Structures for high field magnets ( 14T )  need  to contain Lorentz forces that are over 15 MN/m as well as  prevent  conductor movement. In a recent test of a 12 T racetrack  coil ( RT1) we have used a structure made of cross beams and tie rods that although sufficient to handle the forces could not prevent  a large gap from opening between coils (1.6 mm). Any attempts to eliminate the gap by pre-stress would have caused the tie rods to yield. For a while LBNL has been looking into pressurized "bladders" as a way of applying pre-stress to superconducting magnets. We have now built and tested a 1/3 scale model for a 14 T magnet (RD3) that uses this technology. This concept can provide full pre-stress in coils up to 20 T with no separation, does not require high tolerances, and is simply instrumented. In this talk I shall describe the concept and show test results of the scaled model.