The pilot reported that he was practicing circuits and landings at the time of the accident. During an approach for a touch-and-go landing the aircraft encountered windshear at a height of about 15 ft resulting in a heavy landing. The pilot applied power and continued for another circuit. It was the pilot's normal practice not to retract the gear during circuit training. He noted that the gear indications were still green, despite the heavy landing, during his pre-landing checks for the final circuit. The aircraft's touchdown appeared normal but both main landing gear legs collapsed during the landing roll. A post-accident inspection indicated that the right main gear attachment bracket probably suffered an overload failure during the heavy landing but the leg was held in place by the drag brace thus providing a safe (green) indication. During the final landing the right leg brace failed allowing the leg to collapse. The asymmetric loads then caused the left leg to collapse.