Increasing Moment Resistance on a Slab During Demolition

[DougP100]

I am primarily a Geotechnical engineer. God I feel so stupid because it seems like there has to be a simple answer, but it's been years since I had structural analysis and I don't have time to look this up.

My company is a large contractor that recently took on a major demolition job, a hospital built in the 1950's. Part of the building to be demolished has a one-way concrete slab. To make a long story as short as possible, I analyzed the slab using data presented on the extensive as-built drawings we have been provided using both WSD and USD methods. I have determined that it is too light to take the load of the machine we want to use to do the demolition. We can use smaller machines, other methods etc. but lets just stick with this situation and assume we have to use this machine.

Question: If we were to place 1/2" steel plates over the entire floor between the supporting members without actually attaching them to the slab, can we simply add the moment resistance of the plates to the moment resistance of the floor to increase the moment carrying capacity of the slab - ergo if the floor slab has a maximum moment capacity of 10 Kip-Ft, and the plate has a maximum moment capacity of 2 Kip-Ft, can we say the combination would carry 12 Kip-Ft? I feel embarrassed to ask this, so be easy on me

 

[JedClampett]

It's a little more complicated than that. You'll have to assure that the deflections are compatible. In other words, if it takes 1/8 in. of deflection for the concrete floor to carry its 10 ft-kip capacity but it takes 1/2 in. for the plate to reach its 2 ft-kip capacity, the plate will never deflect enough to help. Think of two springs in parallel. Also, you're adding quite a bit of load (~20 psf) to add strength.
Is there a reason you don't want to add shoring underneath the existing slab?

 

[DougP100]

Thanks for your response! I'll approach it from that angle.

 

[Philitup]

Adding the plate to the slab would only increase the dead load to the one way slab in my opinion. there is no composite action which can be assummed.

Shoring from below to decrease the one way slab span seems like a better angle.