How to Model Flexible Metal Deck Diaphragm

RE: How to Model Flexible Metal Deck Diaphragm

Jason McCool — Wed, 24 Aug 2022 18:40:34 GMT

OK, that's what I thought. We normally use RSS, but there was some concern about the validity of results for horizontal braces in RSS, and I was asked to make an alternate model in RAM Elements to crosscheck and determine the load on them. The building has an untopped metal deck with no interior lateral frame lines, so the idea was to do a horizontal truss along the long side of the building only. But the axial loads in the horizontal truss adds significant load to the vertical braces in the exterior walls making it more of 3D problem. For instance, a horizontal truss web might have a 100k-200k axial load in Elements under wind perpendicular to the long side of the building. That then induces a large load in the vertical bracing in the case where the wind is perpendicular to the brace line, which would not normally be the case. But the horizontal brace loads are MUCH less in RSS.

In RSS, I have a semirigid diaphragm set for the roof so I can play with the stiffness and go from essentially flexible to essentially rigid and see the impact on brace loads and drift. In Elements, I'm using area loads to distribute my roof loads and wall pressures, and a thin shell with a custom material using a modified modulus of elasticity for the roof membrane, which seems to mimic the semirigid diaphragm of RSS qualitatively, but not quantitatively. For instance, if I use a modified E of 1,000ksi up to 24,000ksi in my noncomposite deck properties in RSS and the same values for E in my custom material in Elements, I get similar decreases in brace force and building drift as I increase deck/shell stiffness, but horizontal truss forces in RSS are maybe only 10-20% of the values in Elements. Vertical brace forces seem to be closer, around 70% of the Elements values. Lower brace forces would be nice but I want to make sure that's justifiable and that the deck really is contributing as much as RSS indicates.

Also, in RSS, we've been modifying E to E' as per the example on this site when using semirigid diaphragms. But do we need to be using the stiffness modification values as well to account for the one-way action of the bare steel deck or is it still accounting for the defined deck direction in the semirigid analysis when those factors are not enabled? I've never used those before and just wondering if I should be. Would RSS be treating the semirigid deck as uniformly bi-directional (like Elements) if I don't adjust those stiffness modifiers?
Thanks.

RE: How to Model Flexible Metal Deck Diaphragm

Seth Guthrie — Tue, 23 Aug 2022 22:11:08 GMT

Not in Ram Elements. Diaphragm shells with non-uniform bi-directional behavior are possible in RAM SS and Ram Concept though.

RE: How to Model Flexible Metal Deck Diaphragm

Jason McCool — Tue, 23 Aug 2022 13:51:50 GMT

Is there a way to model shells in Elements with different properties in orthogonal directions or are they only homogeneous? For instance, if you model an untopped metal roof deck as a thin shell in Elements, it would be just as stiff both directions, would it not?

RE: How to Model Flexible Metal Deck Diaphragm

Max Natzet — Thu, 21 Feb 2019 21:29:43 GMT

If you have already selected a deck for your diaphragm and the attachment pattern, most deck manufacturers publish tables to calculate the diaphragm shear stiffness (G' in Vulcraft tables e.g.). I would then determine the size of fictional flat plate elements which match the stiffness. The plates will be extremely thin, but it will capture the load path.

RE: How to Model Flexible Metal Deck Diaphragm

Jason McCool — Thu, 21 Feb 2019 21:03:48 GMT

Any suggestions for when the diaphragm is sloping? I have the same issue of wanting to apply a lateral load to the perimeter beams, but my roof slopes, and Elements won't allow different elevations for nodes in a rigid diaphragm. If I model the roof as flat at the max roof height, my braces on the low end overstress in compression with the size brace I'd like to stay within. If I model it flat at the low height everything works, so I can envelope it that way, but is there a better way in that case? Translate my wind loads to nodal loads instead distributed loads?

RE: How to Model Flexible Metal Deck Diaphragm

Seth Guthrie — Mon, 30 Apr 2018 22:15:56 GMT

You can assign a rigid diaphragm constraint which are called floor diaphragms in Ram Elements in the nodes tab of the spreadsheet.

If flexibility of the diaphragm is critical then using thin shells is a possibility (with some side effects to consider), or simulate the diaphragm with a few diagonal braces in the deck plane.