Abstract: A method and associated preform for forming a structural assembly that defines a complex contour are provided. The preform and, hence, the structural assembly includes first and second skin members and a cellular core member between the first and second skin members. The skin members and/or the core member can be formed to a shape that corresponds to the desired contour of the structural member before the preform is assembled, e.g., by superplastic forming. That is, the first skin member can be disposed against a contour surface of a die to restrain the preform to the contour defined by the surface. A pressurized fluid is provided against the second skin member to urge the preform against the contour surface. A pressurized fluid is also injected in the first chamber to support the cellular core member. The preform is heated to a bonding temperature before or after being disposed against the contour surface so that the core and skin members are bonded to form the structural assembly.