Shear Capacity
The shear capacity is calculated at each POI and is based on the equations of the code. The shear reinforcement as input by the user is used to calculate the capacity. If the shear capacity is not enough, then CIP RC/PT Girder calculates the amount of shear reinforcement needed to satisfy the shear capacity and points it out with a flag in the report. If the required stirrup area is less than the minimum stirrup, then the minimum value is reported. If this value is larger than the maximum allowed value by the specifications, then the maximum value is reported along with its corresponding capacity, and a flag indicating that the section is inadequate. The best solution is to use a thicker web for this case.
The shear 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 shear requirement includes the secondary post-tensioning effect only, and not the primary PT moment. The secondary shear is calculated based on the reactions produced by the post-tensioning, i.e., vertical reaction values.
CIP RC/PT Girder checks the shear capacity at all POI locations, but does not determine the location of the critical section for shear check, nor does it exclude the centerline of supports from the check.
LFD
CIP RC/PT Girder checks if the PT tendon is present at a section or not, and uses the reinforced or prestressed provisions accordingly.
If the section has PT, then equations of Section 9 of the AASHTO LFD code 9.20.2, Eq. 9-27, 9-28, and 9-29 are used to calculate the capacity. Note that CIP RC/PT Girder reduces the effective width of the web by ½ the diameter of the PT-tendons. This provision is based on the segmental guide specifications. The concrete capacities based on Vci and Vcw are calculated. Note that at some sections Mmax might be zero. If this happens, CIP RC/PT Girderuses the ratio of Vu/Mu instead of the ratio Vi/Mmax. However, if Mu is also zero, then CIP RC/PT Girder calculates the capacity based on Vcw only. CIP RC/PT Girder calculates the stirrup capacity (Vs) based on the code equations, and if the amount of shear reinforcement is less than the minimum [LFD 9.20.3.3, Eq. 9-31], then Vs is set to 0. If the amount shear reinforcement is more than the maximum allowed [LFD 9.20.3.1], then Vs is set to the maximum allowed value.
If the section has no PT, then CIP RC/PT Girder uses section 8 equations [LFD 8.16.6.2.1, Eq. 8-48] to calculate the shear capacity. Reinforcement capacity (Vs) is also calculated based on section 8, but is essentially the same as the values in section 9.
In the case of a pot-tensioned section, CIP RC/PT Girder reduces the web thickness by 1/2 of the PT duct diameter, provided that the duct is passing though the web.
CIP RC/PT Girder always requires at least the minimum stirrup from LFD specifications based on LFD 8.19.1.2, Eq. 8-64.
LRFD
The LRFD specifications are based on the modified compression field theory. The equations and tables (Theta and Beta) from Interim 2000 specifications are used. LRFD specifications specify the concrete capacity based on two sets of equations, first when stirrups are not present, and second when they are. If the user has input any stirrups, then CIP RC/PT Girder checks to see if this is more than the minimum stirrup requirement. If not, it assumes there are no stirrups. If no stirrups are input, CIP RC/PT Girder uses the case without reinforcement. CIP RC/PT Girder will then calculate the concrete capacity based on the iterative approach outlined in LRFD 5.8.3.4.2 and LRFD Figure C5.8.3.4.2-1.
If this capacity is more than the demand, then the capacity/demand ratio is calculated and reported. If the calculated capacity is less than the demand, and the case without stirrup was used, then first the concrete capacity is adjusted to the case with stirrups. The required shear reinforcement (Vs) is calculated and checked against maximum and minimum requirements, and adjusted as outlined above.
In the case of a pot-tensioned section, CIP RC/PT Girder reduces the web thickness by 1/2 of the PT duct diameter, provided that the duct is passing though the web.
LRFD V/M Interaction
The LRFD specifications require that longitudinal steel be provided for shear capacity [LRFD 5.8.3.5, Eq. 5.8.3.5-1]. CIP RC/PT Girder reports the required and provided longitudinal steel in the V/M interaction report. If the longitudinal steel is not adequate, then CIP RC/PT Girder will flag this case in the shear capacity reports and provide the details in the V/M Interaction Report.
Note that based on the direction of corresponding moment at each section, the top or bottom steel may be ignored, i.e., if the corresponding moment is positive, then only bottom bars (regardless of their location) and PT tendons (only if located below the CG of the section) will be included as available steel.