Pipe Stacking

This is a good question. What exactly does the "collapse psi" refer to? Compressive stress, or bending stress where two touching pipes press each other? Are the pipes stacked directly on each other (hexagonal), or in layers with 2x4's between?

I retrieved the collapse psi from a chart that shows the various specs of the pipe. I'm stacking the pipe in layers on top of 4x4 cribbing. The cribbing is layed out at thirds of the average pipe length. I believe the collapse psi is related to compressive stress. This is drill casing that i'm dealing with. That makes me think its compresive for the various geological stresses involved with the installation, and extraction of casing. basically I'm trying to figure out my contact pressures so that i can maximize the space in my small storage facility without damaging the pipe on the lower levels. Thanks for the reply.