First, FAC is used differently for B31.4 and B31.4 Chapter IX.
For the Offshore Chapter (IX), FAC should be either .72 or .60. This is something you have to decide.
For the main body of B31.4, FAC can be 0.0 or 1.0, indicating unrestrained, or restrained piping respectively. In addition, if you utilize the Buried Pipe Modeler and let CAESAR II re-mesh the model and apply soil restraints, then FAC is set internally to 0.001. In this case, CAESAR II determines the restrained/unrestrained state of the pipe based on whether the axial stress is tensile or compressive.
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Regards,
Richard Ay - Consultant