welded side laps on metal deck

[gte447f]

Anyone have any experience with welding the side laps on 1.5B roof deck? I have always specified screws, but specified welds this time around for aesthetics (the Arch didn't want to see any screws b/c the underside of the deck is exposed). This is now backfiring on me, because apparently it is nearly impossible to weld the 2 sheets together without burning through. I upsized the deck from 22 to 20 ga to try to mitigate this problem, but the welders still can't avoid the burn through. I have come across a few "references" that say burn through is to be expected or that it is the rule and not the exception, but I don't see how this burn through cannot undermine the strength of the weld, not to mention it looks pretty bad. If this is such an issue, why do all the metal deck manufacturers publish diaphragm strength data for welded sidelaps?

 

[hokie66]

The "burn through" is actually essential for ensuring the strength of the weld, so it is unavoidable in thin sheet.

 

[Ron]

Agree with hokie66. Welding side laps is not a good idea. It can be done, but requires finesse....not something most structural welder possess.

The side laps could have been punched. This is done with a special tool that clips the sheets together. Haven't seen one used in years, but they were available at one time.

Assuming you are in the us, hopefully you required that the welders be certified to the sheet metal code (AWS D1.3) rather than just D1.1

 

[dik]

Button punching the side lap seams would do the trick. In these environs, they are not uncommon.

Dik

 

[southard2]

Theoretically you could put the screws through the labs upward. The heads would only be exposed but it would look clean. Perhaps the contractor, owner, and yourself could come to an agreement to allow this. Its a pain. But I've seen crazier repairs. I had some jobs in South Florida years ago now where the light-gage truss supplier supplied 22-GA top chords. The amount of screws per sheet where thus inadequate and the repair had to be done underneath.

John Southard, M.S., P.E.

http://www.pdhlibrary.com/

 

[southard2]

Oh and you might have them use an extra long screw so that that your extra sure that the direction of the screw doesn't matter.

John Southard, M.S., P.E.

http://www.pdhlibrary.com/

 

[gte447f]

Button punching only works on deck profiles with upstanding seams, right? That won't work in this case, because the deck is already onsite, and it does not have an upstanding seam.

I am considering using screws installed from the bottom side of the deck. That is an idea that the GC has already proposed. Any other thoughts?

Back to the welding issue... upon reviewing AWS D1.3, arc spot welds (a.k.a. puddle welds) seem only to be allowed for sheet steel to supporting structural steel, not sheet to sheet, so why does the SDI allow puddle welds on side laps? Also, the AWS D1.3 qualification test for arc seam welds (a possible alternative to puddle welds) shows a standing seam that has been crimped together so there are 3 plies in contact, not just 2. Figure 2.12 impies that this crimping helps with the burn through issue. It doesn't appear to address just 2 plies lapped in the horizontal plane.

Have any of you actually successfully used welded side laps before? Personaaly, I am leaning toward avoiding them in the future.

 

[wannabeSE]

From the Steel Deck Institute Manual of Construction:
"Burn holes are the rule rather than the exception and an inspector should not be surprised to see them in the deck. The weld develops its strength by holding around the perimeter. A good weld will have 75% or more of its perimeter working."

The manual is online at

http://www.sdi.org/whites/moc2.pdf

 

[dik]

Poprivets? Is there much shear for the deck diaphragm?

Dik

 

[OHIOMatt]

When the diaphragm requirements are such that I determine welded side laps are required, I specify interlocking "B" deck. This is typically the same profile used button punching.

 

[gte447f]

OHIOMatt, I assume you specify interlocking deck so that the deck can be welded with arc seam welds as shown in AWS D1.3, not puddle welds? That makes sense to me.

All, I have decided to increase the number of welds to the supporting structure and allow screws driven from the bottom on the sidelaps to achieve the same shear strength as the original attachment pattern with welded sidelaps. Thanks for the input.

 

[SteelPE]

gtw447f

I have successfully specified welded side laps before... at least as far as I know. However, we had the erector do some tests prior to construction to see if it could be done. I believe we used 18 ga deck though, not 20ga.

 

[haynewp]

I took over a project a few years back that had welded side laps specified and the contractor complained several times about burning through during construction. I remember the guy I talked to at SDI about it said more or less it takes a better than average welder but it would work. After I relayed this to the contractor I didn't hear anything else from them. It says in the paper posted by wannabeSE above that they should not be specified for 22 ga or lighter.