Cold Form Steel Roof Joist To Wall Stud Connection???

[TekEngr]

I am designing a light gauge steel building. I proposed the below connection to client between roof joist to wall stud however outside wall is going up to parapet level as shown.
So I am confused about the strength of this connection does this scheme will work or not under the 6 KN load (joist is simply supported and carrying the UDL load). i am worried about the bottom flange bending of U profile (how I can check) and screw pullout strength.
Please advise how I can make this connection better and what check do I need to analyze.

 

[KootK]

It seems to me that you'd want to use the stiffener to transfer the load to the track web directly, similar to a clip angle connection in hot rolled steel.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[brut3]

Below is a link to a white paper by the Cold Formed Steel Engineers Institute on how to design a single leg of a deflection track for an out of plane load due to wind pressure. The condition in your detail above is similar in nature and may be a good place to start.

https://cfsei.memberclicks.net/assets/docs/technotes/tn-w100-08a.pdf

 

[XR250]

What Kootk said. Also, you have out-of-plane considerations for the connection which are tough to deal with. Also, if you shearwall material is on the outside of the studs, you need blocking to transfer the diaphragm loads (if that is applicable). That detail is not a pretty one but sometimes you are stuck with it. Might also consider platform framing is and attaching a 4" padded out parapet to the face of the studs if it aint too tall.

 

[DaveAtkins]

I would attach the joist directly to the stud. There is no reason to use the rim track.

DaveAtkins

 

[hawkaz]

I usually add Simpson L clips from each joist to track and from the track to each stud

 

[JAE]

I'd agree with DaveAtkins - extend each joist across the web face of each stud and use screws in direct shear to transfer both vertical and horizontal loads.

You might then need to add a small edge angle along the stud-joist interface to help support the floor sheathing/decking/diaphragm.

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[XR250]

You still have to add blocking between the studs to get the diaphragm shear to the outer face of the wall (if that is where you shear resistance is located). It also makes getting the load into the X-bracing tough. Best to platform frame these things when possible.

 

[southard2]

Frame it the way Dave Adkins suggested. Roof joist to the side of the steel stud with TEK screws. No way I would do it any other way if you want the wall studs to form your parapet. The diaphragm will most likely stop at the inside face of the studs so I would run a continuous light-gage steel angle there attaching it to each stud. The roof diaphragm would then attach to the light-gage angle. Just make sure the light-gage angle is at least as thick as the roof deck. Then use flat strapping along the inside of your shear wall below and plywood or OSB sheathing on the outside of the wall. That gives you a nice load path for both in plane and out of plane wind loads. And it helps keep things water tight. You will also have a continuous track at the top of the wall.

As for stud bracing since it is an axial loaded wall and one subjected to wind loads I would make sure both flanges of the studs are braced with blocking or sheathing. I would not rely on the gyp board sheathing for this.

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

https://www.pdhlibrary.com/

 

[TekEngr]

yes your understanding is correct i am using the 1.50mm thick stiffeners as clip angle to transfer the joist load to stud flange.

unfortunately wall studs and joist have already produced, so i cannot connect directly roof joist to stud's web (due to shorter length of joist) as you suggested.
the purpose of using the rim track is to increase the thickness at connection point because my wall stud thickness is just 0.95mm which is not sufficient i think , and rim track will also helpful to control the rotation of roof joist as you can see above i am stitching the roof joist from top and bottom side to the rim track flanges.

I used the Dietrich steel joist design guide tables (page 2.9) to determine the thickness,size of angle and no of screws so i found 1.5mm x 40mm x 40mm angle with 4+4 gauge 10 screws for 6 KN force but these table is only covers the shear force but what about the pullout strength due to tension force in above screws do i need to check the pullout strength of screw with respect to the stud/rim track thickness ???

 

[KootK]

With respect to transferring the joist reaction, I think that the screws can be reasonably approximated as fasteners transferring shear only. As XR pointed out, you'll have wind loads pulling the wall away from the track. That will result in a modest pullout demand on the fasteners as you've shown the detail.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ron]

shahg123.....a few comments:

1. I agree with Dave, JAE and others who noted that the joists should be attached to the web of the studs instead of the flange.

2.

the purpose of using the rim track is to increase the thickness at connection point because my wall stud thickness is just 0.95mm

....this is not true. It doesn't do this. You still have the screw load going through only one thickness of the flange in the line of action of the shear load. You would need a backing strip on the back of the flange to accomplish what you thought was happening. Using the rim track divides the load (potentially); however, does nothing to increase the flange thickness of the studs.

3. I'm not a fan of light gage framing for low slope roofs, particularly with a parapet. The long term serviceability of the structural framing system can be seriously compromised by water intrusion at the parapet, the roof and the interface between the two. Further, depending on the roof drainage design you might have a water load at the parapet-to-roof interface that's greater than any other location on the roof. If you are in a snow load area....even worse! This will significantly increase the shear load on your fasteners. You should add a qualifying statement to your drawings that makes sure the roofing/waterproofing are given critical review for this application.

4. You have a redundant call-out for your screws. A #10 screw is not 10 gauge. Don't confuse the issue with additional information that is not relevant. Also, a #10 screw is fairly small.

5. The wind load on the parapet will put your screws in tension as well as having a vertical shear load from the roof loading. These fasteners are much better in shear only, which is what you would have for both loadings if you had the joist extending through the wall section.

6. Light gage members are cheap. Even though the joists might have been cut already, change them! That's a lot cheaper than a failure!

 

[Ingenuity]

4. You have a redundant call-out for your screws. A #10 screw is not 10 gauge. Don't confuse the issue with additional information that is not relevant. Also, a #10 screw is fairly small.

Ron:
In different parts of the world screw nomenclature gets 'screwed' up. For example, in Australia (Metric country, with some hang-ups with non-metric hardware) "10-16" is referred to as a "10 gauge screw" (with 16 threads per inch). Granted this is NOT the same as a #10 screw (a North American term?), but the OP did not state that, and given that the drawing is detailed in metric units, I think the call out is correct.

Now, if the world was rid of 'gauge' and it was universally metric it would be a better place and we would not be having this conversation

For example: Link

 

[Ron]

Ingenuity....you are correct; however, my point was not necessarily the "gauge" vs. "the #", but moreso the inclusion of both in the reference.....a specification writing no-no! I agree that such terms are confusing and mean different things in different areas. Your table shows a 10-16 and a 10-24, which are generally Imperial designations, but also includes some metric units for the drill sizes and several screw sizes. Easy to be confused!

Strange how the screws get larger as the "gauge" goes up, but for most other "gauge" designations, the smaller gauge number results in a thicker material (wire gauge, sheet metal gauge, etc.).

Yes...let's dump the "gauge" and use thicknesses and diameters.

 

[Ingenuity]

Yes...let's dump the "gauge" and use thicknesses and diameters.

Absolutely agree.

 

[TekEngr]

i am using buildex screws which have some resistance against pull out force with respect to the materiel thickness. total 4 screws are enough in my case for each studs against pull out force according to my calculation ( 0.682 KN/Screw pull out strength total force 2.2 KN/stud.

i am slightly changing the connection detail i am providing 2 horizontal members in wall @ top and bottom level of rim track as shown below which can provide more space for screws.

for this structure we are using the rain screen cladding system (outer face of the wall) which mean cladding material will not directly attached with studs , there will be steel omega profile between the wall stud and cement board

 

[XR250]

Is this wall going to be braced by strapping or sheathing? If strapped, where are the straps attached?

 

[TekEngr]

wall will be braced by 1mm thick double strap (cross bracing)

 

[XR250]

If the strapping is attached to the track at the top of the parapet, you have to get your diaphragm shear load up there somehow.
How tall is your parapet?

 

[TekEngr]

1.2 m parapet wall height from center of joist