Construction staging - Boundary condition activation / deactivation

Question:

Dear Technical Support,


I model a 3-span continuous concrete box girder with segmental construction. The box girder is seating on two abutment and  two piers with bearings having the following articulations:

  -  The girder is fixed at Pier 1 (Joint 137) in x, y, z translation and torsion Rx, which is linked to the master joint 529 in Pier 1;

  -  The girder is fixed at Pier 2 (Joint 373) in y, x translation and torsion Rx, which is liked to the master joint 549 in Pier 2;

 -  The girder is fixed at two abutments in y, z translation.

 

The cantilever construction for the central span will be done in two separate structures linked to Piers P1 and P2. The two structure second structure associated with P2 will be movable if not adding additional restraint. So I defined the following boundary conditions to be used in construction staging:

I defined three rigid link for the same master joint 549:

(1) P2Bearing1: Fix in x, y, z, Rx directions, to be used during cantilever construction.

(2) P2Bearing0: fix in y, z, Rx directions, to be used when the closure segment in the central span is activated for structural behavior.

(3) P2Bearing2: fix in x direction, to be used before closure construction staging


I have two choices:

(1) P2Bearing1 is  first activated  for construction staging before central closure stage;

      when the closure segment is structurally in action, , P2Bearing1 is deactivated and P2Bearing0 is activated.

(2) P2Bearing0 and P2Bearing2 are both activated at the very beginning for construction staging and stay activated until the central closure segment is in action.

    When the central closure segment is in action, remove P2Bearing2 to remove the fixation of the girder in x direction at Pier 2.


The questions I want to ask:


(1) The first choice will not be possible because the program tells me that:

   - P2bearing1 can not be removed because the slave node 373 is active in the structure.


I would like to know why there is this restriction? I would active P2Bearing0 after the removal so that the slave node 373 has a master joint 549. Will it work if I change the deactivation and activation order? How can I do this if it's possible.


(2) Does the second choice work?  I means that the same master - slave node relation definition are splitted into two for different DOFs and applied or removed separately?


I choose not to using any of the above method and I define an elastic link between Joint 549 and a joint near Joint 373 (the joint 374) with a very small stiffness (1kN/m) in x direction to make the structure mathematically stable without inducing any significant fore even I keep this link in the structure for all load cases, as shown in the data file attached in my previous ticket in your system.


Thank you very much.


Best regards,






Answer:

Correction;


 -  The girder is fixed at Pier 2 (Joint 373) in y, z translation and torsion Rx, which is liked to the master joint 549 in Pier 2;



Hello,

Personally, I'd recommend using the elastic link of general type for such case.  Provide a very high stiffness, say 10^8 kN/m for the rigid directions and nominal stiffness for free directions.  These links could be freely activated and deactivated in stages.
Rigid link work on basis of DOF.  Master node governs DOF of slave nodes.  Removal of such link could lead to instability and hence its restricted.  Elastic link however, connects elements of different stiffness.  The load transfer is dependent on stiffness of connected elements as well.
Kindly let us know if further assistance would be required.

Regards,
Nandeep
Technical Manager, MIDAS
Creation date: 10/17/2018 10:01 PM (dklee@midasit.com)      Updated: 10/18/2018 7:32 AM (dklee@midasit.com)
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