Light gauge steel for roof deck bearing support

[Robin7]

We have been asked to review and engineer a steel roof deck support detail proposed by the steel truss fabricator as an alternate to the detail provided by the EOR. The original detail by the EOR is a continuous 1/4” bent steel plate welded to a continuous embed pate with headed studs. The proposed detail is a post-installed light gauge bent plate (similar to a hat channel) attached to the concrete beam using Tapcons. The wind loads are 150 lb/ft uplift (net) and 285 lb/ft diaphragm. We are assuming the wall is braced by the tie beam (not the deck) at this time so no wind loads perpendicular to the deck (we need the EOR to verify this). Has anyone used light gauge steel for steel roof deck bearing or have any other ideas on how the deck support could be post-installed? Sketches of the current detail and proposed detail are attached for clarification.

 

[woodman88]

With the proposed detail having so little strength compared to the original detail I would require payment before doing anything. That being said, I would have the client (or EOR) give you the required forces (up, down, right, left, and parallel) to design a detail for.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Ron]

I have a couple of issues with this detail.

First, I don't like to use Tapcons in tension for a structural application. Here you have the highest portion of your wind uplift applied to an easily compromised connection. In addition, you will have a shear component applied to those fasteners as well.

The second issue I have is light gage material sitting flat on concrete. That can easily be a corrosion issue, again with compromised uplift resistance.

The third issue is that of compatible strengths between the 5/8" puddle weld as compared to Teks screws...check it carefully.

 

[Robin7]

Thank you both very much! At this point, I am being paid to provide preliminary analysis of the detail. Once we determine if it is feasible, I will ask if there are additional loads not listed in the structural notes that the EOR is expecting to be transferred with this connection. I must admit, there are many things I don't like about the detail as well and if I were EOR my detail would likely have been similar to the original detail. Although I have used Tapcons to attach roofing members (such as wood blocking), I don't usually use Tapcons for structural connections either. That being said, I have been asked to analyze the detail.

I appreciate any argument points - pro or con (such as the light gauge sitting on the concrete). My client is a light gauge steel truss and sheet metal fabricator and they can only bend up to 14 gauge steel in house.

 

[hokie66]

I will take the role of the EOR and just say "NO!".

 

[ron9876]

I would go with no also. Be sure to check bending in the horizontal legs due to uplift. Also any lateral loads will bend the vertical legs thru their thickness.

 

[Robin7]

Thank you ron9876. In addition to checking the hat channel as a section I also checked the sloped top face and the vertical legs as individual elements. We reduced the dimensions of the hat channel so the sloped top and inside vertical leg are shorter than shown in the PDF. When checking the vertical legs as a cantilevers with the lateral component of the uplift the deflection ratio is no good using 14 gauge, but okay if a thicker steel is used. (Sorry I should have mentioned that in my original post.)

I was hoping to see if anyone has used a light gauge steel detail for this application; it doesn't sound like it. I think I have enough argument points now for my client to tell the GC the detail will not work (atleast if my client fabricates it in house).

Thank you all for your input!

 

[ztengguy]

Can you just pour the concrete wall with the correct angle and embed a cont plate in the top, that would save all the labor of welding or tap-conning down a bent plate, or light gauge plate.

If not, the original one is my choice. Tapcons can easily be installed wrong.

 

[DST148]

Robin7: Agree with the posts above. But if you are stuck with light gage framing alternative.....
How about fastening clips on the sides of the wall and running a regular shaped member between glulam beams? This member could be fastened to the clips on the wall and glulam at the ends. This would avoid fabricating and analyzing odd ball shaped hat channel.
Can the deck span as a cantilever from the last truss to the end of the wall?

 

[MiketheEngineer]

150 psf uplift?? Are you desiging for a tornado??

 

[DST148]

@MiketheEngineer: May be you missed...The OP says 150 lb /ft net uplift