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Short term and long term settlement analysis

Question:

Hi,
I'm working on a voided slab for highway bridge according to AASHTO code. for this, I have been asked to model the settlement load to short term and long term condition. For the short term, it is assumed that 50% of the settlement will occur with the uncracked section. While, another 50% of the settlement will occur during the long term with a cracked section where we need to reduce the modulus of elasticity to 0.5xE. The differential settlement for this project is 25mm.
Need your advice, how can this be done?

Thank you.

Answer:

Hello,

Thanks for getting in touch.
To do this, the specified displacement option needs to be used instead of the usual settlement analysis in midas Civil.  I'll describe the process for achieving this at one of the supports.
Step 1:  Define 2 load cases with -12.5mm specified displacement in Dz direction at the support location.  Say, Set 1 & Set 2.
Step 2:  Define a boundary group.  Say, E reduction.
Step 3:  Assign reduced Iyy scale factor to the section in the boundary group defined earlier.  So, Iyy scale factor should be entered as 0.5, and E reduction boundary group should be selected.  Now, the purpose of reducing E is to actually reduce the stiffness, i.e., E x I.  We can't scale down material, but we can scale down I.  Hence Iyy is scaled.
Step 4:  Use the Boundary Change Assignment feature and assign this reduced stiffness group to Set 2 load case.  This is shown in the image below.
Step 5:  Combine the results of the 2 cases using the Results>Load combination feature.
The sample model attached here as 2 beams for comparison purposes.  In one case, the reduction is applied and in the other case, it isn't.  It could be noted that the results in both the beams are the same for Set 1 load case, while it's different for Set 2 load case.  Kindly check the "My" bending moment results.
Do let us know if further assistance would be required.

Regards
Creation date: 9/8/2021 11:13 AM (nandeep)      Updated: 9/8/2021 11:14 AM (nandeep)
Files   
Cambodia Airport-Part 1_R5(Settlement short and long).mcb
4.9 MB
Cambodia Airport-Part 1_R5(Settlement short and long)[1].mcb
4.8 MB
DataImage20.png
218.4 KB
Sample.mcb
63.1 KB
screenshot.png
548.9 KB
screenshot[1].png
923.6 KB