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- To: <seaint(--nospam--at)seaint.org>
- Subject: concrete floor strengthening
- From: "Andrew Kester, PE" <akester(--nospam--at)cfl.rr.com>
- Date: Tue, 27 May 2008 13:36:31 -0400
This subject was just brought up, and I need some opinions...
I am nearly done with the design of a project where the client is adding a Nuclear Camera to an existing (1937) reinforced concrete structure.
This is a hospital with a PE on staff who is reviewing my work, including calcs.
The existing floor slab is composed of a T-beam/ ribbed slab type system. The drawings detail the beam portion very well but limited info on the slab. Liberal calcs showed this existing slab could not support the new equipment.
We placed steel beams under the floor slab (between the ribbed beams), and they were designed to take the ENTIRE load of the equipment, patient, and operator, plus misc. dead load. All of the equipment will rest on a new steel floor plate, that rests on steel shim plates placed directly over the wide flange steel beams below. The load is thus bridged over the slab and beam and onto the shim plates. We have some connections for uplift of the equipment from the top flange into the U/S of the slab, but we did not design continuous shear connections to the slab so that we have composite action. The end connections are web welded plates, welded to end plates, epoxy bolted into the existing concrete girders. A layer of grout will be placed between the slab and the wide flange beam.
A majority of the dead load will be removed during installation of the steel except of course for the weight of the slab system. Under worse case loading of one particular steel beam (the worse case beam), the deflection of the new steel beam is approximately that of the existing system, as best as that could be estimated. I did this before at the same hospital a few years ago and it worked fine, and have used this system to shore up concrete floors before with success.
The reviewer is busting my chops about the deflection and stiffness compatibility of the two systems, exstg concrete vs steel. I understand completely where he is coming from, but who knows exactly what the concrete slab/beam system will deflect, or how to calculate its Icr accurately? Or strain in the rebar? How much deflection has already occurred since 1937? Exact fixidity of the end connections? It is my opinion, and I am sure I will get corrected, that since the steel shim plates are directly placed over the steel beams the load will "mostly" go into the beams. Sure some stiffness of the slab system will pass some load into the concrete beams, but not enough to cause any "harm". (Using quotes because I know this is not tech talk.)
To upsize the steel beams to assure that the EI steel > EI concrete would make the shapes quite large and heavy and cause all types of installation problems.
My only other logical option I am considering is composite slab design, but that seems so unnecessary. Plus would we not have to jack up the beams to push up into the slab to get full composite action, or would that just be necessary if we wanted dead load composite action? I could make LL only composite action work and then assume floor slab system carries its own dead load by itself.
Please CC me directly.
Thanks in advance,
Andrew Kester, P.E.
ADK Structural Engineering, PLLC
1510 E. Colonial Ave., Suite 301
Orlando, FL 32803
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