Use caution with this assumption. Most programs that I have tested use "3 * I" (where "I" is the moment of inertial of standard pipe of the same diameter) as the stiffness parameter for rigid elements. I also encountered one program that assumed 12 inch STD pipe to compute "I", but then increased "E" by a factor of 10.

CAESAR II on the other hand uses "10 * thickness" to determine the cross section properties of rigids.