Abstract: The structural member, such as a pipe has a pipe wall, whose inner and outer surfaces are formed by plastic surface layers. Between the surface layers is provided a space, which is filled with a material differing from that of the surface layers. If the structural member is used as a foundation part of a structure, the space is filled with a material or a non-porous plastic for increasing the strength and stability of the pipe, while for other, that is overlying structural members, the space is filled with a lower density material, such as PUR (Polyurethane) foam, or is in the form of a cavity. Thus, the foundation parts are not subject to unnecessarily high loading and material is saved. Playing and gymnastic structures with a random spatial arrangement can be formed from the structural members.