A stiffenable structure is made of longitudinal beams that are positioned around a longitudinal axis. In one state, the longitudinal beams are bendable so that the structure is bendable. As the structure bends, the longitudinal beams slide longitudinally with reference to one another. In a second state, the beams are clamped in a fixed position relative to one another so that the structure is stiffened. In some aspects the longitudinal beams are made of two or more longitudinal laminar elements. As each beam bends, the laminar elements slide longitudinally with reference to one another. Each beam, and accordingly the structure, is stiffened by preventing the laminar elements from sliding longitudinally relative to one another.