Rating Prestressed Concrete Bridges 1

[srbu]

I am currently working on my first bridge rating of a simple span precast/prestressed concrete bridge. I am wondering how to apply S, the unfactored prestress secondary moment, to the rating equation
RF= (?R - (1.3D+S))/(2.17L(1+I)) found in Article 6.6.3.3 of the 1994 AASHTO Manual for Condition Evaluation of Bridges, Second Edition.

Specificly, should S be subtracted from the 1.3D since S is causing compression in the bottom half of the member while D is causing tension in the bottom half of the member? Furthermore, I am assuming that S = (Pe)/S where,
P = prestressing force after all losses
e = eccentricity of the prestressing
S = section modulous of the bottom of the beam
Is this assumption correct?

Any help would be greatly appreciated.

 

[yakpol]

srbu,
If you are rating precast/presstressed girder bridge yours secondary moments = 0, sinse all the prestressing have been applied to statically determined structure. Secondary moments are the total moments due to prestressing forces in STATICALLY UNDETERMINED structure minus primary moments due to prestressing. M2 = Mtotal - P*e. In the case of statically determined structure Mtotal = P*e and M2 = 0.

 

[srbu]

Thanks yakpol.

Everything makes sense now.