After performing three touch-and-go landings on a frozen lake, the pilot set up for a full stop landing on the plowed surface of the frozen lake. After a smooth landing, and while taxiing to park, the left wheel strut collapsed. The airplane tipped over onto its left side, resulting in substantial damage to the left wing spar. Examination of the airplane after the accident revealed evidence of fatigue cracking on the failed portion of the left wheel strut mount. The airplane had been flown about 50-75 hours after its last annual inspection with no problems noted by the owner. The pilot stated that he lands on frozen lakes frequently and that the collapse of the landing gear was unexpected.