Reinforcing Metal Decks 1

[CBEngi]

I have a project where the previously installed metal decking is now under designed for the new roof loading. I am wondering if anyone has encountered this before. I'm looking for a way to increase the moment capacity.

I have a couple thoughts on how it could be done:
-Attach metal bar to the bottom chord of the panels
-Attach metal sheets to the bottom chord of the panels.

I haven't checked through the numbers completely, but i'm looking for what other people have done in the past, or some standard details from manufactures (Which I haven't been able to find).

Thanks in advance

 

[jayrod12]

I usually do a net of angles (or channels) between the joists to cut down the span of the deck.

 

[jike]

Depends on overload, spans, access to the underside of roof deck , etc.

I have many times installed new joists between existing joists to cut down the span of the deck. Joists need to be specified with a bolted mid-span splice OR one bearing end to be field installed. Also new joist must span between bridging since it will NOT generally be attached to the deck for lateral stability. Also specify no camber and Min. Moment of Inertia.

 

[KootK]

I did a bit of this earlier this year and used both strategies mentioned above.

Here are some details that I developed along the way:

1) Channel deck reinforcing: Link
2) New OWSJ top chord deck connection: Link

Notes:

1) My hand sketched versions of these details were glorious. My drafters have a penchant for drawing things at too small of a scale.

2) With the channels, I did my darndest to make it constructable with long slotted bolted connections etc. They welded the crap out of all of it.

3) I'll allow a fair bit of deck overload before I'll go to all this trouble. In my opinion, the consequences of deck failure are not terribly dire.

4)Having thought about it for several months now, I've thought of two additional reinforcement strategies that might work well if the original deck worked in shear / web crippling:

4a) Use light gauge reinforcing members, similar to the channels, that do not quite make it to the OWSJ top chord.

4b) Nest new segments of deck to the underside of the existing deck. Obviously, one has to consider any junk suspended from the existing deck.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[Brad805]

I am not sure if OSHA has implemented the same rules about lead paint as we have in Canada. If your building is older, and OSHA has adopted the same rules, I would add a disclaimer that it is the owners responsibility to investigate for the presence of lead paint and pay for any additional costs associated with same. There is quite a protocol to remove lead paint from the weld locations. It can have a significant impact on costs.

If you elect to add new members all I would add is keep the members as light and short as reasonable. The biggest challenge for these type of jobs is working around existing mech/elec items. The problem with existing features can turn into a real problem.

If the roof membrane is old or planned to be replaced soon, it can be easier and more cost effective to merely lay down another layer if the joists are acceptable.

 

[jayrod12]

Brad,

Good point. When I do channel (or angle) deck reinforcing as shown in Koot's sketch I generally don't weld to the existing. I actually have them make a saddle out of an angle and a piece of plate. The horizontal leg of the angle is sized to go completely over the existing joist to the other side. The piece of plate is then welded to this angle creating a saddle. No welding to existing is required therefore no paint removal is required.

The angles and channels are not getting fastened directly to the deck so there are no uplift concerns.

 

[MGX]

Is it possible to install a new beam to cut down the span of the exist. metal deck?