The present invention is a system for joining tubes in a structure. The system includes hollow tubes and pressure assemblies. The tubes have tool holes spaced from the ends which are to be joined together. The pressure assemblies include members which can be spread apart by threaded inserts threaded into one of the members. The threaded inserts are spaced to correspond to the spacing between the tool holes as occurs when the tube ends are placed together. Accordingly, a pressure assembly can be inserted into adjacent tube ends and then expanded with sufficient expanding force for joining the tube ends in a rigid, strong joint. The system of the present invention may be used to make highly versatile and portable structures such as soccer goals, American football uprights and basketball goals.