A plurality of thin flexible sheet ribs are secured to thin flexible sheet stringer members to form a distortable frame structure. One edge of the ribs lie in a curved surface such as a parabolic cylindrical surface. A thin sheet of boron-silicate glass, preferably formed with a second surface mirror, is annealed and then bent at room temperature to abut the edges of the ribs lying in the curved surface and is retained in place by ribs at opposite edges of the sheet. The inherent strength of the glass under stress, when bent, provides structural rigidity to the combined elements.