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Miss match between Design Forces in the design and forces from bending moment diagram

Creation date: 1/2/2020 1:24 PM    Updated: 1/3/2020 12:16 AM
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Question:

Dear  Midas Team,

I have created new Ticket to let you avoid conflict with the other ticket, I really find a lot of discrepancy  in the results of Midas Design of Steel Bridge, However, While checking the forces of Element 358J in the excel sheet results, I found that the Moment of short term is 84533.711 kip.in while from the Bending moment Diagram is 73438 kip.in, I also did Break Down to the forces and added them manually using the applied load factors found they are matching the bending moment diaphragm result not the design result. (FYI I have removed all the envelope cases in the load combination cause I know it makes conflict), For Element 343i you will find that there is no discrepancy so I'm wondering why this element has this issue (I shouldn't check every element, this is wasting time, once one element gives matching results like 343i, all other elements shall be the same). I have attached the excel sheet and model for your check.
Also below snap shot. for element 358j.




Dear Waleed,

We are sorry fort he inconvenience. The forces that were checked for positive moment design forces for element 343i, we would be reporting this to the developer and let you know the updates.

Yes, if the behavior is verified for one element other elements shall not be checked. We apologize for the issue, we would try to resolve it at the earliest.

Regards
B Swapnil



Yes please, I appreciate your cooperation, Remember we already fixed the same problem for element 343 and I shouldn't have checked element 358 as i was saying the problem has been fixed with element 343 but now we have the same problem in 358.

These kind of issues are not letting the team here trust Midas and they will think to avoid it in the future.

Also when u extract the design of this element 358, it shall be done according to clause 6.10.8 in the AASHTO (because its a kinked girder) but the design is generated it as if its not kinked (using Appendix A), but I'm trying to update the design report to follow 6.10.8, I will send it to you once I finish for your INFO.

I will be waiting your response asap (I hope today) COB regarding this mismatching.

Thanks,
Best Regards,
Waleed A. Rashed


May be live load moment coming from the analysis needs to be updated to match the one in the design, Because we have a doubt about its value.
The reasons of this doubt is that we did a line girder model using another software we found that the difference in the moving load after using distribution factor is huge around 2000 kip.ft which is very big difference. we were expecting that the difference can be around 15% not almost double because grill-age analysis is less critical than approximate method from AASHTO.

Could you transfer my message to the developer, to have alook on it .

Thanks,
Best Regards,
Waleed A. Rashed



Hi Walid,

Please find some other application results comparison (MIDAS - MDX or 3D FEM - grilllage).

You can find other engineers' differences range. If you find differences greater than their range, there probably is a difference between the configuration of the two models.



Thank you very much Angela, I will check them.

But The issue here is  the difference in the Midas results it self not with other software, Midas output is different than Midas design value, I have explained in the previous snap shot.

Moment in Midas analysis (bending moment Diagram) is 217615 kip.in while the moment used in Midas design itself is 228711.444 kip.in.

they should match., I hope your team can fix this issues as soon as possible as it shows in alot of elements in the model.

Thanks,
Best Regards,
Waleed A. Rashed



Understood. Swapnil, please check and follow up with Walid asap. Thank you.


Swapnil,

my answer below doesn't answer WAlid's question on the fl. Please still take a look at his question related to fl.

Hi Walid,

Regarding the kinked girder can you please confirm if this workaround works for you after you try it out? I am going to request the update/fix on the software as shown below:

1) modify the feature so that users can mark it as kinked or curved directly
2) display a warning message that kinked girders also require curved bridge information input and change "Curved Bridge Info" menu name to "Curved or Kinked girder info".



Yes Angela, I agree.

I suggest to add an option in the design tools defining girders to be curved or kinked or straight. I prefer to have it optional for each girder in the model as you can see in my model I have 8 girders straight (they should be designed as straight) and two girders kinked ( they should be designed as kinked) so to have this feature optional for each girder will be good.

This is just an suggestion.

Thanks,
Best Regards
Waleed A. Rashed



Dear Waleed,

Yes, even when I was performing the comparison for the expected moment in the design report the discrepancy is occuring due to the component of moving load. When it is removed the value matches, we have asked the developer to solve it at priority basis.

Regards
B Swapnil Agarwal
Technical Support Engineer
Global Technical Center



So which value is the right value, moving load considered in the design or the one considered in the analysis.
FYI we did a line girder analysis for moving load considering AASHTO approximate method we got moment 5200 kip.ft compared to Midas analysis 3200 kip.ft (huge difference), we were expecting difference around 15% as it is grill-age analysis.

Please update me asap.

Thanks
Best Regards,
Waleed A. Rashed



Hi Walid,

For girder line analysis vs. MIDAS analysis comparison, our support team cannot give any meaningful feedback without knowing in-and-out of the comparison and also if your midas Civil or other analysis case were set parallel in terms of inputs and condition set up. Swapnil will review your question later but I wanted to mention this in advance.



I understand, All what I want know,  which value of live load is right between Midas design and Midas analysis, Because it seems that the moving load is the issue, Then later we can compare with line girder analysis and see how much percentage is different.

Let us see what is the developer will say.

I hope to get it asap.

Thanks,
Best Regards,
Waleed A. Rashed



Dear TS team,

I delivered the feedback from the development team to Walid, but he was not able to agree with the feedback. Walid will share his justification soon, so I would greatly appreciate if you could assist him further.

Moreover, Walid mentioned to me that his display value is different from the values from the analysis result (table) and design. Walid will soon share his screenshot, so please review and take this issue into consideration. Thank you very much!


Dear Walid,

As we discussed over the phone, I would appreciate if you could share your justification for our developer's feedback written below. Also, I would appreciate if you can post the screenshot of the diagram, analysis result (table), and design result. Thank you very much.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Incorrect concurrent forces cause the difference between analysis results and design forces due to moving loads. This issue is caused partly by the wrong definition of traffic line lane and partly by the program error.


  1. Wrong definition of traffic line lane

To apply two trucks to find the critical negative moment, the user should specify ‘Span Start’ at the interior support to separate individual spans. In the user’s model, this is not specified in some lanes.

  1. Program error

When ‘span start’ is not specified so that there is just one span in the traffic line lane, two trucks should not be applied. This is not working when '7th dof ' is taken into account.


Therefore, the user should correct the input error in the traffic line lanes, which will provide the same values between analysis results and design forces. The program error is fixed with which the values between analysis results and design forces will be the same, but still, the correction in the traffic line lane should be done for accurate results.

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Best Regards,
Hope Kang
CAE Consultant
hopekang@midasoft.com
MIDASoft Inc.



Thank you very much Hope for your message.
What I found that the values in the result table match the value in the design but not matching the value in the bending moment diagram.








I have attached the design tables, the first one match the result table but not matching the design diagram, the second table doesnt matching any one.


Could you please check and let me know, Do I need to send you the model ??, I think it will appear in the same model you have.

Thanks
Best Regards,
Waleed A. Rashed



Hi Walid,

Can you please send the model and excel files to ensure we have the right information/bases?



Here it is, Design Sheet and Model.


Thanks,
Best Regards,
Waleed A. Rashed



Dear Walid,

After the application of the patch the discrepancy has been removed between analysis results and the design results, kindly apply the patch then the design results are matching the analysis results.




The patch is now available.


To apply the patch, please follow the steps below:
1) Download the patch by Clicking on above patch link
2) Exit any open project and close all midas Civil windows
3) Extract the content of the file
4) Paste them at the location where midas Civil is installed. The default location is - C:\Program Files\ MIDAS\midas Civil
5) When prompted to replace existing files, please click yes
We are attaching the modified file and excel sheet for your reference.

Kindly let us know if further assistance is required.

Regards
B Swapnil Agarwal
Technical Support Engineer
Global Technical Center



So I will be having two patches one in ticket number 

#24911980  


and the other one here, or both are same ??


Gents,

I applied the two patches and still getting different in design forces between two tables (shown below). Also I would like to know why the moment has been reduced from 259360 to 240867, what was the reason behind the moving load application, because if span start feature was not applied and now its applied so the predicted moment should be go to the higher value not the lower value




Dear Waleed,

So I will be having two patches one in ticket number 

#24911980  


and the other one here, or both are same ??
Ans: Both were the same patch.

I applied the two patches and still getting different in design forces between two tables (shown below). Also I would like to know why the moment has been reduced from 259360 to 240867, what was the reason behind the moving load application, because if span start feature was not applied and now its applied so the predicted moment should be go to the higher value not the lower value
Ans: The reason for the reduction in the value is that the software was incorrectly applying the two truckload even when "start span" wasn't applied when we had a section with 7 dof option checked on. Thus, now it is rectified hence we are getting the moment according to the proper lane support negative moment.
We are getting the design forces and the element forces same in our model, kindly send the modified file so that we could check at our end.

Regards
B Swapnil Agarwal
Technical Support Engineer
Global Technical Center



Okay, I'm sending you the specified model  and btw the maximum bending moment for Load combination ULS-I-Max-TRKR-2 you took TRK-L not TRK-R.



Dear Waleed,

The reason for the difference between the element forces and design forces for element 358J is due to the fact that the critical negative moment is not occuring at 358 J rather it is occuring at 357 I as shown below:


As in the design forces it takes the most critical moment for design between the bracings.



We are attaching the excel sheet for your reference.

Kindly let us know if further assistance is required.

Regards
B Swapnil Agarwal
Technical Support Engineer
Global Technical Center



Agarwal, I'm extracting the design of element 358J, so the design results shall be consistence with forces in that element not designing forces for another element regardless which is the critical.

The engineer shall decide which element to design regardless which one is the critical, However even thought this is not matching other ticket.



Dear Waleed,

Agarwal, I'm extracting the design of element 358J, so the design results shall be consistence with forces in that element not designing forces for another element regardless which is the critical.
Ans: The design forces taken for the element according to the codal clause 6.10.1.6 which states that the most critical value between the unbraced length must be taken to account for the lateral-torsional buckling:

 
Thus for element 358 J, the design forces would be the maximum moment arising between the braces which happen to be at 357 I among the elements 356to359.

The engineer shall decide which element to design regardless which one is the critical, However even thought this is not matching other ticket
Ans: Yes, in ticket #24956320 there is a display issue that exists. The developer is working on correcting it.

Regards
B Swapnil Agarwal
Technical Support Engineer
Global Technical Center



Bridge 04_R09_No Cross Frame_Adjust Bearing Condition Center to center_Pouring One Time_Center Top_Iner0.365.xlsx
Bridge 04_R09_No Cross Frame_Adjust Bearing Condition Center to center_Pouring One Time_Center Top_Iner0.365.mcb
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Midas Webinar Nevling 03 04 2013_MidasDesign.pdf
Client After Patch_4.xlsx
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unmodified.mcb
After Patch.xlsx
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Bridge 04_R10_No Cross Frame_Adjust Bearing Condition Center to center_Pouring One Time_Center Top_Iner0.365_G9&10 Allignment Change[1].mcb
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Bridge 04_R10_No Cross Frame_Adjust Bearing Condition Center to center_Pouring One Time_Center Top_Iner0.365_G9&10 Allignment Change.xlsx
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Bridge 04_R10_No Cross Frame_Adjust Bearing Condition Center to center_Pouring One Time_Center Top_Iner0.365_G9&10 Allignment Change.mcb
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