Composite girder bridge deck

Question:

Dear Sir,


I have been trying to run temperature difference load cases 'Heating' and 'Cooling'  on the attached MIDAS model we have created to design a simply supported bridge deck with concrete slab. We have bridge deck about 11.000m wide supported by 4 plate girders and 250 mm thick deck slab.


I have input  the temperature difference loads contained in the load cases using 'Beam Section Temp'. For the input for these load cases I have used  0 as initial temperature and provided temperature gradient using top of the deck as the reference.


For the 'Heating' load case the analysis gives sagging moments in the internal longitudinal members and the hogging moments in the external longitudinal members.


For  the 'Cooling' load case the analysis gives sagging moments in the external longitudinal members and hogging moments in the internal longitudinal members.


W can not understand why we are getting opposite sign bending moments in the external and the internal longitudinal members which are running along the global x direction.


Answer:


Hi,

Let me explain with the 'Heating' load case. 
I removed all the transverse members connected to the main girders and after running the analysis the moments are almost zero for all four girders as expected.

And the center of all girders are moving upward as shown below.

If you check the rotations about the global Y axis (RY), we can see that the rotations are larger in the external girder than the internal girder. In the grillage model, the main girders are connected to the transverse members. Thus, the difference of rotations between the internal girder and external girder is restrained by each other. Therefore, the rotations of the external girder are decreased, which leads to sagging moments. The rotations of the internal girder are increased, which leads to hogging moments.

By the way, I found one error in the temperature input. For the composite section with Composite-General section, material properties (Young's modulus, Thermal coefficient) of concrete slab and steel should be entered separately using the 'Input' option. In your model, the 'Element' option is used. Then, same material properties are applied to both concrete and steel. 

Regards,
DK 
Creation date: 8/15/2018 8:47 PM (dklee@midasit.com)      Updated: 8/16/2018 6:19 AM (dklee@midasit.com)
Files   
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Stage_2 - GAST3 LC 27 Heating.pdf
Stage_2 - GAST3 LC 28 Cooling.pdf
Stage_2 - GAST3.mcb