Plastic hinges and forceBeamColumn elements

[nitadorakis]

Hi!

I would like to examine the plastic hinges' formation during pushover analysis and I'm using forceBeamColumn elements with fiber sections.
Which is the most appropriate way I could work with?
Is a plastic deformation recording a solution?

Thank you

[vesna]

Yes, plasticDeformation is the response you can use for this purpose.

[nitadorakis]

Thank you very much

As far as I know, plasticRotation recorder records plastic rotation at each end of an element (epsP,theta_P1, theta_P2).
My questions are:
1) is this only the plastic part of the rotation or it includes the elastic part as well i.e. total rotation?
2) what exactly is the epsP?
3) are the values in rads?
4) I get values such as 1e-05 at the beginning of pushover analysis and 1e-3 at the end. Are they logical?


Looking forward for your reply.
EN

[vesna]

1) These only include plastic part of rotations
2) epsP is axial plastic deformation
3) yes, the values are in radians

[nitadorakis]

Thank you for your help.Your reply was very useful!

I was wondering, since there are values for plastic rotation at both ends of all the elements at all storeys even at the beginning of the analysis (1st step), how should I know where and when 'plastic hinges' are formed?
Is there something wrong with the analysis or is there any lower limit for rotation (i.e. e-03) above which I should assume that a hinge is formed?

[vesna]

With a forceBeamColumn elements there is no sharp boundary which will indicate formation of plastic hinges. You will have to use your judgment to set the limit for this. If the elements are designed well plastic rotation of hinges start after the reinforcement yields. You can look at stress-strain relationships for reinforcement bars in tension to find a lower limit for plastic rotations.

[nitadorakis]

Thank you.


So, I suppose I should:

1) create a recorder like this:
recorder Element -file Element1_9StressStrain.out -time -ele 1 9 section 1 fiber 0.10 0.10 stressStrain.... (but there are about 8 steel fibers at each section!)

2) plot the results and see for which Base shear value the stress-strain relation is not linear any more, in those elements

3) see the value of plastic rotation for that value of base shear and set this as a limit for the rotation that indicates a plastic hinge formation


I don't know if my thoughts are correct, but do you know if there are any specific limits for plastic rotation set by other researchers or in seismic codes that I could use with forceBeamColumn elements?

[vesna]

Yes, this is what I meant.

I do not know of any limits set by other researchers or seismic codes.

[nitadorakis]

Thank you!
Time-consuming, but since there is no other option with distributed plasticity elements, I have to try it.

[amir70007]

Hi...
I have a few question about recorder command:


"stressStrain – record stress-strain response.
example: recorder Element -file ele1sec1StressStrain.out –time -ele 1 section 1 fiber $y $z <$matID> stressStrain
$y
local y coordinate of fiber to be monitored*
$z
local z coordinate of fiber to be monitored*
*NOTE: The recorder object will search for the fiber closest to the location ($y,$z) on the section and record its stress-strain response

If I have a fiber section with 10*10cm dimensions with 8 subdivisions in each direction there is 16 fibers in my section.
1-Can I get the ouput for a specific subdivision or all of them in one line command?
I do not understand what the *NOTE means in this command exactly.
2-For example when I assign "fiber 0 10" for the central fiber in my section, which subdivision of the fiber is monitored in the section?the center subdivision in the top layer?
3-Is there any way that I can get output for all sections of an element in one command line?
Thank you...

[vesna]

1. As of now you can record response of one fiber only.
2. If you create output in -xml format you will see the exact coordinates of the fiber whose response you have recorded.
3. I believe you can get output of all sections if you omit section numbers.

[amir70007]

Hi...
If Plasticdeformation records only plastic part of rotations why the results are not zero for elastic response of the element?
(I have a recorder for stressstrain of fiber sections that have Fy=2400kg/cm2 to check when fiber section is inelastic)
In other words if the stress in fiber sections is less than Fy, Plasticdeformation should record 0 for rotations and when Fy exceeds 2400 it should record nonzero results.

[vesna]

Many users have complained about plasticDeformation recorder argument for FBE. Non-zero values before yielding may be a consequence of round-off error. If the values of plastic deformation are not very different before and after yielding I would suggest you to avoid this recorder.