Bridge piping is checked for fatigue due to waves. The piping is analysed with one end fixed and other end with the displacements, for example there are two platforms interlinked with bridge, pipe running between these two platforms via bridge. Specify in your model the fixed end and displacement at the other end where bridge ends and check for displcaement stresse and fatigue as well. You might require good size loop on one of the platform to accomodate the displacements depending on your pipe size and thickneses, material etc. If your doing this type of analysis get help of anyone who has done analysis earlier for bridge piping. You might need to consider the wind as well so the need of guides on the bridge.