Waffle Slab Penetration

[SteelPE]

I have a client who wants me to go look at a project with them. The project requires the installation of a new RTU onto an existing building. I have not seen the project yet, but I am being told that the existing floor framing is a waffle slab system. I don't know much about these systems other than I don't think it's a good idea to cut any of the ribs in a waffle slab.

I am scheduled to meet the client onsite next week to review the work overall. Once I heard the framing was possibly a waffle slab system I expressed a good amount of concern to the client about making penetrations through this type of system. There may be a possibility to avoid the penetrations all together, but I just need to know how hard I need to push the client away from making any penetrations throught this slab.

Has anyone had experience in the past with making penetrations through this type of system?

 

[KootK]

1) A waffle slab is really just a permutation of a two way flat slab. As such, most any penetration that you could make in a flat slab could also be made in a waffle slab. See this free publication for guidance: Link.

2) The concentration of capacity at the ribs has both benefits and drawbacks:

a) It can be handy to know where the important reinforcement resides.

b) If a 6" conduit absolutely must pass through a rib, that obviously sucks.

c) There is some potential for services to pass horizontally through the system before passing vertically through it. This can also be handy.

3) As you've recognized, a penetration that would interrupt no ribs at all is pretty much a slam dunk.

 

[JLNJ]

If you really push an HVAC engineer, you can get them to agree to let your structure run through the ductwork. It's not cheap, but it's cheaper than trying to reinforce a cast-in-place concrete building.

 

[SteelPE]

So just to we are on the same page here.

-Cutting a rib of the waffle slab it not recommended at all (pretty much what I thought)
-Cutting the thin slab porting is much more desirable as long as it does not disturb the ribs.

I am currently going to try to push for the HVAC to either

-use an existing opening
-run down the outside of the building and enter the building from the side (doubt this will work).

I have considered putting columns around the opening, but I don't like adding support in the middle of a slab like this.

 

[JAE]

Agree with above but a couple of other points:

1. Openings through the slab portions (non-rib areas) would be preferable through the slabs near end supports to avoid diminishing any flexural strength provided by the slab (i.e. T-beam type behavior in the rib-slab system at points of larger positive bending moment).

2. Having said that, however, holes/openings through the slab near column centerlines or bearing walls may conflict with top flange negative moment reinforcement or tendons. Getting the slab scanned for the location/presence of such reinforcing/tendons would be a good idea.

 

[KootK]

So just to we are on the same page here.

Not me. While preserving the ribs is definitely preferable, I'm willing to interrupt some ribs strategically if analysis indicates that would be viable. Like I said, I'd be willing to do most of what I'd be willing to do with a 2-way slab for a waffle slab. And that includes interrupting some reinforcement when I feel that is sensible.

 

[GeorgeTheCivilEngineer]

The ribs are quite a lot wider spaced with longer lever arms than reinforcement in a flat slab so I'm team 'protect the ribs'.

 

[KootK]

The ribs are quite a lot wider spaced with longer lever arms than reinforcement in a flat slab so I'm team 'protect the ribs'.

The waffle slabs that I've dealt with have had ribs spaced so close together that their concentration of flexural capacity wouldn't be much more severe than bundling a few of bars together in a flat slab. And I don't see the relevance of the increased lever arms. Flexural resistance is flexural resistance.

I feel that it is easy to get the impression that these systems are made of "beams" and, therefore, assign each rib the significance that a "real" beam normally gets. These systems are not normally designed as beams, however. They are designed as two way slabs. That's their nature. They are often even solid near columns in acknowledgement of two way punching shear concern.

 

[JLNJ]

That's some serious late 60s early 70s *BLING*. Is that exposed aggregate I see on the columns? Purdy!