Precast/Prestressed Girder computes rating factors for the prestressed concrete beams, as per the AASHTO Manual for Condition Evaluation of Bridges, Art. 6.6.3.3. Both inventory and operating ratings can be obtained for any user defined vehicle or predefined vehicles in the library. In inventory rating moment capacity and stresses (concrete and prestressing steel) are computed under both positive moment and negative moment envelopes. Shear capacity is checked under both inventory and operating ratings. A summary table showing the controlling ratings is printed for each member for which rating is performed.

When the option to use cracked section analysis for computing tension in prestressing tendons is checked, Precast/Prestressed Girder calculates this value based on a simplified assumption as follows. It is reasonable to calculate the dead load stress in tendon based on gross section, which would be f_pe = f_{p_jacking}-f_{p_loss}. So we find the stress in the strands due to live loads by interpolation from dead load stress to ultimate levels. We then use this to compute the rating factor

f_LL = M_LL x (f_pu - f_pe) / (M_n - M_DL)

RF_OP = (0.9fy* - f_pe)/ f_LL

Where:

f_LL: interpolated stress due to live load in the strand assuming partial cracking

f_pe: effective stress in prestressing strand after accounting for losses

f_pu: ultimate strength of prestressing steel.

M_LL: moments due to all live loads

M_DL: moments due to all dead loads

M_n: nominal Moment strength of the section

RF_OP: rating Factor, operating, prestress steel tension

The rating values are computed at all tenth points along the span, and also at H/2 and Transfer Length distance. Different set of trucks can be run for inventory and for operating ratings. Since shear rating involves re-running the trucks and computing capacities for individual trucks at every point along the beam, the run time on multi-spans is slightly longer. When axle loads are used, the program automatically computes the gross vehicle weight (GVW) and provides the controlling rating in Tons.