Steel-Composite Construction Staging Troubleshooting

Question:

Hi,

I am working on a model of a severely skewed steel composite girder bridge and I have created two separate models which I attached to this email.

My first model (m-3) tries to follow modeling Method C (composite action without sequential analysis) and my second model (n-2) tries to follow modeling Method B (sequential analysis with a long‑term modular ratio of 3n). I say “tries” because I am new to these modeling methods and the online help is very unclear. I have been searching through the various Midas tutorials as well to make sure I am modeling things the way I intend but I am getting some bad results I think.

The first model (m-3) relatively closely matches the service girder moments and maximum service reactions of a comparison model in another software. The values are also generally in line with what I would expect based on some line girder analyses. However, the chord forces in my cross frames are tremendous (some about 3x what I get with my comparison model). I am not sure why this would be.

I thought perhaps it was just a crude analysis method and so I created a model according to Method B with discrete construction stages. This model gave me service reactions that are about 63% higher on average and service moments that are also about 63% higher on average. I don’t know why this would be. Perhaps I am double-counting some loads unintentionally… The chord forces did come down as I was expecting but only slightly. They are still tremendous with some still about 3x what I get with my comparison model.

Would you please take a glance at my models? Perhaps you can point out my errors. I have searched through the online help and the tutorials but I can’t figure out what I defined incorrectly.

Thank you,


Answer: Hello User,


Thanks for writing to us.
1)
I was checking your model 'XXX.mcb' where the connection between the girder, cross-frame, supports are provided as shown below.


Its better(intuitive) to provide the supports at the bottom of the girders and connected using rigid links. as shown in our default tutorial: C:\Program Files\MIDAS\midas Civil\Tutorial>23 Curved Steel Composite Design_methodA model.
So I have made the modifications accordingly to correct the positions of the supports as shown below, PFA the revised model.


2) If we check the cross-frame forces(truss forces) in the model: 'XXX.mcb' are shown in below for service case.



if we dissect the service combination, to check which particular load comb is causing a maximum effect in cross frames, then its gLCB24, further HL93 (B lanes) case is causing maximum force effect in the chords. (further increase due the load factor of 1.3).



Now using moving load tracer feature to identify the vehicle position which is causing max/min force effects on the particular chord. Considering element 182, vehicle position causing max effect is shown in below snapshot;


So because of the eccentric vehicle loading, there are large forces that are getting generated in the cross-frames. Please check with your comparison model for this live loading results.

kindly reply back if you have any further issues.
Regards,
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Thank you! I have modified my boundary conditions as you suggested and I believe I am now getting more reasonable output.
Creation date: 10/30/2017 2:03 AM ()      Updated: 10/30/2017 11:39 AM ()
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