Positive Restraining Moments

Restraining moments are caused by long-term creep and shrinkage deformation. When the beams are restrained for rotation at the diaphragm, restraint moments are reduced. Restraint moments are calculated using the method described in Reference 4, the PCA Method.

To consider restraining moments, click the Project Parameters button on the Analysis Tab to activate the Project Parameters. Click the Restraining Moments tab to display the appropriate screen. Select the method you want to use. There are two options available, Manual Restraining Moment and PCA Method (Calculated Restraining Moment).

If you select Manual Restraining Moment, the program will use the values entered in the fields on the screen to computer the required reinforcement.
If you select PCA Method (Calculated Restraining Moment), the program will first calculate the restraint moments using the PCA Method given the age at which continuity is established and the continuity factor and then, calculate the required reinforcement.

To disregard restraining moments, select the Full Continuity, Disregard Restraining Moments option. Note that the NCHRP method of restraining moment's calculations will be included in a future upgrade of Precast/Prestressed Girder.

The restraint moments at the left and right pier are the sum of moments due to prestress, creep, dead load, and shrinkage. These are calculated assuming the exterior supports as pinned and the interior supports as fixed. This is an approximation performed by the program, but you may take into account the moment distribution by entering the factors for the left and right pier manually. (Select the Restraining Moments tab on the Project Parameters and input the desired values.)

As specified in Reference 4, the program calculates the required reinforcement to connect the ends of the beam to the diaphragm. When restraining moments are considered, they are also added to the final stresses. In most cases, the restraint moment will be positive, which will introduce additional tension stress in the bottom of the beam.

In Reference 7, it is recommended to use an allowable tension stress of $- 3 \times \sqrt{f c `}$ , without considering restraint moments. If restraint moments are considered, you might want to increase the allowable bottom tension. (To do this, select the Allowable Stresses tab on the Project Parameters.) The following is a sample of a portion of the output for calculation of restraining moments:

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The phi factor is calculated by multiplying the five numbers shown. The first four numbers are taken from the tables in Reference 4. The program uses the larger of the Ms (or 1.2M_cr) and the computed Positive Restraining Moment to compute Asb.