Buckling Restrained Braced Frames in Risa 3D

[IngeIvan]

Hello,

Im starting with the design of a one story building with a rigid diaphragm (SDC D) and we are going to use buckling restrained braced frames.

Do you think risa 3D is a good software to do this? Are there any design tips for modeling the right stiffness of these frames? Any advice would be a lot of help! Are there any examples or design guide lines to do this in risa 3d?

Thanks!

 

[JoshPlumSE]

Well, RISA isn't going to design the BRB members, but analyzing them shouldn't be a problem.

I've had conversations with both the CoreBrace and StarSeismic folks at one point or another. Mostly about what it would look like for RISA to add a specific BRB member into the program. Based on these conversations there are some tips and such that might be helpful.

1) At one point, I was given a database of BRB members. This was essentially the core area of the brace, but with an artificially high moment of inertia so that buckling wouldn't control. That way, the axial code check on the member would be meaningful.

2) There is usually a stiffness adjustment factor (KF) that accounts for the fact that most of the cross section is larger than the core area. This would be accounted for by using a larger Young's modulus for the steel material assigned to the member.

3) You'd often use a rigid end offset at the ends of the brace to model the distance between the work point of the brace and where the "elastic length" of the brace actually begins. Either that, or you incorporate this difference into the KF factor.

At that point, you would get pretty good analysis results. You'd still have to obey seismic detailing provisions and design the connections and such. But, your brace forces, your drift calculations and such would be pretty good.

 

[IngeIvan]

Thanks, Josh.

I was in a star seismic seminar not too long ago and they had some RISA 3D frames up in the screen with specific sections showing the KF designation and stuff like that. Is this the section database that you are talking about? Is that available in any version of RISA? or do we have to go down the path that you mentioned (about manually increasing moments of inertia, etc)?.

 

[jayrod12]

Forgive my ignorance (I don't design in seismic zones) why not just pick an appropriate sized tube brace such that buckling isn't a concern anyway?

Based on my 2 second google searching you end up having to wrap the steel core with a tube to fill with concrete anyway.