Metal Deck Diaphragm ASCE Wind Case 3
Metal Deck Diaphragm ASCE Wind Case 3
(OP)
I have looked through the forum for guidance on this question but did not see this topic specifically addressed. If I did overlook a thread then please bring it to my attention:
I am looking for some guidance or reference that describes in detail out how a metal deck diaphragm that is properly fastened to a building roof can address ASCE 7-10 Figure 27.4-8 Case 3. This wind case is to simulate wind at a diagonal to building. For my example, I have laid out a simple 80’x40’x20’ tall building (see attachment). I have also thrown in some basic loads for windward and leeward wind at the roof level on both sides. This is not a real life situation and is just an example as a talking point. Keep in mind I am asking this question because I traditionally use steel braces instead of a metal diaphragm to transfer loads to the vertical stability system. Generally, this has to do with the fact that the majority of the structures I work on are large industrial structures. I am curious more about metal deck diaphragms because I have seen many references that state that metal diaphragms are very commonly used. In my research I have seen only examples that show wind in one direction (along the long face of the building) but never the other side (short face) or on both sides. I have spoken to senior engineers at my work and they have stated that they do not know the answer to my question but did confirm that metal deck diaphragms are used in many industries. I look forward to anyone who can provide some clarity to this situation.
I am looking for some guidance or reference that describes in detail out how a metal deck diaphragm that is properly fastened to a building roof can address ASCE 7-10 Figure 27.4-8 Case 3. This wind case is to simulate wind at a diagonal to building. For my example, I have laid out a simple 80’x40’x20’ tall building (see attachment). I have also thrown in some basic loads for windward and leeward wind at the roof level on both sides. This is not a real life situation and is just an example as a talking point. Keep in mind I am asking this question because I traditionally use steel braces instead of a metal diaphragm to transfer loads to the vertical stability system. Generally, this has to do with the fact that the majority of the structures I work on are large industrial structures. I am curious more about metal deck diaphragms because I have seen many references that state that metal diaphragms are very commonly used. In my research I have seen only examples that show wind in one direction (along the long face of the building) but never the other side (short face) or on both sides. I have spoken to senior engineers at my work and they have stated that they do not know the answer to my question but did confirm that metal deck diaphragms are used in many industries. I look forward to anyone who can provide some clarity to this situation.






RE: Metal Deck Diaphragm ASCE Wind Case 3
Then add the two shears back together to get back to the effective shear in the metal deck for deck shear and deck fastening design?
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RE: Metal Deck Diaphragm ASCE Wind Case 3
- I am curious if this was what the SDI testing for metal decks is meant for a bi-axial load.
- How does the metal deck react to deflecting in two directions.
- Also, how does the perimeter beams and fasteners react when loads overlap since traditional the side beams take shear load while the top and bottom (chord) beams take tension or compression. When the load is in two direction these forces overlap which complicates the situation.
I guess I could design in both directions and keep the direct sum of the unity ratios under 1.0 but generally with steel the direct sum is overly conservative. I find it odd that all examples of this situation do not address this situation even though it appears commons based on ASCE wind requirements.
RE: Metal Deck Diaphragm ASCE Wind Case 3
RE: Metal Deck Diaphragm ASCE Wind Case 3
RE: Metal Deck Diaphragm ASCE Wind Case 3
RE: Metal Deck Diaphragm ASCE Wind Case 3
For what it's worth, I don't know that I've ever actually seen anybody design a diaphragm for the diagonal wind case. That's not to say that it shouldn't be done. It just doesn't seem to get done. You may well be on your way to becoming a pioneer of high quality diaphragm design. Of course, I practice in a technical backwater so... there's that.
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.
RE: Metal Deck Diaphragm ASCE Wind Case 3
So any piece of deck within your diaphragm simply sees "shear".
But for load path into collectors and braces you need to break it down directionally.
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RE: Metal Deck Diaphragm ASCE Wind Case 3
KootK - I think the method JAE proposed is logical. I would be curious to see an example worked out. I question why SDI, Vulcraft, New Millenium, etc. would not have this as one of their example problems (unless I overlooked an example problem). I am hesitate because the SDI method values are reliant on testing done by SDI and the results are slightly black box (in my opinion). It already seems kind of odd to me that you can just place a metal deck spanning in either direction then go to some charts and lets say choose a 36/4 puddle weld pattern with 2 sidelaps and you get a shear value that is good for loading in either direction. It took me a while for me to just not get too detail orientated about that situation and just move past it and accept their results. As for the second part of your response, I am also curious if there is some provisions in IBC or ASCE that allows the wind load design to be simplified to only consider one direction at a time. I have noticed for the most part that these types of buildings that use this design are relatively short less than 60 feet and also about a 2 to 1 length to width ratio or less. That just might be a pattern that I am seeing that means nothing but I am curious if there is any benefit to simplifying either the wind loading or potentially relying on another means for wind load distribution along the short face (such as a steel beam or a precast concrete wall). Also regarding the pioneer of "high quality diaphragm design" that made me laugh :) I am far from it. I generally like to know what I am getting into with my designs so I can avoid structural engineering "gray" areas as I refer to them. Personally, I like to feel confident about my designs and point to solid references or examples. I guess in doing so AISC & ACI have seen their fair share of questions from me...I guess I will add SDI to that list too.
BTW, thanks for everyone's responses. It is helpful for me to critically talk it out.
RE: Metal Deck Diaphragm ASCE Wind Case 3
I think that it's a tricky position for those folks to be in precisely because diaphragm design is a grey area, at least with respect to what individual consultant design practice is. I imagine that they want to show you enough that you'll know how to work with their products but not so much that you'd ever mistake their guidance for being complete and absolving you of the need to exercise your own judgement.
If you ever have a hankering to get a sense for just how deep this particular rabbit hole goes, consider procuring a copy of the book referenced at the end of this article: Link. It's for wood diaphragms but most everything is applicable to steel deck as both utilize the same shear panel philosophy. Unfortunately, once you've seen it, you can't really unsee it. Prepare to either hemorrhage fees or relax your stance on avoiding grey areas.
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.
RE: Metal Deck Diaphragm ASCE Wind Case 3
JAE - I am following your logic. I threw together a basic loading layout to see if you had any more thoughts. I assumed that the vertical bracing is along the perimeter in both directions. I added color this time to make it clear. Purple is the deflection (scaled up for clarity), Green is the loading on the long face along with chord reactions, Blue is the loading on the short face along with chord reactions. The red clouds are corner areas that appear to have local load concentrations. What are your thoughts?
Thanks again!
RE: Metal Deck Diaphragm ASCE Wind Case 3
RE: Metal Deck Diaphragm ASCE Wind Case 3
Assuming a 200 ft. x 100 ft diaphragm.
Total E-W shear (perp. to the 200 ft. side) = 300 plf x 200 / 2 / 100 ft = 300 plf shear on diaphragm side along N and S sides.
Total N-S shear (perp. to the 100 ft. side) = 300 plf x 100 / 2 / 200 ft = 75 plf shear on the diaphragm side along the E and W sides.
Shear in the corners (where your red clouds are) - shear added together directly = 375 plf.
Now look at the singular direction of wind (Case I) -
(300 plf)/0.75 x 200 ft. width / 2 / 100 ft collector = 400 plf > Case III wind shear (375 plf) in terms of diaphragm shear.
So for the diaphragm - in this case, it may not ever have Case III controlling for diaphragm shear.
For other irregular buildings that may not be the case.
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RE: Metal Deck Diaphragm ASCE Wind Case 3
RE: Metal Deck Diaphragm ASCE Wind Case 3
A good example (I think) might be this situation - a very interesting case technically as well as ethically: Case Study - The Citicorp Center Design
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RE: Metal Deck Diaphragm ASCE Wind Case 3
The amount of questions I have asked about diaphragm design and the amount of silence I have received from experienced engineers is very interesting.
Makes me wonder why I do the calcs... (no one else seems to), but it makes me feel good to be able to put a number to important items rather than just guessing.