Concrete Tube Slab - partial demo & construction loads

[RHTPE]

Back in the 1960s an engineer in my area (Northeast US) had patented and was promoting a tube slab. It is essentially a one-way joist slab where, instead of using pans, fibreboard tubes were suspended between the top & bottom bars to displace concrete and reduce the dead load. Earlier on in my career I was involved in the construction of such a slab and it proved to be a challenge as the clear space between the tubes was only 2" and resulted in numerous areas of honeycombing or (even worse) areas where the concrete did not fill the space between & below some tubes.

Fast-forward to today. I am trying to evaluate a 14" tube slab for demolition and construction loads. This is a 14" thick slab using 10" diameter tubes at 12" c/c spacing. From what I can see from underneath, the slab appears to be well constructed with no obvious evidence of the honeycombing we had previously experienced.

My client needs to accomplish 2 primary things: 1) Cut a new 22'-4" square opening through this slab, and; 2) remove a 1-3/4" topping consisting of a grout setting bed with quarry tile.

I need to feel comfortable that this slab (originally designed for 100 psf LL) can safely handle the weight of the equipment to be used to perform the necessary tasks. The primary piece of equipment would be a 6K skid-steer and a 4K mini track loader. Not yet specified is the weight of the saw-cutting equipment. The current plan is to remove the concrete for the new opening in 2' wide X 22'-4 long pieces, lift them above the slab and place them onto 2 H.D. moving dollies and roll them outside for subsequent demo. Removal of the tile + setting bed will be done with a skid-steer mounted apparatus similar to that used to take up carpet & composite tile.

I do not have any information on the design of a tube slab and I am hoping that one of the well-seasoned followers of this forum might have something. To me, it's a one-way joist slab having a funky-shaped joist. Based on the original design loads I'm finding that the neutral axis of the section is very close to the top surface. Concrete fc' was specified as 3ksi and fy was specified as 40 ksi.

Any words of wisdom out there?


Ralph
Structures Consulting
Northeast USA

 

[KootK]

The one way joist analogy seems apt to me. If you feel good about the quality of the quality of that soffit concrete, you can make use of the low flange for improved negative flexure strength and stiffness.

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.

 

[Ron]

Agree also that the slab will act like one-way joists. Essentially you have closely spaced, shallow joists with a topping slab.