Moment Capacity According to the AASHTO Method

The ultimate capacity calculations are done in accordance with the unified approach for the design of prestressed and reinforced concrete, as specified in LRFD Art. 5.7. Only the prestressed steel, sufficiently adjusted in the development length zones, will be considered and no deck steel or any steel input by the user for improving transformed section properties will be considered in ultimate moment capacity calculations.

The factored resistance (M_r = φM_n) and nominal resistance (Mn) are calculated using Eq. 5.7.3.2.2-1 for flanged sections and modified according to Art. 5.7.3.2.3 for rectangular sections. At all tenth point sections, the amount of prestressed tensile reinforcement provided is checked to see if it satisfies the minimum reinforcement provisions as specified in Article 5.7.3.3.2.

The minimum reinforcement ratio is calculated as the ratio:

M_r-provided/min M_r (should be greater than or equal to 1.0), where min M_r is equal to the lesser of:

• 1.2 times the cracking strength, M_cr

The calculation of M_cr is based on the following equation, adapted from the LFD Specifications, Art. 9.18.2.1:

where

fr	=	Modulus of rupture =$0.37\sqrt{{f^{\prime }}_c}$
fpe	=	Compressive stress in concrete due to effective prestress force only (after allowance for all prestress losses) at extreme fiber of section where tensile stress is caused by externally applied loads.
Md/nc	=	Moment due to non-composite dead loads, ft-kips.
Sc	=	Composite section modulus for the extreme fiber of section where the tensile stress is caused by externally applied loads.
Sb	=	Non-composite section modulus for the extreme fiber of section where the tensile stress is caused by externally applied loads.

Moment Capacity by Strain Compatibility

The algorithm used in Precast/Prestressed Girder for computing moment capacity by method is described under the "Moment Capacity by Strain Compatibility: LFD" section.