The pilot reported that he landed hard on the initial touchdown and heard a loud noise, which he attributed to a blown tire. After the aircraft ballooned up, the pilot reduced the power and the aircraft impacted the runway and subsequently slid off the runway. A witness to the accident reported that after touchdown the aircraft began to porpoise down the runway. The porpoise progressively deteriorated until the landing gear collapsed. Flight control continuity was established on-scene and no discrepancies were noted with the landing gear tires.