Virtual Joists 2

[Struct_Dre]

Hello All!

I am currently using virtual joists in my RISA3D model, because I am using 18k5 joists in my design.
Unfortunately, I've read info from SJI and talked to RISA support and I cannot figure out how to determine the equivalent/corresponding virtual joist for the steel joist that I am using.
I understand that VJ18-1 will be an 18" deep joist with "chord combination" 1, but I have no clue what the chord combination number represents. Has anyone used virtual joists before? If so, how did you determine which corresponding virtual joist to use?

I feel like there's such a simple solution, but no one I've asked seems to know. Any help is appreciated!!!

 

[r13]

I am the dummy here. What is "virtual joist"? Are you modelling the joist to its real configuration?

 

[JoshPlumSE]

I believe this is usually done the other way. You end up figuring out which "virtual joist" is required for your project to get it to pass deflection and stress checks. Then you tell the joist guys this and they come back to you and tell you what the actual joist is going to be. Maybe also suggesting you use a different virtual joist.

Essentially, it just allows the engineer and the joist fabricator to communicate better. When you tell them what you're getting from your analysis as far as virtual joists go, they know exactly how you modeled the joist and have a good idea what the stress and deflection requirements are.


Note:
To me, it's not as necessary for regular joists. I tend to think the Virtual Joist Girders tend to be a little more useful.

 

[r13]

Josh,

Is the virtual joist an analytical tool available in commercial software? Haven't heard it before.

 

[JoshPlumSE]

r13 -

It's reasonably new. It's been around for at least 3 years. Probably more like 5. If you go to the SJI (Steel Joist Institute) website and search on the term, you'll find more information.

I only know about it because they approached RISA 3 to 5 years ago (when I still worked there) and asked RISA to do for them what STAAD already had. Essentially, it was just adding in a series of I sections that would approximate some of their steel joists. That would allow the engineer and the joist fabricator to have less back and forth.

"We're using this particular "virtual joist" in our structural engineering model. Here is the loading and forces and such that resulted from that."....

"Okay, now I know exactly what self weight and moment of inertia you used. So, to resist those forces I can properly determine what joist I need to select for your project. If it matches close enough to the virtual joist that you used, then we're good. If not, I'll give you the name of a different virtual joist to plug into your model instead."

I don't know that this the interaction btw engineers and joist guys was all that difficult for regular K, LH, and DLH joists. Even the ones with some non-standard loading. But, it seems (to me) to make the most sense for Joist Girders that aren't as regular or which are part of the lateral resisting system.

 

[r13]

Thanks for the info.

 

[Struct_Dre]

Thanks so much for help! @JoshPlumSE That makes a lot more sense b/c I could not find any info on it.