UPLIFT ON METAL ROOF DECK 2

[sedesigner06]

I am looking into the uplift capacity for a 3'' 20 gauge metal roof deck with supports at 14'-0'' OC. In zone 3 of my components and cladding wind load with and effective area of 50 SF (I am considering a 3' wide sheet 14' long) I am getting an uplift of 92 PSF. If I put that into a an ASD load combo of 0.6D + 0.6W net uplift is about 54 PSF.

Am I considering this correctly? Are a majority of the load table out there for metal deck allowable to downward & uplift load?

If I am considering this correct, I am thinking about cutting down the spacing in the corner beams to 9'-3'' since the decking does not have the allowable capacity for 54 psf at 14' spacing.

 

[Hokie93]

With the information given, I would say you are correctly analyzing the situation. You may be able to use a slightly larger effective wind area (65 square feet) by taking advantage of the definition of effective wind area in Section 26.2 of ASCE 7-10. Have you correctly assessed your dead loads? You should only count on the dead load likely to be present and then multiply by 0.6 but don't ignore dead load that is likely to be present just to be conservative. To answer your other question, I would say that most steel deck span tables are based on allowable or service-level loads, not ultimate or strength-level loads. Reducing the span seems like a reasonable approach. I don't usually see anywhere near 14'-0 spans for roof deck but, then again, I typically design with 1.5" deep Type B roof deck.

 

[MotorCity]

Depending on the type or profile of deck you have, the width of the flutes may not be the same on the top and bottom of the deck. As a result, the deck has a different section modulus depending on which side is facing up when it is installed.

In the Vulcraft catalog, this is noted by Sp (positive section modulus) and Sn (negative section modulus). If you install the deck as it is shown in the catalog and have a downward load, use Sp. If you install the deck as it is shown in the catalog and have an uplift load, use Sn. If you install the deck upside down from the way it is shown, then use the opposite values.

 

[Ron]

I would not span roof decking that far. Check your deflection under the loading and see what effect that has on the overlying insulation and membrane. Further, I don't know what type of attachment you are considering, but that's a lot of load to put on a puddle weld of dubious quality.

 

[Teguci]

This is probably not directly your problem but may be applicable and worth thinking about. Any ductwork, electric distribution, fire protection piping, etc... that is supported from the roof will also need to span the 14 ft which may get fairly expensive since some of these systems are good to about 10 ft max without custom supports. 6 ft is very comfortable.

 

[theshearstud]

The allowable load tables published are for gravity load only. I reached out to my Vulcraft rep a while back and asked about his. Your rep should be able to provide you with uplift load tables if you ask.

 

[JLNJ]

You might look at Factory Mutual for their deck span recommendations. They are conservative, but they have vast experience in deck uplift and roof/roofing damage.

 

[mike20793]

I use Profis Diaphragm from time to time with large uplift forces to check my deck and connections. I believe it does an interaction of uplift and diaphragm shear on the connections and deck.

 

[sedesigner06]

For the 28' bay I have revised the spacing to 9'-4 instead of going with the rather aggressive 14' spacing. As Tequci noted intermediate supports would likely be required for MEP systems (10' is the max span I am told by our MEP engineers). I spoke with the Vulcraft Rep and basically told me the same thing MotorCity stated. Most decks are actually stronger for uplift and downward load.

He is sending me a document to show the procedure of how the connections are checked for high uplift. I will share once I receive this document.

I was not familiar with the Profis diaphragm software. I will download and give it a shot.

Thanks!