Usually one does not consider the refractory as granting rigidity to the system... only added mass. To that end, refractory comes in different flavors, depending on the contents of your pipe. You can expect 1/4th to 1/3rd that of metal to be a starting point, or you could just go all out and assume that of metal itself, and accept some amount of failure for a handful of lines to be acceptable. I would caution you to caution your structural folks that you're incorporating such a safety factor until better information comes your way.