Moment Capacity
The moment capacity is calculated at each POI, and is based on the equations of the code. However, compression steel is ignored by CIP RC/PT Girder. Therefore, the rebars that are designated at "Top" affect the negative moment capacity only, and the rebars that are designated as "Bottom" affect the positive moment capacity. Furthermore, if the PT tendon is located above the CG of the section it is considered to be on top, and affects the negative moment capacity only, and if located below the CG, affects the positive moment capacity only.
The moment capacity demand is based on the load factor combinations requested in Final and Final w/o Sway cases. These include LFD-1 and LFD-1B for the LFD specifications, and Strength-1 and Strength-2 for the LRFD specifications. However, it should be noted that the combined moment requirement includes the secondary post-tensioning effect only, and not the primary PT moment. The secondary moment is calculated based on the reactions produced by the post-tensioning, which can have both vertical and moment reaction values.
LFD
When a section is considered for positive or negative moment capacity, and the applicable tendon and rebar are determined, CIP RC/PT Girder determines if PT is included at this section or not.
If the section has PT (as tension reinforcement), then equations of Section 9 of the AASHTO LFD code [LFD 3 9.17.3, Eq. 9-14 and 9-14a] are used to calculate the capacity. The section is checked for being over-reinforced, and if so, the capacity is limited to the over-reinforced value [LFD Eq. 9-20 and 9-21]. The section is also checked for cracking moment (Mcr) as per specifications [LFD 9.18.2.1 and 9.18.2.2]. The 1.2*Mcr value is reported in the moment capacity report. The capacity/demand ratio (C/D) is calculated based on the demand (Mu), and the capacity, Phi*Mn and the 1.2Mcr. The demand is the larger of Mu and 1.2Mcr, however, 1.2Mcr is limited to 1.33 Mu.
If the section does not include any post-tensioning, then the equations of Section 8 [LFD 8.16.3.1, 8.16.3.2, and 8.16.3.3] are used to calculate the moment capacity. The section is also checked being over-reinforced based on the 0.75 of rho-balance. If over-reinforced, the capacity is reported as 0. Note that the cracking moment (1.2Mcr) condition is not applicable to reinforced sections.
LRFD
The LRFD specifications do not distinguish the reinforced and prestressed cases. Therefore, the prestressed equations [LRFD Eq. 5.7.3.2.2-1] are used for all locations. Furthermore, the over-reinforced condition is checked based on the equations within the commentary [LRFD Eq. C5.7.3.3.1-1 and C5.7.3.3.1-2], which are the same as the LFD specifications. The cracking moment is always checked for the LRFD moment capacity. The Phi factor is based on the value in the Design Parameters dialog box. The value for Prestressed and Reinforced are used for the cases that include PT or are purely reinforced.