evaluating holes in a draped mesh goulash conc slab

[samdamon]

I am reviewing a 1930's era floor in a NY city building that was designed using the empirical formula in the NY code of that time. Having some difficulty deciding how to rationally decide when a hole in the floor of what size needs to be reinforced with steel angles etc under the slab.

The mesh reinforcing is supposed to be acting as catenary cables rather than as tension reinforcing in the normal modern engineering sense of the word. My understanding is that the empirical formula for determining the slabs reinforcing was based on testing. I can assume that the load formerly carried by cut wires spreads to adjacent undamaged slab and reinforcing, but to what extent?

Is anyone aware of a design method or approach for dealing with post-construction holes in this type of floor slab? My particular slab has 1/4" dia wires at 4" o/c, typical spans of 5' to 6.5', overall thickness of 4" of cinder concrete. There are existing intermittent holes up to 15" +- wide in the slab now and new ones contemplated the same size or slightly smaller. No strengthening has been done to the floor and it has held up ok so far. The live load on the floor is not going to increase, I just am trying to understand and want to be able to justify rationally how much intervention and strengthening is needed for these holes.

 

[Ron]

Not sure of any particular standard, but my rule of thumb is any hole 12"x12" or larger requires angle framing.

 

[bookowski]

Is there transverse wire? If so then you should be able to count this as distribution steel taking the load to the adjacent strips. With a 15" opening you are looking at less than doubling the load on the adjacent strips if you looked at a 1' effective strip on each side.

You can check the catenary strength, although a lot of times the profile ended up not actually being the proper shape - usually more smushed down than they should be for a proper catenary, still the same principle though - discount the cinder and count the wires in pure tension.

Those load tests were typically to very high values so even with the additional load at each side of the opening my guess is that at 15" it would be fine. That being said, for new openings why take the chance/liability. The cinder concrete itself often almost useless, sometimes it's ok but you can't count on it - if there isn't distribution steel you need to count on arching around the opening. If they are just existing openings I would say to leave them. If they are new I would consider reinforcing anything over 12" (3 wires?).