load, provide a steel support system (plates and beams), tight to the underside of the concrete deck.

 

[GalileoG]

Thanks for the feedback. Are you suggesting a flat plate for the entire footprint of the equipment? How would you ensure there is uniform contact between the plate and the underside of the existing slab?

 

[Teguci]

Is full contact a design requirement? Can you let the existing slab deflect down a bit? How big of an area are we talking about? Can you remove the existing equipment (and load) before installing the new support system?

You can assure some load transfer by jacking the new steel system up into the existing slab. Further contact can be obtained by using short, simply supported spans for the new basement steel system. Does the existing slab have a decent amount of top reinforcement? If yes, you can jack the new supports up and put negative bending on the old slab. Trying to get a new system to uniformly support an old system is a balancing act that I'd shy away from. Full demo should still be on the table.

 

[hokie66]

GalileoG,

I don't think anyone is suggesting a steel plate under the entire footprint. But if you install new columns under the equipment, and possibly steel beams between these columns, I think it probable that a 12" slab originally designed for 250 PSF live load would be capable of distributing the loads to the new structural steel.

But have you actually checked the capacity of the slab for the new loading? If your slab is designed for 250 PSF, and you have some new areas above 350 PSF, that does not automatically mean the slab is not strong enough. It depends on how much of the area is loaded, and would require new analysis to determine adequacy or otherwise.