A superplastically formed structural assembly is provided, as is an associated preform and method for forming such an assembly. The assembly includes a skin member and a support member that are joined to define a space between the members and between the joints. The assembly can be produced by joining the members in a flat configuration, and then forming the resulting preform to a predetermined shape of the structural assembly. The support member defines at least one aperture in communication with the space between the members. Thus, the skin member can be formed by delivering a pressurized fluid through the support member to the skin member, e.g., to superplastically form the skin member against a die that defines a contour surface corresponding in shape to the predetermined configuration of the assembly. The support member can extend in a substantially direct configuration between opposing portions of the skin member.