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Midas Negative Temp difference

Question:

Pls see attached file - it has compression only elastic links

The following loadcases are giving weird results:
a) Negative Differential temperature is very low - when data is copied into another model with normal elastic links, it solves correctly (moments at pier in the order of (hog -5000kNm).  I thought it was because negative diff temp causes the deck to lift off the abutments and as it is a compression only spring, then there is no restraint to the deck and the restraint induced moment is hence small.  Is this the case?

Pls review and advise?

b) Centrifugal load case ( Load case 79) has crazy results (when data copied into another model wiht normal elastic links, solves correctly) This is not a big issue as its one case in 123 so am not fussed but would like to understand why it happens





Question 1 :  Negative Differential temperature is very low - when data is copied into another model with normal elastic links, it solves correctly (moments at pier in the order of (hog -5000kNm).  I thought it was because negative diff temp causes the deck to lift off the abutments and as it is a compression only spring, then there is no restraint to the deck and the restraint induced moment is hence small.  Is this the case?

Answer: Yes, the negative differential temperature will cause the uplifting of deck at the abutments, thus there is no restraint and the restraint induced moment is very small.
Moreover, I would suggest you to use general elastic spring instead of compression-only spring as in reality, bearing would already be in huge compression due to dead loads and when the uplifting occurs, the amount of uplifting will be less than the compression already caused by the dead loads. Thus, negative diff. temperature will only reduce the effect of compression at the bearing. This reduction is thus neglected  when we use compression only spring which will give 0 value and when load cases are combined, the resultant will not give correct value.

For example: Let's say compression due to dead load is 100kN . Now, uplifting  is causing -30kN (opposite) .
When we use general spring, when the uplifting occurs, the total reaction after uplifting and dead load combination is 70kN (100-30).
Now, when we use compression only spring, force in compression only elastic link due uplifting will be 0 and due to dead load is 100kN. Thus, total is 100kN. 

As spring was in compression earlier when uplifting force was applied, thus it's true value should be 70kN instead of 100kN as dead load will always exist and thus compression due to dead load will always be there.

Question: Centrifugal load case ( Load case 79) has crazy results (when data copied into another model wiht normal elastic links, solves correctly) This is not a big issue as its one case in 123 so am not fussed but would like to understand why it happens?

Answer: I was not able to observe  any significant different behaviour in load case 79 from other cases. I verified the displacement (10^-1 mm ) as well all the forces obtained for whole structure. 
Can you specify which behaviour you feel is not okay?
Moreover, I would again suggest you to use general springs instead of compression springs for the same particular reason.


Question:

The reason I went for compression only springs is because:

a) this old bridge was provided with 5 bearings at each of the abutments - 4 free sliding and the central one fixed in transverse direction BUT not taking vertical load.

b) when dead or live load is applied, due to the curve of the bridge, torsion is induced at the abutments and this is taken out in a push pull effect between the 4 bearings and the reactions have to be manually resolved to get the correct reaction.

c) by specifying compression only bearings, I was able to get sensible reactions for most loadcases except Negative temperature and some of the centrifugal load cases


Specifying general springs will not get rid of this reaction issue (will be same as vertical pin supports.  Can you think of a way around this problem?

Answer:
Boundary group change assignment option can be used for this purpose where we can change boundary group for specific load case. 
Boundary group can be changed for negative diff. temperature case.

For this what we can do is we can apply general spring support at the locations of elastic link with very less stiffness (0.1 kN/m) with a boundary group name (let's say CS general support) for analysis of model but we will change the general spring support from 0.1kN/m to 10e7 kN/m (let's say boundary group -negative general support) for the negative diff. temperature case. 



So, for whole model compression only links will work except for negative temp grad case, the general spring support will act.
The model has been attached for review. (Some cases have been deleted in model which we not required).

Creation date: 5/27/2021 11:52 AM (jatin2010)      Updated: 5/27/2021 12:07 PM (jatin2010)
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SN6193 Rev 4 Test bearings.mcb
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