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RE: flat plate strengthening

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I had a similar experience on a shopping center.  We increased the LL from 40 psf to 100 psf.  We added wide flange beams at the column lines to pick up the beam load.  The slab was OK as it existed. 
 
We anchored the beams at the columns and suspended the end of the steel beam from anchor rods to the slab above.  The beams were preloaded with load cells and hydraulic rams so that we knew how much load we were preloading into the steel beam.  We then welded in WT's between the bottom of the slab and the top of the wide flange beam that locked the preload into the beam and transmitted horizontal shear to make the beam composite.  Thus the dead load was picked up by the wide flange steel beam and the live load was then shared in the composite section.  It was then load tested.  The anticipated deflections were calculated and it performed pretty close to the calcs. 
 
That was done in 1982.  The shopping mall is doing just fine. 
 
I like FRP, but you have to waive fire resistance.  I am not aware of any protection on FRP to get a fire rating.  I can use a lot of different materials to protect structural steel.  And, obviously, concrete is pretty good in fire resistance.

Regards,
Harold Sprague




From: akester(--nospam--at)cfl.rr.com
To: seaint(--nospam--at)seaint.org
Subject: flat plate strengthening
Date: Wed, 16 Jul 2008 11:40:47 -0400


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


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