A Heavy Structure in any material is one that is made stable by a skeleton that is able to stand by itself as a rigid structure without depending on floors or walls to resist deformation. Frame structures can be made stronger by using struts. A strut is a solid bar joined into a structure to make it more stable. Struts are used in roof trusses, bridges, cranes and pylons. Skeletons are frame structures made of a system of struts.