Question about decking.

[jay156]

I have a client who wants to put a 1.5 inch decking on the bottom of some bar joists to be a ceiling for this one room in the building. He also wants the maintenance guys to be able to walk on it. I spaced the joists at 4' so the loading on the deck shouldn't be a problem, but I'm worried about the strength of the screws holding the deck onto the bottom chord of the joist.

Is there anywhere I can find published holding strength of screws so I can size and space them properly? Is there anything else I'm overlooking?

 

[SteelPE]

I hope you informed/plan on informing the joist mfr of what is happening. Some problems I might be concerned with:

1) local bending of the bottom chord.
2) drilling holes through the bottom chord.
3) minimum deck bearing (1-1/2" in my area).

There may be other problems.... but those just popped into my mind.

As far as screw values, ITW Buildex is where I go to answer all of my screw questions:

http://www.itwbuildex.com/index.html

 

[woodman88]

Yes. Do not do it.

Unless they use very large washers with bolts it is not a good situation. Even then it is not a good idea.
The added cost of placing some 2x or 3x on top of the bottom chords (of the bar joists) at whatever spacing is needed for the maintenance guys will be worth it in the long run.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[jay156]

Are you saying I shouldn't even put the decking on the bottom chord at all? Or put decking with the 2x wood on top spanning between the bottom chords?

 

[msquared48]

Over the top of the bottom chord, then fur down with 2" flat members tthat the deck can be secured to so it can pass beneath the bottom chord. DO NOT screw anything into the bottom chord of the joists.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[woodman88]

Sorry I meant that to support a walking load, from the maintenance guys, not to do it. I would require something that clearly shows where the maintenance guys should walk.

As a ceiling, the decking can look nice. But it is as good as gypboard is for a walking surface IMHO when hung from joists.

If you (or if the client insists and you are willing to go along) still want to design it, make sure (if you are in the USA) to follow the NDS or an ICC ES report for the values. I would recommend that you use at least 40 psf live load with only 50% of the connection values if you go this way.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Hokie93]

Is the top chord (compression chord) of the open-web steel joist laterally unbraced for the entire span? Or is the maintenance worker going to be stepping over intermittent joist bridging?

 

[jay156]

Okay, thanks for the advice. I'm thinking of tack welding a 3/16 metal plate to the bottom chord so they can screw to that. I didn't think I would need bridging since the joists are so close together, though with the compression chord unbraced by a roof I guess I probably do.

 

[hokie66]

I think some are being unnecessarily negative about your proposal. This is just a ceiling structure, with a bit of access by maintenance staff. The screws won't have much affect. I would rather depend on screws than on welds to the chord. If you want to be conservative, just downrate the joist capacity by how much metal is drilled out for the screws. Another way might be to bolt between the chord angles, if they are that type joist. I do agree that you need bridging for the top chord.

 

[IsaacStructural]

I'm not exactly sure how the detail would look, but this seems like a case where a system like lindapter might work. A little friction clip that doesn't require welding to or screwing through the bottom chords.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[hokie66]

We typically screw metal roofing to light gauge purlins to resist 60 psf wind uplift, so there is no reason similar type fasteners wouldn't work for gravity loading as in the OP's project. Predrilling will probably be required, and the difficulty of working overhead is involved, but it can be done.

 

[XR250]

@hokie66

Why would pre-drilling be required? I have never seen a joist chord that a TEK screw cannot go thru.
Contrary to what others say, you are not going to hurt the joist by adding some screws to it - unless it is designed to bare minimum code. Many are sized for deflection anyway. You are also not going to hurt the bottom chord by stepping on it - sheesh, how conservative can you guys be?
As long as your decking is 22ga or greater, you are not going to pull a screw thru it either - even if you had a 300 lb worker up there, there is some much redundancy with all the fasteners that a complete failure is unlikely.

 

[hokie66]

Excel,
You may be right that predrilling would not be necessary, but we don't know the span or the thickness of the chord members. I wouldn't like to try screwing overhead into 1/4" steel without predrilling.

 

[XR250]

Drilling overhead is no easy task either.
A Tek5 self-drilling screw will drill just as fast and easily as a normal bit.
Most Bar Joist flange thicknesses are only about 1/8 to 3/16" anyway

 

[hokie66]

I am sure the workers would soon find the best way. When I have screwed to hot rolled steel members, I have found drilling pilot holes to be easier than doing all the drilling with the TEKS screws. But then, I am not a tradesman.