Determination of the life span of a structure is a complex issue, check this article which includes useful information about the life span of structures, http://www.concreteconstruction.net/Images/The%20Life%20Span%20of%20Structures_tcm45-345529.pdf

Generally amusement park structures are not heavy duty structures , and are ornamental, meaning looking attractive. The main weak link comes where there are joints, mechanisms like roller coasters and other specific structures. In certain places they are movable and makeshift structures while in big places assume permanence.Corrosion is a major life threatening factor for which no life span estimation is necessary because the structures are lightly loaded but corrosion control is required. The intricate parts namely moving and mechanisms are to be designed with safe margins and periodically inspected and certified. Life span can be assured by this process rather than go forr a life span assessment.