member forces creaded by concrete shrinkage and creep

Question:

Dear Technical Support,


I have two questions about the member forces created by concrete shrinkage and creep:


(1)  The member forces at Stage 12 (Closure) First Step (activate the closure segment with 4 days maturity) for shrinkage load case: primary and secondary


       I get very large axial compression and small  bending moment near the pier as shown in the following Excel sheet. The girder is not restricted for axial deformation and it can contract or expand freely, why I got the large axial force in the girder? How are the member forces (primary and secondary) calculated for shrinkage load case ?

       For prestress, a clear definitions for primary and secondary load effects are given in text books. What are the definitions for the primary, secondary load effects on the structure for shrinkage and creep load cases? 

       In most cases, only the combined load effects are considered for these load cases. They are different from the load effects created by prestress, because the primary prestress loads are considered to be part of the resistance, and the secondary load effects are considered as a load.




(2)  The member forces from Stage 12 (Closure) First Step (activate the closure segment with 4 days maturity) for Tendon Secondary load case


The following diagram is the moment for a part of the structure.


The secondary moment is normally in straight lines created by the reaction forces acting on the structure, which are created by the actual load. 


The above diagram contradicts with the normal wisdom and I want to know how the secondary load effects due to prestress are calculated.


The structure has compression-only springs as falsework  in the side spans for cast-in-place construction for the side spans, and they are deactivated in Stage 10. Do these springs have any impact for the secondary member forces due to prestress tendon forces?


Thank you very much for your help.


Regards,




Answer:

Hi,

1) You are supposed to check the forces by CS: Shrinkage Secondary. Please refer to the online help for details. 

2) Moment diagram for the tendon secondary load case.

The compression-only springs are deactivated at the first step in Stage 10. The moment diagram at the time is as shown below.

This moment diagram is obtained by the sum of the two results below.
1) The moment diagram at the last step in Stage 9.

2) The current step (or incremental) moment diagram at the first step in Stage 10.

The current step (or incremental) moment diagram at the first step in Stage 10 has occurred because of the deactivation of compression-only springs.

The secondary moments due to prestress occurs when the girder is statically indeterminate. The moments are negative moments as shown in the first image. The girder becomes statically determinate at the first step in Stage 10 when the compression-only springs are deactivated, which leads to the condition that the secondary moments will be lost in Stage 10. Thus, the moments due to CS: Tendon Secondary in Stage 10 are very small. Therefore, the moment diagram for tendon secondary load case makes sense to me.

Regards,
DK Lee


The current step force option can be activated by checking on the option below.

Regards,
DK
Creation date: 9/2/2018 11:20 PM (dklee@midasit.com)      Updated: 9/3/2018 8:52 AM (dklee@midasit.com)
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