MIDAS Analysis Result

Question:

Hi,

I have tried to model my bridge using grillage method and plate method. I have three query regarding my model's output.

1. I've noticed that the bending moment envelope due to "Shrinkage Secondary" in both model differs significantly (in terms of shape and value) as shown below.

Shrinkage Secondary BM Envelope for Grillage




Shrinkage Secondary BM Envelope for Plate





2. I have also checked the envelope for Creep Secondary. The two model doesn't have big difference but there is positive sagging moment for the grillage model, but not for the plate model.


Creep Secondary BM Envelope for Grillage



Creep Secondary BM Envelope for Plate




3. As for the other static and moving load cases, it can be noticed that the results obtained from the grillage method is more critical (higher moment) as compared to plate method.


SW BM Envelope for Grillage





SW BM Envelope for Plate





Since the difference is only on the method used in modelling, I assumed that results should be more or less the same. I have tried to diagnose the problem, however, I've failed to find the cause for this. I am unsure whether it's my loading definition issue or other issues, which causes this discrepancies. I hope to hear some comments from MIDAS team on this.

Attached together is the model using grillage method for your reference. I will attach my model with plate in the subsequent reply. Hope to hear from you soon.

Thank you.

Regards,

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Answer:

Hi Alvin,

For all beam elements

If we want to consider composite action, (where Girder + Deck properties need to taken together for calculation of design forces), then we select a composite section in a grillage analysis. In this case, the effect of the concrete deck is taken into consideration via composite stiffness values as shown below:


Now if we want to model deck via plate elements, then



then plate elements stiffness will also be considered additional to the composite section. Hence we need to alter the stiffness via stiffness scale factors. Such that double stiffness of deck should not be considered.


kindly let me know if I missed anything.

Thanks,
Suman


Hi,
In the case of the deck to be completely modeled as plate elements, which can be used for design, we need to ensure that girders are not composite type but instead should be steel girder sections.

In the below snapshot, I have highlighted the plate elements which represent the deck slab, and the girders are steel sections. PFA the example model




If we consider the composite section for girders and model the deck slab with plate elements, then we will consider the double stiffness of the slab in the analysis calculations.

kindly let me know if I missed anything.
Thanks,
Suman

Dear Suman,
Hi,