huangyhg

rc slab strengthening
please advice on the best way to detail the connection of a steel beam to an existing rc slab. the slab is about 400mm thick and the steel beam is used to strengthen the slab and reduce loads to other steel beams within the frame. my main worry is how to ensure that the new beam picks up the slab load and suitable connection details.
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if you only need to ensure some bigger limit states load, drilled connectors tying such thick slab to the beam can be enough,
if, as it seems, you want the newly installed structure to partake in some predictable amount the current load, and of course its share of anything later loading on the whole structure, use jack ring cushions to transfer the wanted loads to your beam, then lock the loads by concreting.
in this process, inflatable jacks having the shape of blood cells would be put at a number of positions, say, 3 of them at quarters of the span, or at fifths etc, and are inflated, the pressures giving exact readings of the loads at the moment being transferred to your new structure, that acts as support of your jacks. when the readings indicate the loads that you want, by similarity of the diagram of moments scheme to what wanted, you can lock the loads concreting the space between the top flange of your beam and the slab with small shrinkage mortar, then after the mortar gaining strength, deinflate the jacks and complete the infill of mortar at the jacks position. by then you have locked standing loads in a known amount.
ishvaag,
can you be a bit clearer. i need more suggestions.
i would install the beam to within an inch of the underside of the slab. drive steel wedges at 1/4 points tight to the slab and then tack weld them to beam so they will not work loose. continuously dry pack with non-shrink grout between slab and beam.
if you anticipate that the slab could possibly lift off the beam (due to loading or some other affect) then i would recommend that some fasteners be installed from the beam into the slab.
ishvaags suggestion is similar to something we did recently
two steel dowels were installed between the beam and slab near the centre of the beam, the ends of the beam were then jacked up to predeflect the beam, then the columns under the ends installed after. then the remaining gaps were grout packed.
the slab was holding brick walls above and the steel beam was installed to allow removal of a brick wall under. predeflection was important to stop the brick walls over cracking.
for a heavy slab like the one in the op, ishvaaag's way is best, as it allows the best control of the preload placed on the beam. the jacks he described are normally called "flat jacks".
for small beams, they can simply be wedged, with the deflections measured and correlated to the preload.
the method chosen, shim tight or preloaded, should reflect your analysis. in other words, if the existing beams carry all the dead load and share the live load only witht the new beams, then i would use the "shim tight" method. however, if your intention is to have all beams (existing and new) share dead and live load then use the "preloaded" method.

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