Tension Only Bracing with Seismic Analysis

[akef.amro850095]

Has anyone have problems with tension only bracing when seismic analysis is carried out. I am getting unrealistic forces which doesn't correlate to the base shear or floor seismic forces. I am setting the compression members as inactive members to get the forces but I wonder if anybody have a solution

Can anyone advice

Thanks

[dratsiox]

I have discussed this problem with some colleagues.This is not a problem of Robot, rather a theory problem.

If you are using the response spectrum method, then the problem is dynamic linear.That means that all of your members should behave linear,too.When you are using tension only members, then these members are non-linear.Thus, the results should appear some problems.

In order to combinate both tension only members and dynamic analysis, you should make a non linear dynamic analysis (time history).Another way is to change these members into truss members and design them for both tension and compression in the response spectrum method.

There is also a nonlinear response spectrum method but as far as I know it has not been implemented in any commercial software

[zhukov_25]

My opinion is: Don't use tension only elements in analysis, go with regular members. When analysis is done take the compresesion and tension force and make summary, and with the sum force design the member like tension element. that solve your problem. If you make tensinon only elements you have new redistribution of the internal forces.

[rigid_joint]

as dratsiox right said above, it's not a robot problem cause every software deals the same with this issue. modal nonlinear analysis has been proposed but it doesn't get much popularity.so. at the time, time history analysis is the solution to this problem

[waldemar.okapa]

You must be very carefull performing modal and spectral analysis on a non-linear elements. Keep in mind , that in case of non-linear element the modal and spectral analysis is carried on the precedent case stiffness matrix !
For example there is a wind load case, where some tension-only element were switched off. Then the next is modal analysis, giving vibration modes for spectral, seismic analysis. In fact you get vibration on that state calculated in wind load case only.