Drag Strut connection to existing sttel roof deck

[Schneidly]

I am looking at installing a drag strut in an existing CMU building. The roof consists of a 1 ½” deep 18 gage steel roof deck spanning perpendicular to the new drag strut. The roof joists are “16H5” spanning 27ft at 6ft oc. I have to install the drag strut from under thus the fasteners would go through the steel member. The drag force is on the order of 17.8 kips ultimate. The length of diaphragm I have to attach to is 58 ft.
There is 1 ½ inches of rigid insulation above the roof deck and above that built-up roof. The roof is basically flat.
What member connection should I use and what sort connection should I use?
Thanks.

 

[msquared48]

Is it an option to locally remove the roof to weld the deck to the drag member so a top down weld can be done? If not, then side welds will have to be done along the side of the member.

I would consider a steel tube member of sufficient depth to clear the bottom chord angles of the OWJ, and use a section of flat plate as a connector to be welded to the tube members on either side of each OWJ. I would not disturb the OWJ in any form or fashion.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[a2mfk]

So the drag strut will be parallel and between the joists? Then Mike's idea is even easier.

An 18ga deck you could carefully weld to from underneath, but I think in that case an angle may be easier to weld to as the curved of the HSS would have to be a tricky overhead groove weld.

Alternatively, and probably much easier, you could attach a piece of light gage track or stud upside down over the HSS (on the ground)to form a cap, PAF it to the HSS through the top, place the built-up member up against the roof deck, and then TEK screw up through the track/stud and into the ribs of the deck. Use 1-1/2" long TEKs max and you'll stay under the top of the ribs so you won't be anywhere near the roof membrane.

You would have to play around with the geometry so they can get the fasteners in there you need for shear, like using a HSS3x3 and a 8" wide track for example, you could get multiple fasteners per rib depending on the shear...

You could do this in lengths cut to fit in the field to get around MEP stuff in the way, bracing, etc., and give them a typical splice detail.

 

[Schneidly]

The drag strut is parallel and between the joists. I cannot have the roof removed.

 

[a2mfk]

So will my idea or something similar work?

 

[Schneidly]

a2mfk,

I think your idea is really good. I think it will work. Thanks!

 

[Ron]

Agree with a2mfk....use stud track and screw to the deck.

Welding heavier structural members to sheet, even 18 ga. is tricky and requires two qualifications...D1.1 and D1.3. The joint is not pre-qualified, so a procedure qualification would have to be done if you are using AWS and most any of the codes in the US. Further, the welding will be done in the overhead position...a difficult position.

 

[a2mfk]

Good points Ron, I have avoided specifying welding light gage my entire career other than puddle welds through deck, but I know its done sometimes...

 

[BAretired]

Don't the existing joists act as drag struts? Why add another?

BA

 

[Schneidly]

BA,

The existing joists do not line up with the shearwalls I am dragging the force down into.

 

[BAretired]

Schneidly,

Can you add a six foot long member tied to each joist and anchored to the shearwall? That would seem to be an easier fix.

BA

 

[a2mfk]

You could post a roof framing plan if you want us to dissect your whole scheme But BA or someone else may have an easier way to skin the cat that you did not see, the ole "another set of eyes" phenomenon...