Steel Deck on Wood Joists

[preynolds]

We are in the preliminary stages of a project and the architect inquired about the use of steel decking supported by open web wood joists. We notified them that on the surface, things seem OK, BUT we were concerned on whether or not information was available to provide diaphragm capacities as this was a system we have not come across. I did some more digging: talked to contractor's, steel deck suppliers and also open web joist suppliers, none of whom could recall such a situation. We sort of all came to the same agreement, that without load information for the diaphragm, we would basically have to strap the roof.

So, the question is, has anyone had a similar situation? If so, what was the consensus on the design? We have plenty of information available for wood-to-wood, wood-to-steel and steel-to-steel diaphragms and shear walls but this one is escaping us.

 

[BAretired]

Steel deck should be fine as a diaphragm on wood members. The key is to fasten the deck down properly. Probably you would use wood screws.

BA

 

[beton1]

Actually I saw this last week. Stopped at a gas station and the cashier/store/coffee building had steel deck on a unique wood/steel tube truss system. The joists consisted of top and bottom wood chords (2x4 I believe) and tubes (steel?)for diagonals (looked like they were clinched and bolted at end through wood). Location was in Burlington, Ontario (west of Toronto) and it was a 'Pioneer' gas stop if I recall. Like BA states, it is all about the connection of the steel deck to wood, but obviously has been done.

 

[preynolds]

so, it has been seen, but can someone point to diaphragm capacities? I don't think you are going to get a wood screw to self-tap through the steel. I question whether or not a self-tapping steel screw will be usable in wood at the point end due to the thread difference between a wood screw and a self-tapping screw. Can you point to a diaphraghm capacity table for a steel diaphragm supported on wood?

 

[MiketheEngineer]

Never seen any tables to support this. Will it work - probably.

 

[msquared48]

Watch the side seam welding and starting fires. May need or want to use button punching. Similarly, no plug welds for obvious reasons.

Believe it or not, there is lateral stiffness in a corrugated plexiglass roof over wood joists, it just hasn't been quantified to my knowledge. So I see no reason why metal could not be used in a similar manner. The question is the lateral load rating, if you need one. In that case, the in-plane "X" strapping and blocking would be an appropriate solution.








Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[preynolds]

I am sure it will work but I have a hard time submitting a design if I can't point to something with tangible diaphragm capacities. Hard to hold up to a standard if I can't point to one.

 

[ToadJones]

DO NOT try to use self tapping screws or "Tek" screws in wood.

Tek screws are not suitable for use in wood to my knowledge and "self drilling" portion of the Tek would be too big a pilot anyway.
As BA suggests, if you do go this route, use wood screws.
The seems can still be done with "lap screws" as they never reach the substrate anyway.

There is a restaurant in my area that I frequent (for their excellent steaks and fresh ice cold IPA's on tap) that has a steel deck roof on a portion of the building. The steel deck is fastened to strapping (like little purlins) on top of 2X rafters

 

[a2mfk]

The deck and diaphragm doesn't know what the supports are made out of... If your sidelap fasteners match a given deck diaphragm table, then could you not replace 1:1 the support fasteners, though like others have said with wood screws vs screws/puddle welds. The support fasteners are for uplift and to prevent buckling of the diaphragm (?). I think if you were very conservative in this area you would be OK.

If you have a strict building dept that does not let you use any judgment outside of the box than you are probably out of luck, or if you have a critical diaph. deflection you would need tables for that also...

What is driving this architecturally, is it exposed? Will the trusses be spaced out like 5' o.c. plus?

I think we have all seen these types of structures, but who knows if/how they were engineered or justified. Come to think of it, my folks' garage is wood framed with wood trusses and all metal corrugated panels for the walls and roof. But that is also a contractor homemade-special POS....

 

[ash060]

I would be careful as the diaphragm tables out of deck catalogs assume that you are attaching to a steel sub-structure. And a diaphragm's capacity is largely based on the connectors. A arc-spot weld is good for well over a 1000 # in shear while a wood screw is good for a couple hundred at best.

I run into this problem when dealing with light gauge sub-structures because the welds are not as good so I don't get the capacity I was hoping for.

If you are going to place pliot holes for wood screws thru the metal deck why wouldn't you use nails? They are easier to install.

 

[preynolds]

you guys are confirming my thoughts.....while it may seem doable, we can't point to anything of substance that provides capacities.

The architect has since switched to wood trusses with plywood. Problem solved.....for now.

 

[csd72]

every second house built in Australia in the last few decades has this type of system though usually they will use steel strap bracing underneath it.

As others have said make sure it is adequately screwed down. Also ensure that it as not a concealed fixing system as these usually allow movement.

 

[a2mfk]

Well, that's good news for you. I try to always fall somewhere between too conservative and guinea pig. Me thinks they were trying to get you to be the latter.

I went through this once on a project that was being driven by a GC who did mostly PEMB. They wanted to try using glorified standing seam metal roofing that was some type of thin-gauge stuff you could only use for gravity, and after many attempts of explaining diaphragms they finally gave up and went with a traditional system. They were used to in-plane X bracing at the roof and steel moment frames. They could not get their head around a diaphragm and CMU shear wall box structure, though they went with it and everything worked fine. I could not understand any cost savings in what they were proposing, I kept telling the architect, "You know most buildings are constructed this way, only PEMB don't use diaphragms..."

That last comment was over-simplified for discussion's sake..