Placing concrete slab over an existing Roof 1

[Dijin]

Hi,

Please guide me about the adverse effects of placing a slab on existing Roof Arrangement . Both top(250mm thk) & bottom (400mm thk) slabs are having 16 dia rebars @150 mm spacing. This modified slab will be used for crane access (operating load 300tons). The crane is of All terrain type with Outriggers.A bonding agent Concresive 1414 or equivalent shall be applied at the interface of two surfaces.

 

[hokie66]

"Adverse effects"? A 300 ton mobile crane is a big thing, and one adverse affect would be for the crane to fall through the roof.

Seriously, your structural engineer should be evaluating this design. An internet forum is good for ideas, but not for evaluating a structural design sketchily described.

 

[Ingenuity]

Agree with hokie66...and with a picture worth a thousand words, this is what happened to a 250t crane on a suspended slab that was designed for the weight for the crane traversing but the shoring for the outriggers was not placed in the correct location.

 

[msquared48]

Well... the crane WAS oversize.

Proof that outrigger loads are not to be ignored.

Mike McCann, PE, SE (WA)

 

[KootK]

Let's see:

1) Your existing slab will need to be able to handle the wet weight of the new slab in terms of both flexure and punching shear. Of course, a 400 thick slab is pretty darn sturdy.

2) If you need the two slabs to act compositely in order to make a go of things, take great care to convince yourself that what you're constructing will truly behave as a composite slab, for both bending and punching shear.

3) Obviously, columns and footings need to be able to handle the new load.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Dijin]

Actually there are beams in transverse direction in every 4 meters of size 600mm x 1800mm and end walls are 13 meters apart. The axle load of crane is 12 tonnes (6 no.s axles)and the lifting weight is 23 tonnes.

 

[hokie66]

Dijin,

That is still well short of the information needed by your structural engineer to assess the proposed structure.

 

[Dijin]

Thanks for the info hokie, would you mind suggesting how this scenario has to be approached ?
Basically we are checking span moment, shear & support moment, shear due to the self weight of crane, counter weight & lifting weight. Dead load of concrete & a live load of 13.5 kN/m2 is considered apart from this. 11m x 13m is the overall size of slab.

 

[dcarr82775]

Dijin,

The outrigger load is the load that matters, not so much the total weight of the crane and load being picked. You can estimate the outrigger load, or better yet get it from the crane provider.

 

[jike]

I would prefer to use vertical dowels to transfer shear between slabs rather than bonding agent. This type of bonding agent can be very sensitive to application time and can actually become a BOND BREAKER, if not applied in time.