Reinforcement of SOD MEP Penetration

[TylerM94]

I recently came across a detail for a project a principal in my office is pier reviewing that calls for two #4 bars at all sides of an MEP opening in slab on deck. Rebar is to extend 3’ past the opening on all sides and max opening is 18” perpendicular to deck span.

Can someone explain how this works structurally or how I would go about proving it? Is the idea that the rebar acts as a header in flexure and spans perpendicular? What if the contractor installs the rebar close to the top third of the slab?

My office has a typical detail which allows an opening in slab on deck 2’ wide perpendicular to the deck span provided that a C5 channel is welded at each side of the opening spanning perpendicular to the deck. I haven’t run the numbers on it but conceptually it makes sense to me because you are heading off the load of the slab and using the channel to point redistribute the load to the ribs adjacent each side of the opening.

A conceptual explanation of the rebar approach would be much appreciated and if you would be able to describe the approach to proving this works structurally that would be appreciated!

 

[KootK]

1) The rebar may be for crack control, structural capacity, or both. The author of the detail may be the only one able to say for certain.

2) If the rebar is used for structural capacity, then your assumptions are probably correct:

a) The bars perpendicular to the flutes would serve as short headers and distribution members (like the channels that you mentioned.

b) The bars parallel to the flutes would serve as added, spanning direction flexural capacity. That said, there would be two challenges:

i) As you mentioned, the depth at which the bars are placed matters.

ii) If the bars do not run the full span of the deck, then they cannot be relied upon in the regions of the deck where they do not exist.

I recommend digging around in SDI literature and manufacturer catalogs as those documents often address this stuff. My gut feel is that your set up is just crack control and distribution steel as shown below.

 

[MSUK90]

a) The bars perpendicular to the flutes would serve as short headers and distribution members (like the channels that you mentioned.

Doesn't the tributary load distribution will be as per the picture attached in the following image?
Also, I always wonder about the width in which the extra rebar around opening should be provided? Does any code mention that?

 

[TylerM94]

Thanks very much for your helpful input. I appreciate you affirming I’m on the right track and will also check out SDI.

 

[KootK]

Doesn't the tributary load distribution will be as per the picture attached in the following image?

I see it more like this with the flexural demand flowing around and past the opening.

Also, I always wonder about the width in which the extra rebar around opening should be provided? Does any code mention that?

I don't know of a code limit but I've not looked to hard for one either. Practically, these bars really can't be too close to the slab edges so I just space them at 3" oc.