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Re: flat plate strengthening

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Andrew,
 
        I just did something similar with a one way slab.  In my case I had a building with the basement extended out under the city sidewalk by about eight feet for the entire 130 feet length of the building.  The structure consisted of concrete encased steel beams at various spacing (about eight feet) with the slab designed as simple spans spanning parallel to the building.  Some of the steel beams had corroded to the point where failure was imminent; and, in my opinion, the reinforcing was marginal (to say the least) for the spans considering possible load which might accidentally be applied.
 
        My solution was to widen the beams by up to two feet on each side of the existing beams using reinforced concrete (also roughen the underside of the slab where required).  This a.) reduced the slab spans, and b.) provided new beams which were themselves adequate to carry an H20 truck wheel load.  I used a very liquid, shrinkage compensated concrete mix (about 4,000 p.s.i. plus high early strength) that could be pumped through 4" holes drilled through the slab.  The slab has since been subjected to a crane pad and a fire truck loading without problems.
 
        Hope this helps.
 
Regards,
 
H. Daryl Richardson
----- Original Message -----
To: seaint
Sent: Wednesday, July 16, 2008 9:40 AM
Subject: flat plate strengthening

Does anyone have any advice on methods of strengthening a flat plate slab with drop panels in an existing hospital? We cannot likely remove any live load except from a very limited area. It may be possible to go underneath and place steel or additional concrete. I am dealing with spans of about 26ft and 20ft.
 
The slab is not working in negative bending at a column strip over the column (not enough top bars), punching shear is not a problem. It would work if there was more rebar in the slab initially... The building was built in 1960, we do have some of the original drawings and are working on getting additional drawings. We are adding about 9000lbs total of medical equipment to a room that works out to about 70psf, but the capacity we are getting thru calcs is about 50psf. I know that uniform load does not equal the effects of point loads, this is just the first step and quite a bit off the required capacity.
 
Though I have never done it before, if you could get a majority of the LL off of the slab, could you maybe cut the slab and epoxy/grout in rebar, or use carbon sheets for additional negative reinforcement? I am thinking the problem in our application would be that it would be necessary to add this outside of the room, out into the hallway...
 
Thanks in advance, please CC me directly.
Andrew
 
Andrew Kester, P.E.
Principal/Project Manager
ADK Structural Engineering, PLLC
1510 E. Colonial Drive, Suite 301
Orlando, FL 32803