Wood floor on steel joists

[SteelPE]

I need to design a mezzanine system that will consist of a wood floor on open-web steel bar joists. The project is a new mezzanine inside of an existing building and I have calculated the required seismic loads on mezzanine. I have calculated a maximum shear load of 80plf under the seismic loads (I think this is very small). I am planning on using a plywood floor to support the gravity and diaphragm loads. I can't seem to find allowable shear loads for a plywood diaphragm supported on steel members, is there such a table or am I approaching this the wrong way?

 

[XR250]

80 plf is nothing for 3/4" subfloor on just about anything. How is the plywood attached to the steel?

 

[msquared48]

Yea. That's my question too, as I know of no way to bolt anything to a steel bar joist without compromising the joist design. Welding a steel deck to the joist is usually the case.

You might need a 3X wood nailer, full with of the top double angle section of the joist, with nelson sill plate anchors welded alternately to the top angles of the bar joists in order to attach the plywood to the joist.

Mike McCann
MMC Engineering

 

[SteelPE]

The attachment is what I need to come up with. I was thinking about using self drilling screws (ITW Buildex makes a wood to steel screw with a flat head) but I don't think that will be the easiest thing to accomplish. I was considering to use a double layer of plywood to but I am not sure that is the best idea.

 

[azcats]

What's the joist spacing? Don't need subpurlins or anything?

Link

In the details section (59 & 60) of the link above, they show the attachment of a nailer to the joists. Seems like overkill for your situation though. Especially if you're running the joists at 24" or 32" spacings.

 

[msquared48]

SteelPE:

My point is that I do not think that you can drill the top of the joist flanges without compromising them. You have to weld something to them as I suggested in bold above.

Mike McCann
MMC Engineering

 

[azcats]

I think the joist mfr can send the joist with the wood nailer already attached. Make it part of the deferred submittal and specify the shear you need b/w joist and nailer.

 

[SteelPE]

Azcats,

My joist spacing is planned to be 2'-0" o.c with a few instances of 2'-1".

msquared48

I understand your concern with regards to compromising the TC of the joists..... however, this is really no different than screwing steel deck to the TC of a steel joist which, as far as I know, is allowed. In the details azcats provides in his post, the holes are pre-drilled into the TC which I would agree degrades the TC. However, with a self drilling screw, would your really be compromising the TC? Wouldn't you be replacing the material you took out with the shaft of the screw? and in a member that is under compression would this cause a problem?

 

[EngineeringEric]

You can attach to steel bar joists with #10/#12 tek screws. The screws can be self-tappers/self-drillers to make life easy. Then for the attachment calculation you can base the plywood sheat strength on NDS diagphram values using #8 (or #10 or #6, i don't have in front of me) screws... Then just check that the single shear value for 3/4" to 1/4" steel is greater than 3/4" to 2x material.

But to be sure call Vulcrafts South Carolina office and talk to the Joist guru, Mr Mike Antici.

If i can specify to attach a metal deck at 36/6 with #12 screws to steel joists then i should be able to do the same thing with wood deck at 6" o.c... But spacing (the Field) attachment is still important in this configuration

 

[EngineeringEric]

SteelPE, Look online at page 89 of attached

 

[EngineeringEric]

89 in PDF PAges = 87 of booklet

 

[azcats]

Is squeaking/noise a concern with the plywood directly on the steel joist?

Also - it might be wise to coordinate with the joist manufacturer no matter what connection you choose as there are likely bracing forces for the top chord of the joist than need to be resolved and I imagine that's not in your scope.

 

[XR250]

I would fasten with winged self drilling teks. They go in real fast. I agree with these not compromising the top chord.
@azcats - I cant imagine 3/4" plywood not providing sufficient bracing for the top chords

 

[azcats]

@EE - I tend to agree but I'd discuss whatever I'm planning with the joist designer.

 

[Triangled]

West Coast USA Steel joist manufacturers routinely provide joists with 3x wood nailer attached. They design the chord to accommodate the screw holes which they provide. Design floor system as nailed diaphragm.

 

[Triangled]

See bottom illustratin of attachment.

 

[Triangled]

If you choose to have the manufacturer design and install the wood nailer, be sure your design intent and their design understanding are coordinated with regard to lateral load paths. For example, if you are utilizing a 32' joist I wouldn't assume they will provide a continuous 32' wood nailer, but rather the nailer may consist of 12' (or some other economical)lengths and the nailer joints may concur at random unforeseen (unless specified and coordinated) locations. This may not be of concern if the nailer is merely required for diaphragm shear transfers from plywood sheet to plywood sheet. If however, one intends to anchor the wall to the joist utilizing something like a Simpson PA strap nailed down into the top face of that nailer, and coincidentally that random nailer joint occurs at the outstanding tip of the PA strap, it is quite possible that simply a single #14 screw may be in the load path between the steel joist and the PA strap. All of this can be handled by coordination.