I Trying to model a tower piping system where we have about 7 lines going up a tower. We have one 8" line, about 4 1" lines, a 2" line and a 4" line. We were thinking about guiding the smaller pipes by the 8" pipe but the 8" pipe wouldnt support any of the other lines' weight. Wind and Seismic are factors.

The only thing i can think of is finding the force that is generated by wind or thermal forces and then to input forces at the location where they would be connected on the 8" pipe to determine if this scenario would overstress the 8" or any of the other pipes.

The reason we are trying to do this is because trying to guide the lines individually would be more difficult and there is only cross bracing for the tower at 20' intervals.

Does anyone have any idea the best way about going about doing this or a better alternative idea on supporting the small lines which have large displacements due to wind.

Use CNodes...
Let's say that you want to hold the 4 inch line with the 8 inch line in the X direction. The 4 inch node is 450 and the 8 inch node at the same elevation is 850. Add a restraint at 850 in the X direction with a CNode 450.
Read up on CNodes, that's what they are for.