Metal Deck Attached to Wood Nailers

[Eng_Struct]

Hi Group,

I am replacing an existing wood deck with a metal deck for one of the projects. The existing roof system consists of OWSJ with 2x6 wood nailer attached to the joists using 5/16" dia. x 6" bolts at 2' and 2" wood deck attached to the nailer.

My question is can I simply take out the wood deck and attach metal decking to the wood nailer? What are the things I should be considering? Any watch-its? The metal deck will need to have a certain fastening pattern to develop the diaphragm action. I would think that the nailer-to-joist connection will need to be checked to resist the shear in the deck as well as resist 2% of the lateral load form the joist chord in compression. Will there be any particular requirement for the type of connection between the metal deck and the wood nailer? Speaking to one of the metal deck suppliers, they recommend two screws at every 6" at the nailer location.

Any guidance will be helpful. The diaphragm shear varies from 5 kN/m to 9 kN/m.

The other option is to take the nailer out and attach the deck directly to the joists below however this will result in other detailing issues at the interface between the new and existing deck as there will be a step in the two surfaces. Using a filler member i.e., hss or plate of some sort will be costly.

Thanks!

 

[XR250]

To be clear, you are replacing wood sheathing with a metal deck product as a roof?

 

[Eng_Struct]

The existing decking is not a plywood if that is what you mean by sheathing. The existing is 2" thick T & G wood decking.

 

[XR250]

Are those shear values ASD or Ultimate? They seem pretty high.
As long as your numbers for the connections work out (in the field and at the perimeter), seems like you should be ok.
Are there chords on all 4 sides?

 

[Eng_Struct]

I have thought about this and now thinking that I should take the nailer out and replace it with HSS attached to the top of the joists as shown in the sketch below.
members. This will be additional material but this will simplify deck connections as it will be between metal to metal.

As long as the HSS is strong enough to take the compression from the deck or the framing members. I think this could work.

Feedback will be much appreciated.

 

[bigester93]

why about weak axis? turn that around.

 

[Eng_Struct]

@2bigster93 The existing floor level in the surrounding is only 1.5" above the top of the joists. The HSS is orientated to bring the new deck to the same level.

 

[BAretired]

Transferring shear from deck to joist requires a shear connection on two planes, top and bottom of existing nailer. The bottom connection is already in place, so if the wood deck can be removed without excessive damage to the existing nailers, then fastening metal deck to nailers with self drilling screws would seem to be the most economical plan.

Replacing a bolted 2x6 nailer with an HSS results in a connection problem. You can't use the existing bolts and field welding on the curved radius of the HSS is a difficult weld.

I don't know why you are replacing wood deck with steel deck. Have you considered fastening steel deck to the existing wood deck without removing it? Alternatively, have you considered replacing the existing wood deck with a new wood deck?

 

[Eng_Struct]

The wood deck is essentially a portion of the roof that is now being revised to be a mechanical penthouse. Since there are fire safety-related concerns, the existing wood deck is being replaced with a metal deck and a checkered plate on top as a floor surface.

There is no guarantee that the existing nailer will remain undamaged after the removal of the wood decking. Leaving the wood nailer may not resolve fire safety-related concerns.

In terms of welding the HSS to the top chord, I understand your point but the amount of welding is not going to be significant. At the most critical location, the deck shear will be around 12 kN/m. Even a 1" weld at every 2' along the length is more than enough per the design.

 

[Brad805]

How many joists are there? If more than one I would remove the nailer and deal with the step at the addition.

If they do not like the deck for fire reasons do they like the wood nailer?

Simpson make a nice screw that would screw thru the deck and into the joist in a single go. They have data for the use of that screw with metal decking.

 

[BAretired]

Okay, for fire safety concerns, I agree that the nailer and wood deck must be removed. If a step in the mechanical penthouse is acceptable, I agree with Brad805's suggestion. Otherwise, the HSS fillers seem to be doable if the existing roof joists are adequate to serve as a mechanical room floor.

 

[Eng_Struct]

@Brad805

The unfortunate part is that within the area where the deck is to be removed, there is already a patch where they had replaced wood decking with metal back in the 1980s. This portion of the metal deck is on top of a wide flange beam that has the top of the flange higher than the surrounding joists (i.e., the beam is placed 1.5" higher to account for the nailer thickness). I cannot lower those wide flange beam and putting HSS on top of the joist in place of the nailer seemed a good idea to bring the deck back to one level everywhere.