Moment Frame and Cantilevered Column Analysis or RISA 3D

[jeffhed]

I have a project that is an open canopy but has drop shades around the perimeter so it can be an open or an enclosed building. There are existing concrete walls that must remain in place that are in between my transverse frame columns so I don't have a true moment frame in the transverse direction. Due to existing site conditions, the column locations cannot be changed. The frame would be like a true moment frame in the longitudinal direction with beams framing into the sides of the columns. The client wants to use an HSS frame, so I know I could get a size to resist the torsion at the top of the column and transfer it to the transverse beams. I have recently done a 2D HSS frame on another project with similar loads,so I know I can get the frame to work in the longitudinal direction. However, I am on the fence on the method of analysis. My first thought was to run an analysis on the 2D longitudinal frame. Then run a transverse analysis as cantilevered columns cantilevering from the beam at the top. The beam would resist the transverse loads through torsion and transfer the loads to the two transverse beams that are approximately 2'-1" away from the center of the column. I would calculate sidesway by summing the cantilevered column deflection and adding the amount of drift I get from the torsional rotation. But then I thought that this might be a good reason to move from RISA 2D to RISA 3D. I have attached an image of my RISA 3D model that I created in a trial version to see how I like the 3D version of RISA. I have done all of my frame design in the past with RISA 2D and I really feel like I won't use the 3D version very much after this project, so I am leaning towards the cantilevered column and torsion method rather than purchasing a RISA 3D license. Has anyone else designed a frame without transverse beams framing directly into the sides of the columns? Is my cantilevered column and torsion method along with a 2D frame analysis a sufficient way to analyze this frame? This frame is in a non seismic region.

 

[JoshPlumSE]

Well, good and bad news for you. You cannot buy a copy of RISA-3D anymore, the new ownership / management did away with that option. The best you can do is purchase a 1 year subscription. This is much less expensive up front, but much more expensive long term. Therefore, for someone like you (who won't need it much after this project), the subscription may be a good option for you.

That being said, you're basic concept of using 2D to design a 3D frame should be fine. Personally, I prefer using 3D models, because it ends up being less work on my end. And, because it's easy to demonstrate to plan check exactly what you've done.

 

[jeffhed]

JoshPlum,
I understand the subscription thing, I have a subscription for RISA 2D and realize that I would need to purchase a subscription for RISA 3D if I decided to go with it. With RISA 3D, does it actually generate the wind loads, or do I need to generate my own loads with spreadsheets, etc. and then enter in all of my loads on the model like I do in 2D?

 

[JoshPlumSE]

You would almost certainly have to do the wind load yourself. Last I checked RISA-3D could only generate wind loads for building type structures that have rigid diaphragms at each floor level.

Granted there are ways to area loads to the structure to represent the wind. But, these can a bit more clumsy than you'd think. So, if it were me, I would calc the wind loads outside of the program and enter them in manually.

 

[jeffhed]

Thank you, Josh.