Moment-curvature

Question:

I am trying to do a simple column moment-curvature analysis. It's a 4.5' dia. RC column. Longitudinal bars are 40 - #6 bars. Spirals are #5 @ 3" pitch with 2" clear space to the surface. Axial load is 1000kips compression.  I have the following questions below.

 

 

1.    The help manual doesn’t provide guidance on the appropriate inelastic material models to use for moment-curvature analysis.  I have defined the inelastic material properties using the Mander model for concrete and the Park model for steel. When I try to choose the concrete model in the moment-curvature window, the Mander model previously defined is not available. See image below and attached model.  Please advise on proper input definitions for moment curvature analysis in Midas.  Is there an example?  YouTube video examples?

 

2.    The first time I ran the model, I used the Kent & Park model for concrete, but the moment-curvature curve that it produced was incorrect.  As defined using the Mander model, it won’t perform a moment curvature analysis.  If Midas doesn't allow use of the Mander model, how do we input confined/unconfined concrete regions within the section (i.e. concrete within the spiral and concrete outside of the spiral)?

 

3.    As for the rebar inputs used to define column reinforcement, what do the Size 1 & Size 2 inputs correspond to? I only have one size of rebar for my longitudinal reinforcement. Also, what does the "Ties/Spirals Number" input define?  Moment curvature is a sectional analysis, I am unclear what this input defines.  Is it the number of spirals in the cross-section (i.e. interlocking spirals)?  Please advise. 

 

4.    What is shown in the Section Shape window?  It appears that only longitudinal reinforcement & section perimeter are shown.  Does Midas show confined & unconfined concrete limits within the section?  Spiral reinforcement?  Please advise, it is not clear from the input window what is being shown.

 

Thank you for your help in this matter.  I really appreciate your time and consideration on these issues/ questions.






Answer:

Hello,

Thank you for your patience.  Kindly refer the response to each issue below.
1.  The help manual doesn’t provide guidance on the appropriate inelastic material models to use for moment-curvature analysis.  I have defined the inelastic material properties using the Mander model for concrete and the Park model for steel. When I try to choose the concrete model in the moment-curvature window, the Mander model previously defined is not available. See image below and attached model.  Please advise on proper input definitions for moment curvature analysis in Midas.  Is there an example?  YouTube video examples?
A.  Mander model should be available!  The currently provided inputs are proper.  I'll ask our development team to look into the issue. 

2.  The first time I ran the model, I used the Kent & Park model for concrete, but the moment-curvature curve that it produced was incorrect.  As defined using the Mander model, it won’t perform a moment curvature analysis.  If Midas doesn't allow use of the Mander model, how do we input confined/unconfined concrete regions within the section (i.e. concrete within the spiral and concrete outside of the spiral)?
A.  This could be checked quite easily in midas GSD as well.  If that is available with your midas Civil license, kindly check it out for the time being.  In the mean time, we will get the Moment curvature issue resolved. 

3.  As for the rebar inputs used to define column reinforcement, what do the Size 1 & Size 2 inputs correspond to? I only have one size of rebar for my longitudinal reinforcement. Also, what does the "Ties/Spirals Number" input define?  Moment curvature is a sectional analysis, I am unclear what this input defines.  Is it the number of spirals in the cross-section (i.e. interlocking spirals)?  Please advise. 
A.  Software permits the use of two sizes of rebar for each position in each layer, i.e. Rebars with two different diameter can be provided at a single position in a single layer. Both the rebars are provided equally in number. Lets say we have 4 number of rebars in position 1 with Size 1 as P10 and Size 2 as P8, then 2-P10 and 2-P8 will be provided in that particular layer. Likewise the rebars are decided for the other position as well. If we have odd number of rebars say 3, then 2-P10 (Size 1) and 1-P8 (Size 2) will be provided.  If there is only 1 re-bar size, then the other box could be left empty.  Tie/spiral number basically means the number of legs (bars) to be used for shear check.  Actually it doesn't matter for moment curvature, but the same dialogue box is used for RC design.  Thats the only reason for the input. 

4.  What is shown in the Section Shape window?  It appears that only longitudinal reinforcement & section perimeter are shown.  Does Midas show confined & unconfined concrete limits within the section?  Spiral reinforcement?  Please advise, it is not clear from the input window what is being shown.
A.  This window only shows the section shape and longitudinal reinforcement.  Confined area is not shown here.  Again, this is shown in midas GSD, if that is available with you.  It wouldn't show the spiral reinforcement.

I'll update once I get a response from development team about Mander model.

Regards,
Nandeep
Technical Manager, MIDAS

Hello,

We have a response from the developers.
To use the Mander's model, properties have to be defined for both confined and unconfined concrete separately.  Also, the graphic view would display the confined and unconfined region.  Sorry for the earlier response to point 4!

Once the defining for both confined and unconfined is done, the Moment Curvature dialogue could be updated as shown below.


Hope this helps.  Kindly let us know if further assistance would be required.

Regards,
Nandeep
Technical Manager, MIDAS
Creation date: 10/4/2018 1:09 AM (dklee@midasit.com)      Updated: 10/4/2018 10:39 AM (dklee@midasit.com)
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Column_Moment_Curvature.mcb
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