Mechanical coupling for mechanical and structural tubing

A coupling used to connect structural tubing includes segments surrounding a central space which receives the tubing. Teeth on an inner surface of the segments project toward the central space and form zones of deformation in the tubing upon engagement with the tubes' outer surfaces. Ends of the tubing abut one or more tongues on the inner surface, and observation apertures in the segments permit visual confirmation that the tubing is seated. Relief pockets on the inner surfaces of the segments receive portions of the tubing which deform outwardly. An insert may be present in the central space. The insert defines a space between itself and the inner surfaces of the segments which receives the tubing. The insert has relief channels which receive deformed regions of the tubing corresponding to the zones of deformation.