Semi-Rigid Diaphragm for Metal Roof Deck 2

[STR04]

Has anyone modeled a semi-rigid metal deck diaphragm in RISA-3D? I would like to find some guidance on adjusting my steel plate thickness and shear modulus (G) to account for proper deck stiffness with fasteners. I asked RISA support and they did not have any specific quidance on this topic other than tell me to adust these values. TIA.

 

[structSU10]

There is a tech note from bentley that could be helpful:

http://communities.bentley.com/prod...tructural_analysis_and_design__wiki/3348.aspx

At the bottom it has a calculation and discussion for metal roof deck.

 

[JoshPlumSE]

I believe the Steel Deck Institute has a design manual of some sort which gives a method of calculating a revised G value based on assumed fastener slip.

We might have the reference somewhere around here (I know I've seen it before). But, I don't have it close at hand.

If memory serves me correct, the procedure is to use the gage thickness of the deck as the plate thickness. Then you just have to modify the G value based on fastener spacing.

 

[STR04]

I found this in the Diaphragm Design Manual on pg 3-3 where the effective modulus G = G'/t; where G' is the adjusted stiffness and t is the deck thickness. I looked at the example RAM provided and they were calculating an adjusted E' - Effective Modulus. I'm not sure why you would change both G and E for your material properties in RISA.

For my RISA model I plan to revise the effective Modulus G based on my deck and attachment properties and leave E at 29,500 ksi. I will set my density to 0 kcf and keep the plate thickness as 20 GA (0.358") plus check the plane stress. Does this sound reasonable? Do I need to back into an equivalent deck thickness based on the moment of inertia for a ribbed deck?

 

[structSU10]

That sounds reasonable. I am not a RISA user, and RAM doesnt let you mess with G for plates, so you modify E and have no shear deformation to simulate the deck. With the case with RAM, i keep the thickness as the actual thickness as well.

 

[JoshPlumSE]

Sorry for the slow response. My computer is having conflicts with Eng-Tips lately. Not sure why.

StructSU10 has it basically correct. Some program (which force you to use isotropic plates) will ignore the G value entry for the plates. RISA used to be like that. But, at some point (version 9.0-ish) we started considering the G value for plate elements. So, in the current version of RISA you would only have to modify the G value.

 

[STR04]

Thanks, Josh/structSU10! Great info.

 

[STR04]

Josh,

To close the loop on this, do you know if RISA automatically adjusts G based on the plate thickness you enter or do I need to make this adjustment myself like I describe above per the Diaphragm Design Manual on pg 3-3 the effective modulus G = G'/t; where G' where G' is the adjusted stiffness and t is the deck thickness?

 

[JoshPlumSE]

You will need to make the adjustments yourself.


At some point (if RISA does eventually do this in a future release) then we will have to ask for deck definition data where you specifically tell the program what type of deck is being defined. Maybe how often the connectors are spaced.... that sort of thing. RISAFloor is close to having enough information to do this, but not RISA-3D.