Problem with Flexible Diaphragms in Risa 3D/Floor Interaction

[IngeIvan]

Hello,

I am modeling a pop-up roof with flexible diaph that is sticking out of another flexible diaphragm (see attached). I am having trouble getting the lateral load out of the pop up into the main braced frames of the lower diaphragms. I think we can achieve this just by transferring the forces through the deck in reality, but in Risa I think we need some kind of additional bracing to get the load out. I've tried rigid links and rods, but none have worked. Any ideas on how to solve this issue?

Thanks!

 

[JoshPlumSE]

So, the flexible diaphragm load at the "pop up" roof level attributes to the columns that directly support it at that level.

What you're trying to do is then have those columns transfer their shears into the lower diaphragm level to be pulled out to the exterior braced frame. Right?

If the lower level is a flexible diaphragm in RISA, then it has no stiffness. Which means it's not going to pull any shear out of those interior columns. If the lower level is a rigid diaphragm, then it has infinite (or near infinite stiffness) and it should pull out virtually all of the shear from those columns. So, that may be an option for you.

Now, in reality your "flexible" diaphragm has some rigidity and is somewhere between infinitely flexible and infinitely rigid. Therefore, you could model in some rigidity for that diaphragm. You could use a mesh of plate elements to do that. This is some extra modeling work, but it should be clear what to use for your plate elements (thickness and material properties). Alternatively, you could model in some fictitious X bracing to approximate the stiffness of the diaphragm and drag those forces into the braced frames. But, the tricky part with this is coming up with rational / appropriate stiffness properties (A,E) for those braces.

 

[IngeIvan]

Thanks Josh,

Do you know if RISA is at least thinking to provide a stiffness for flexible diaphragms based on connection pattern and metal deck in the future? Or are there other reasons why they are not doing this?

 

[JoshPlumSE]

What you describe is NOT a flexible diaphragm, but a semi-rigid or semi-flexible diaphragm.

We're not just thinking about this, we have already taken our first steps. RISAFloor + RISA-3D have already added an automated semi-rigid diaphragm for concrete slab floors.

There are a few steps to go before we get to what you're talking about. Extending this concept out to beam supported floors with concrete decks. Then extending it out to cover wood and metal decks as well.

I can't give you definitive time frames on when these items will get finished (which means definitely not for the Fall 2015 release). But, I can tell you that they're definitely on our To Do list.