Pipes Through Composite Metal Deck 1

[Nulukkizdin]

We've been asked to account for mechanical piping running through our composite metal deck on a project. Our initial intent was to have the pipes run below the slab, but it appears that this will be a major cost for the mechanicals that could be mitigated if they were able to go through the slab instead.

Details:
[ul]
[li]Composite steel beams[/li]
[li]2" VLI deck with 3.5" of concrete above the flutes[/li]
[li](2) 1" pipes will be passing through the deck in various places. We will encounter areas where the penetrations are perpendicular to the beam span and also areas where the penetrations are parallel to the beam span.[/li]
[li]Beam sizes are designed and fabricated. No options for up-sizing or redesigning beams at this point.[/li]
[/ul]

The most specific resource we have found as of now is (

http://www.newsteelconstruction.com...mposite-floors-effects-on-slab-beam-design/).

You might need to open it with IE, for some reason Chrome/Firefox were having fits with it.

In this resource, the author mentions that pipes running parallel to the deck WILL have an effect on the concrete compression block considered for composite design. It appears that the article states that pipes running perpendicular to the deck WILL NOT have an effect on the concrete compression block. This seems a bit odd, as your compression profile (3.5" above the flutes in our case) would have a big chunk across it's whole width (1" in our case) taken out of it by the pipe.

The author does mention the effect on the shear studs as well. He mentions that pipes should be kept a minimum of 1.75 * (stud length) away from the stud centerline, or else that stud contribution should be ignored. Keeping these pipes as far away from the shear studs does make sense to me, but I'm not 100% sure about the conical shaped failure surface that he is discussing?

Additionally, we will have issues of both avoiding large moment conditions (makes sense to avoid passing a pipe over the midspan of a beam) as well as maximum shear flow conditions (at locations of max shear, we also have locations of max shear flow). "Losing" a stud at either of these max locations seems like it would be rather problematic for our design.

Soooo, after that long-winded introduction... Does anyone have any additional information on dealing with pipes running through a composite deck? Does anyone have any thoughts on how to quantify the effects of these pipes on the adjacent shear studs? Any idea on best practices (besides the obvious "don't put pipes in your slab")?

Any thoughts would be helpful!

 

[KootK]

Huh, I didn't even know this was an option. Here's what I've got:

1) I wouldn't sweat the high shear locations. In composite design we make the assumption that horizontal shear transfer into the slab is uniform along the span. It's an assumption that's long bothered me but that's how it's done.

2) Given the choice, I'd prefer to have the pipe closer to the exterior sides of the studs than the interior. Interior being closest to midspan and exterior being the other side. My understanding of the mechanics of the studs is that concrete struts come down out of the compression block and terminate roughly where the studs meet the beam flange on the interior side. As such, you're critical on the interior side and less so on the exterior.

3) For the flexural design of the deck slab, pipes perpendicular to the deck mess with your compression block. However, your compression block may never get deep enough for it to be a problem.

4) For the flexural design of the composite beam, pipes parallel to the deck mess with your compression block because pipes parallel to the deck are perpendicular to the beam. I this case, your compression block will be deeper and pose more of a problem.



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.

 

[Brad805]

This is a bad path to go down. Do you know where all of the pipes are going and will you get any say with what the mech trade does? We did a precast slab with a topping one time where the EOR allowed the electrical conduit to be cast in the topping for the same reason. The topping in this case was the primary diaphragm. When I went out on site to look at the precast components I was shocked at the forest of electrical conduit that was grouped in certain areas. The mech piping will be much less of a problem, but getting that trade to follow directions from us is like pulling teeth.

If you are going to consider this you need to talk to make sure the mech consultant appreciates the complexity of this. The mech trade is unlikely to open a structural drawing so you need them to indicate the importance of this.

 

[Nulukkizdin]

Thanks for the comments! It is currently our (structural) understanding that IF we do go this route, the mech will be following our requirements quite carefully. The building of interest is the shape of a large "Y", and each wing of the Y has various rooms with a long hallway separating them. Each room needs access of (2) of these 1" pipes (no insulation). With this requirement, I think we can be pretty specific about directing the mechanical pipes where we want them. We were thinking of either drawing out specifically the pipe locations for them or possibly just overlaying a plan with shaded "no-zones" where they couldn't run their pipes.

I realize this is NOT the most desirable solution from a structures point. We're hoping to explore it a bit at the request of the owner and CM, as it apparently will save a bunch of money if we can come up with an acceptable solution.

Any thoughts on ways to specifically quantify the effects of these pipes on the design?

 

[Brad805]

I agree their is a cost savings to this. I know of a major BIM project in the US where they prefabbed the mechanical mains and they saved 6% of the mechanical budget by significantly reducing the number of times the mechanical trade needed to go up and down a ladder. It was interesting to learn and considering the steps taken to track the time spent on a ladder I tend to believe the claim was legitimate. Whether or not the trade passes the savings on to the owner, well that is another question entirely.

I would be careful your contract for services has an allowance for the added time you will need to spend. Regardless how well the mechanical engineer communicates the design I would anticipate a lot of additional questions from the field where they want to move things to make it more practical. The mechanical consultants we work with generally produce fairly generic plans showing the basic layout. The trades determine the specific locations based on what is most practical in the field and we both know how important those specifics will be to you. Maybe it is different in the US. If you have the mechanical trade on board now much of that problem could be avoided, but as we both know that is uncommon.

Analytically, the case where you have a single pipe does not seem that complicated once you have determined the shear, flexure, and compression forces in the slab at the pipe locations. We do voided slabs in the precast industry all the time. Yes, you have studs to deal with, but so long as they don't want to tie the pipes to your studs I doubt the difference will be significant. Just as Koot mentioned you have two cases to consider.

The area where I see the complexity is near the mechanical room. Pipes usually start from the mech room and the quantity of pipes near the mech room could be vast if the architect has not allowed for multiple mech rooms. That is the area of most concern as your compression and shear regions will be impacted the most. The design of that region will not differ from the areas with single pipes, but I suspect these regions will play a major role in your slab design unless you intend to thicken the slab for some region around the mech room.

 

[haynewp]

SDI Code of Standard Practice has the following concerning the deck:

4.10 Conduits In Deck Slabs:
Conduits are permitted in deck
slabs subject to local code
requirements and fire rating
considerations. When conduit sizes
are 1" (25 mm) or less in diameter,
or less than 1/3 the concrete
cover, and no crossovers occur,
and conduit is spaced at least 18"
(457 mm) apart with 3/4" (19 mm)
minimum cover, conduit may
be permitted in the slab unless
further restricted by the design
documents.

I have dealt with this before as it came up during construction, we analyzed the beams as non-composite at areas significantly affected by the conduits/piping and they didn't meet that above SDI paragraph at all. We had to add a new beam adjacent to some of the ones affected. I would take the paper you found and apply that with judgement for the beams.

 

[Nulukkizdin]

Unfortunately in our case, we will not be able to alter the steel design near these locations. Our design has already been submitted and the beams are either up on site or fabricated. If we are able to justify the penetrations (albeit in our specified locations), we will be able to accommodate the request of the mechanical to save money as mentioned above. If not, they'll just run their pipes below and take the extra cost.

I don't think we'll have that much of an issue regarding the mechanical room. The only pipes that need to run through the slab are these 1" runtel (?) pipes. They will be coming down vertically from the level above and contained in the walls until they hit the floor of interest (our 2nd floor). At the 2nd floor, they will need to route (2) pipes into each room, where it will hit a small mechanical box (one in each room) to provide heating to that specific room. That said, each room will get (2) pipes, but we shouldn't have to worry about a huge mass of pipes running through the slab anywhere.

Regardless of the possibilities of mechanical options and scenarios, I am really just curious of the effects of the couple pipes themselves. Has anyone actually done this design before? If so, what were your major concerns and assumptions? What are your thoughts on the conditions listed in the document I listed above? To me, some parts seem reasonable, while others maybe not so much, so engineering judgement will definitely come into effect. Let me know what you think!