Do Forces Recorded via Element Recorder Include Damping?

[salehi]

Dear Vesna,

Actually I have a fundamental question! I usually use Element recorder to record "force" in the first and the last sections of nonlinearBeamColumns. The question is whether the forces recorded through this recorder include damping forces assigned via Rayleigh command or they are only representing K*Delta (due to stiffness)? I'll be so grateful to you if you answer my question as swift as ever .

[vesna]

No, they do not include damping forces.

You can record damping forces at the nodes following this example:
recorder Node -file Node_2_DampingForce.out -time -node 2 -dof 1 rayleighForces

[tsteele]

Hello,

I have a curious problem when capturing the hysteretic response of a buckling brace using force-based beam column elements. This is based on the question by the first poster, salehi. When I plot the hysteresis on the brace in a system with only mass-proportional damping, the hysteresis from dynamic analysis is the same as from a similar incremental static analysis based on the displacement history from the dynamic analysis. However, when initial stiffness-proportional damping is included or used without mass-proportional damping, the hysteresis has some rounding that looks suspiciously viscous in nature; the forces are also slightly higher than in the static analysis. So with respect to the first question, I think the element recorder "localForce" for forceBeamColumn elements may include some force from the stiffness-proportional part of rayleigh damping (i.e. F = k*u + beta*k_init*v).

Please correct me if I am wrong, and help me determine some sort of solution. I have been puzzled by this response for a while now. If I am not wrong, is there a way to separate the damping force into mass-proportional and initial stiffness-proportional components using the rayleighForces recorder so I may correct my hysteresis? Is there maybe an element version of the rayleighForces recorder?

*Edit: Higher stiffness-proportional damping coefficients lead to more rounding of the hysteresis,

Best,
Taylor

[fmk]

the force beam column does not return any part of the damping force when "localForce" is asked of it. you can ask for the damping forces: "raylighForces" .. to be sure have a look at the source code. the part you want concernes the setResponse() and getResponse() methods.

http://opensees.berkeley.edu/WebSVN/fil ... lumn2d.cpp