Explain tendon primary/secondary load cases and how it should be used in load combinations?Explain shear design. 



Tendon primary effect is the moment, shear and axial force generated by the direct application of the force in the tendon on the section being considered. 

Secondary effects occur when the structure is statically indeterminate and restraints on the structure prevent the prestressed member from freely deflecting when the prestress is applied.

Coming to Load Combinations,
Tendon Primary should not be considered in ULS combination. It should only be considered in SLS combination. 

Prestressing is treated as part of resistance. In midas Civil, in ULS, we should not consider Tendon Primary, as the demand gets altered. The bending moment capacity, Mu must be greater than the ULS external effects bending moment plus the tendon secondary effects bending moment. Demand without Prestressing is compared with Capacity with Prestressing considered as material (like reinforcing steel). 

Example :-

1) When Tendon Primary Included in ULS

2) When Tendon Primary NOT Included in ULS

3) Tendon Primary Moments Only

So, Tendon Primary is considered as Part of Resistance (Capacity is increased) and is not included in ULS (which would rather decrease the demand, thus creating double dipping.)

Coming to Shear :-
For the shear resistance check the program internally considers the effect of tendon primary automatically, along with the other Loadcases which we have provided in the ULS combinations. Therefore if manual creation of load combination is required then do not consider tendon promary in ULS Combinations. 

Creation date: 2/15/2021 12:29 PM      Updated: 9/7/2023 6:11 PM
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