# GSD Hinge vs. Midas Hinge

Question:

I need someone to walk me through how the hinge created in GSD is recreated in Midas Civil when exported.

Axial Load:

For my case, hinge yield surface properties in Midas Civil show a Pmax of 9,785 kips and the component properties show an axial force of 15,000 kips.  That 15,000 kips is there because I could not get GSD to export a hinge for axial loads in the normal range, only for very large axial loads (is this normal? acceptable?).  Throughout the pushover analysis, a given column sees no more than 2,000 kips.

Are the hinge properties being recalculated at each pushover step using the correct axial load or is the hinge always using a very large axial load, thereby showing an exaggerated capacity?

Idealized Yield Curvature:

What yield curvature is being used as the governing limit in Midas?  Is the idealized yield curvature being used to set the point at which first yield occurs in the FEMA hinge?  What is being used as the ultimate limit?  Rebar or Concrete Curvature? Each are given in the GSD Moment-Curvature analysis.

I have attached my GSD and Midas Pushover files.

Answer 1:
Hi Kyle,

While Nandeep is looking into your query, we've developed some Q&As and technical materials while working with Caltrans on nonlinear analysis. You may not find them helpful for your immediate issue but for your reference in the future.

Answer 2:

Hello Kyle,

I believe the documents that Angela provided should solve the issues.  However, if any doubt still persists, kindly let me know.

Regards,
Nandeep
Technical Manager, MIDAS

Question:

What I am still missing is how the idealized yield gets imported from GSD.

Midas shows this:

That 3642 kip-ft is not the idealized yield strength at P = 1757.  Rather it is the moment when P = 0 kips as shown on the interaction curve.  So, for the given Axial Force shown, when M/MY = 1 @ Point B, is this suggesting that My = 3642?  I think probably not, but I can’t see where the idealized yield moment and curvature generated in GSD are being employed in Midas.

Answer:

Hello Kyle,

Thanks for getting back.  As you said, the moment being displayed there is for 0 axial force.  The yield moment is taken up based on the axial force from the PM curvature as exported from midas GSD.  Kindly refer to the image below.

Now, about the interaction curve values, these are taken up from GSD as shown below.

I hope this clarifies the doubt about the moment interaction in GSD & midas Civil.  If clarification is required in some other aspects, kindly let me know.

Regards,
Nandeep
Technical Manager, MIDAS

Question:

Nandeep,

Thank you, but you did not answer my question.  I cannot see where the actual idealized yield moment/curvature that GSD calculates for a section at a given axial load is employed in Midas.  In the directional properties dialog you show below, where point B = 1 M/MY, that should correspond to the idealized yield moment for that axial force, not the MY for P=0 that is shown in gray below.  Where does Midas reveal the idealized values pulled from GSD?  In other words, how can I be sure that when my GSD Imported FEMA hinge hits 2.0 (1st yield) in my pushover analysis, that this corresponds to the idealized yield strength shown in GSD?  If there is nothing that explicitly shows this, could your team create a simple model as a "proof"?

Also.. why does Midas only pull every other point from the GSD P-M interaction curve?  Doesn't that make for a much coarser analysis when linearly interpolating?

Below are 2 images from my Midas pushover analysis using a GSD Imported Hinge and 1 image from GSD M-phi analysis.
1. At Longitudinal Pushover Step 26 (1st FEMA Yield) I have an axial load at the top of far right column (top hinge) of P = -1374.9 kips
2. In the next image, you can see the associated moment at which that yield occurs, M = 4622.9 kip-ft
3. In the last image, you can see my GSD moment-curvature analysis for that axial load, Idealized Yield M = 7062 kip-ft

What am I missing?

Answer:

Hello Kyle,

Yes, this seems to be taking yield moment for design (P-M) rather than yield moment from moment-curvature.

This is an issue that the development team is working on.  In the meantime, the solution here is to manually modify the P & M values for the imported GSD hinge.  Kindly refer to the image below.
Step 1:  Right-click on the hinge exported from GSD to Civil
Step 2:  Click on Yield surface properties button.
Step 3:  Click on show value button.  The button will now change to Show ratio as in the above image.
Step 4:  Note the axial force value.
Step 5:  Update the moment value in accordance to the exported axial load.  Moment value is the yield moment as obtained from GSD for the mentioned axial load.
Step 6:  Change moment to Z axis and again, fill up the moment column.  To obtain yield moment about local Z axis, enter the neutral axis angle as 90 degrees in midas GSD.  The image below shows this and the yield moment in Z direction for the first axial load.  This needs to be done for all the axial loads.
I understand its a bit time-consuming process, but that's the only way out now.  The development team mentioned that they can't make this a quick fix and this improvement would be done in the upcoming version.
Hope this helps.  Kindly let me know if you have any further questions on this.

Regards,
Nandeep
Technical Manager, MIDAS
Creation date: 10/31/2019 3:29 PM      Updated: 11/6/2019 10:28 AM