DURING FLT, THE GLIDER SUSTAINED IN-FLT STRUCTURAL DAMAGE WHILE MANEUVERING AT AN ALTITUDE OF APRX 17,500 FT MSL IN THE VICINITY OF CALIFORNIA CITY, CA. DURING A PRELIMINARY VERBAL STATEMENT TO AN FAA INSPECTOR, THE PLT STATED THAT HE HAD DONE A COUPLE OF LOOPS. HE SAID HE HAD JUST RECOVERED TO LEVEL FLT WHEN HE HEARD A COUPLE OF LOUD BANGS, THEN FELT A SEVERE VIBRATION IN THE CONTROLS. LATER, THE PLT INSISTED THAT NO AEROBATIC MANEUVERS WERE PERFORMED, BUT THAT HE HAD ENCOUNTER TURBULENCE & HAD 'FLOWN THRU A ROTOR.' WRINKLES WERE FOUND IN THE SKIN ON THE AFT FUSELAGE & EMPENNAGE, THE VERTICLE STABILIZER WAS DEFORMED & THE LEFT HORIZONAL STABILIZER & ELEVATOR HAD SEVERAL AREAS OF DEFORMATION. SURFACE WINDS IN THE AREA WERE FROM 270 DEG AT 22 GUSTING 28 KTS. PEAK SURFACE WINDS WERE FROM 290 DEG AT 42 KTS. STANDING LENTICULAR CLOUDS WERE REPORTED TOPPING THE MOUNTAINS & SIGMET JULIET 4 WAS IN EFFECT CAUTIONING OCCASIONAL SEVERE TURBULENCE BELOW 16,000 FT MSL.