Positive Restraining Moments (LFD)

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

To consider restraining moments, select the Restraining Moments tab on the Project Parameters screen (on the Analysis screen). Then, on the screen that appears, select the desired method. There are two methods available: Manual Restraining Moments or PCA Method (Calculated Restraining Moment). If you selected the "Manual Restraining Moment" option, Precast/Prestressed Girder will use the restraining moment values entered on the screen and compute an area of steel. If you selected "PCA Method (Calculated Restraining Moment)", the program will calculate the restraint moments using the PCA Method given the age at which continuity is established and continuity factor.

To disregard restraining moments, select the Full Continuity, Disregard Restraining Moments option.

Note: The NCHRP method of restraining moment 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 interior supports as fixed. This is an approximation done by Precast/Prestressed Girder, but you may take into account the moment distribution by inputting factors for the left and right pier. (To do this, select the Restraining Moments tab on the Project Parameters.)

As 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 4, it is recommended to use an allowable tension stress of, 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:

The phi factor is calculated by multiplying the five numbers shown. The first four numbers are taken from the tables in Ref. 7. The program uses the larger of M_s (or 1.2 M_cr) and the computed positive restraining moment to compute A_sb.