Cast-In-Place Slab Turned Into Diaphagm

[mateogrin]

I am working on a renovation of a building built in the 60s. It is 3 stories high, with a 3" concrete deck that has a very thin steel deck underneath. This is in the Boston area. I have a very limited structural plan set

As far as I can tell, the steel deck is only there to form the concrete. It does not have any weld patterns that suggest that it was designed to be a diaphragm, nor are there studs that suggest that it is a composite deck.

The building is undergoing a major renovation and needs to be brought up to comply with current wind and seismic loads. There appears to be no cross bracing to deal with either load cases in the structure. I am looking to see if I can use the concrete deck as a diaphragm to help resist the wind and seismic loads that the building now needs to be designed for.

I've thought of a few various options such as overhead welding of the deck to the joists, or mechanical fastening of the deck to the joists. Neither technique has been thoroughly enough tested to comply with any manuals out there, so they are out of the option.

The client does not want to replace the slab, nor do they want to pay to core the slab and create diaphragm welds from above.

Does anyone have any ideas of how to turn this slab system into a diaphragm? Also, if anyone can point me towards any manuals that discuss the use of concrete slabs as a diaphragm, that would be useful as well.

Thanks!

 

[hippo11]

How about horizontal X-braces laced through the floor framing, maybe one X-brace per bay woven through your joists (if you can pass the kl/r<300 limit for tension members suggested by AISC?

 

[mateogrin]

We've thought about this. The issue is that we still need to find a way to tie the slab to the joists and the beams, because right now, it is just resting on the joists.

 

[hippo11]

If you have the horizontal X-bracing transmitting lateral loads to your shear walls/vertical bracing/moment frames, etc., then you aren't using the concrete slab for diaphragm action, so therefore you wouldn't need to worry about the slab-to-joist/beam connection, right?

At least not for lateral resistance...are you saying that you want to look at the slab-to-joist/beam connection for gravity reasons?

 

[ataman]

Is it possible to add some additional vertical bracing/shearwalls etc so that you shorten the span of the diaphragm. Then the 3" of concrete might work.