Tension-only members in RISA 3D
Tension-only members in RISA 3D
(OP)
Hello,
I am modeling some X braced frames and I want to use 3/4" threaded rods as my braces. I am trying to set them as "tension-only" members in RISA 3D, but as I apply any gravity loads, the model just goes unstable. I already set their Self Weight to zero. Also I have tried the Euler buckling strength, but as they are just rods, this strength is almost zero. Is there any other way to model these braces??
Thanks
I am modeling some X braced frames and I want to use 3/4" threaded rods as my braces. I am trying to set them as "tension-only" members in RISA 3D, but as I apply any gravity loads, the model just goes unstable. I already set their Self Weight to zero. Also I have tried the Euler buckling strength, but as they are just rods, this strength is almost zero. Is there any other way to model these braces??
Thanks






RE: Tension-only members in RISA 3D
RE: Tension-only members in RISA 3D
Remember that RISA is not doing a true non-linear solution for those tension only members. Rather it is iterating a linear solution. By that I mean that the program first assumes all members are active, then solves the model. Based on the results, it decides which tension only members are in compression and turns them off for the next iteration. At the end of the next iteration, the program may turn some members off and other members back on. The program determines convergence when there are no load reversals in these tension only members.
If the program experiences an instability for an iteration, then it deals with it in the normal way (by locking certain degrees of freedom). What is likely happening for your gravity only solution is that all braces go into compression (and get turned off) then the model is re-solved.... only now it doesn't have any lateral force resisting system. Hence the program reports an instability.
There are a number of ways to deal with this. For rods or cables (like you have in your model) I would suggest adding in a negative thermal load to give the X bracing some pre-tension because they will likely have pre-tension in reality.
RE: Tension-only members in RISA 3D
Mike McCann
MMC Engineering
RE: Tension-only members in RISA 3D
RE: Tension-only members in RISA 3D
RE: Tension-only members in RISA 3D
---Sorry for hijacking!
RE: Tension-only members in RISA 3D
RE: Tension-only members in RISA 3D
Eric's suggestion (adding a small lateral load) is akin to a "notional" load such as those used by AISC for their direct analysis method. The only sticking point is that it has to be large enough to overcome the gravity loads tendency to cause compression in the braces. So, it should work most of the time. But, it won't work ALL of the time.
Bookowski's suggestion (adding a lateral spring) can work as well. It will work well for all gravity only load combinations because it truly adds lateral stability. Make the spring too stiff, though, and it will also affect your lateral load combinations. Make it too soft and you will still be prone to P-Delta type instabilities in your frame.
Any of these solutions are okay. I prefer the pre-tensioning because it feels more robust and more realistic to me. But, that's a personal preference based on my on personal engineering judgment. Sometimes, if pre-tensioning isn't enough, I will add in notional loads to the gravity LC's as well.
RE: Tension-only members in RISA 3D