20230228002129I

A Piper seneca on an instruction flight within the traffic pattern in and out of sgj. The crew stated they made multiple landings without issue. With regards to the last flight all indications were normal without incident and on the downwind segment of the flight to land the student placed the gear selector in the down position however, only two light illuminated green accompanied by unsafe gear warning light. They changed the bulbs, checked breakers followed by yawing and recycling the position switch. The crew notified the tower with the issue and requested to perform a low approach and pass to verify if the gear was down. After multiple passes it was confirmed the gear was down. An emergency was declared, and once emergency vehicles were staged, the crew performed a soft field landing but once the left main touched the surface began collapsing. The crew initiated directional control and the aircraft came to a stop on the left side of runway 31 and they cleared the aircraft. Further examination shows damage to the bottom surfaces to include the trailing edge wing tip, lower left side fuselage surface and stabilator lower surface(scarring), whereas it was listing left after collapse. Also, with regards to the gear collapse, it rotated rear causing damage to the flap. Visual inspection shows the forward landing gear trunnions may have failed causing the gear to collapse. Further inspection, and parts required that are necessary to return the aircraft back to service. Incident closed concluding a mechanical issue and not pilot related and will be amended as needed.