Wood Posts on Concrete Topped Metal Deck

[jdgengineer]

We have a condition where we have wood framing built on top of a concrete topped composite metal deck (2.5" topping over w2 deck). We have conditions where we have 6x6 posts within 2x studwalls that sit on top of the metal deck. We have steel beams located directly below the posts and below the deck but the load has to transfer through the wood sill plate to the concrete topped metal deck to the steel beam below. The posts are not especially highly loaded.

How have you evaluated the capacity of the deck to transfer the load? We have conditions where the deck may be parallel or perpendicular. I know that deck manufacturers give web crippling data for loading through an untopped deck, but not sure how to evaluate a concrete topped deck for the load transfer.

Worst case we could cut out the deck at the post locations so we have solid concrete support below the posts to the steel beams but want to avoid this where possible.

Thoughts?

 

[jayrod12]

We usually just specify solid blocking in the flutes from below, if they can live with combustible blocking.

 

[jdgengineer]

Hi Jayrod -- That could be a good option. Seems like it works when the flutes are perpendicular. What about parallel flutes? Seems like you may not always be able to access the space to install a block?

 

[jayrod12]

Existing construction I assume. Since it's concrete filled, could you make the argument that the load spread through the 2.5" gets you to the deep flutes? It seems highly unlike however that you would have a situation that you couldn't access enough of the flute to block down to the beam.

 

[txeng91]

What are the flues spaces at? If it’s only 6” centers seems like a decent portion of the post would land over a flue and you would have concrete below it down to the steel beam. If we’re just talking a couple thousand pounds then I wouldn’t think it would be an issue. If the flues are at 12” centers and you think shear/web crippling might be an issue you may consider using a steel baseplate that will span between flues in lieu of cutting the deck. For the parallel beams is there a condition where supporting beams not located under a flue?

 

[jdgengineer]

Thanks guys. New construction with 12" flute spacing. Seems like there would either be space for installing the block between the flutes or we would have a filler plate if the flute would have been high at the steel beam. Maybe not really an issue. Our loads are somewhere between 2-6 kips.

 

[jayrod12]

Ya I'm game for just figuring on them being able to install the blocking. at 12" flute spacing, I can't in all honesty see a situation where you have a beam with a flange wider than 8". If the random situation happens to present itself (my money is on extremely low probability) then you just figure it out after that. Perhaps make sure it is clear on the floor framing plan and not just in section so maybe you stand a chance of it being coordinated ahead of time.

 

[KootK]

For those kind of loads, my first stop would be to see if perhaps the deck could take the punching shear even if the load didn't land over a beam. By way of logical extension, if that works, so does the post on beam scenario. 2K probably works; 6K probably doesn't.

 

[jayrod12]

That's a good point, there's many deck and topping options that will support the code prescribed pointload of 2kips.

 

[jdgengineer]

When checking punching shear would it be reasonable to use d = 2.5" (full depth of slab above flutes)? Seems like the "steel" reinforcing is the deck itself so this seems reasonable. If that is the case, seems like punching shear strength would be 0.75 x 4 x 1 x ✓3000 x 32" (bo = 4 x (5.5+2.5)) x 2.5" = 13,145#.

If that's the case seems like perp conditions could likely be ok without blocking but parallel conditions should be blocked to make sure we have a straight bearing load path to the beam.

Does that sound reasonable?

 

[jayrod12]

Punching shear might be ok. But at 12" flute spacing what happens in the perp condition when the load is dead centre of a high flute. You potentially have a bending failure potential as well.

I'd be specifying blocking in all cases.

 

[jdgengineer]

Jayrod, I agree realistically I'll specify the blocking or solid concrete down to the beam for either case. I just wanted to get a sense of the controlling conditions.

For the perpendicular condition, I'm not sure I can visualize the flexural failure. With a 12" flute spacing, the width of the narrow portion of the flute is only 5". With a 6x6 post on a 2x sole plate it seems like the load could spread to bridge the high flute and form compression struts to the low flutes that land on the beam.

 

[jayrod12]

Oh geez, my bad, I was envisioning a 12" wide high flute.

I agree with that assessment.

 

[KootK]

Dissenting opinion: I don't love the wood blocking.

1) 2" flutes with a rib on top. So how do we get contact? 2X + 1/2" ply routed on top? Possible, yes.

2) I feel that there is a fundamental material stiffness incompatibility at play here because, even with at tight fit, the wood perpendicular to grain will be soft relative to the deck. My money says you'd punch through the concrete well before you engage the blocking. Sure, afer a punching failure, it would be a nice way to restrict further movement.

3) Wood shrinks.

4) I do the wood blocking thing sometimes on untopped metal deck. That's a different beast though.

At the end of the day, I feel as though the wood blocking is just going to wind up being a loose nuisance to whatever zealous electrician decides to knock it out of the way. If you're bound and determined to stuff something in there, I'd make it 2" HSS chunks or something more compatible.

Does that sound reasonable?

Your procedure is pretty much a spot on match for the official SDI procedure shown below. Apparently, you intuition rocks. That said, I don't agree with the procedure and think that it will be quite unconservative. I recommend doing your calculation based on plain concrete punching, also shown below.

If it were me and the punching numbers worked out, and it was new construction, I would do this:

1) Perpendicular flute condition: not a damn thing.

2) Parallel flute condition: toss a 16" x 16" x 5/16" flange plate on top of your girder so that you're catching two deck troughs that straddle the load. Obviously, the success of this depends on your ability to locate the posts with a modicum of accuracy.

 

[KootK]

In another thread, somebody also posted this spreadsheet that you might find useful.