The reason for guiding the expansion joints at 4D & 14D is because when you introduce an unrestrained expansion joint, the pipeline changes from a tensile member to a compressive member. As a compressive member, the pipeline is subject to buckling the same as a column with a compressive load. As such, a perfectly straight & infinitely rigid pipeline would not require any guiding, however, for "real world" applications guiding is required and the load is unknown. The industry has settled on the 7%-15% of pressure thrust value.

In regard to misalignment, if the pipe is installed in accordance with either B31.3 or B31.1 then the alignment is considered in the above factors. If the alignment because significant, i.e. say over 1/4" then the compressive load will begin to cause eccentric bending in the pipe. This would require a more rigorous analysis with CAESAR II.

Hope this answers your question, if not please contact me at pipesol@connectnet.com for a private conversation.