Secondary Effects on Integral Bridges

[bhushan7]

I am designing an integral bridge of 5 span (4x30m + 26m) with Post-tensioned Composite I girders, rectangular piers, and pile foundation. I have performed the analysis in MIDAS Civil software including the effect of creep and shrinkage in construction stage analysis. For this bridge, the main forces for design are due to the imposed deformation generated by temperature rise/fall, creep of prestressed girders, and shrinkage of concrete. As per Indian Roads Congress, Eurocode and British standards, there is a reduction of these forces due to creep in concrete. Attaching a screenshot (also quoted below) of the NZ Bridge manual for effects of creep, shrinkage and prestressing effects.

In the derivation of forces imposed on the structure due to these effects, consideration shall be given to the likelihood of cracking occurring in reinforced concrete piers and the influence this will have on their section rigidity. An appropriately conservative assessment of the forces to be adopted for the design of the structure shall therefore be made. The effects of creep in the pier in reducing the forces may be taken into account.

Can you explain how to appropriately assess the forces for cracking and for effects of creep in pier?

In the Indian code of bridges, we reduce the forces generated due to uniform temperature rise/fall by 0.5 to account for the creep effects. I think a similar factor for forces generated by creep and shrinkage should be applicable. But, am not able to find a direct explanation of this in any code.

Please share your opinion.

Regards,
Bhushan Raj

 

[ukbridge]

From experience, the behaviour of the bridge will be quite sensitive to the EI of the piers.

In simple terms, make sure you use an appropriate long-term modulus for the piers for SDL, soil ratcheting earth pressures etc. Its not an exact science at the end of the day, play around with the E in your model and see how much your design moments change in your superstructure and substructure, make some conclusions, stick it in as an Appendix in your calcs.

I think MIDAS calculates the E for your as it does its own creep/shrinkage analysis - something I'm not a fan of as its very black-boxy and gives the engineer less control in performing their own little sensitivity studies.

gl