THE PLT FLEW TO A NEARBY ARPT TO PRACTICE TAKEOFFS & LNDGS. HE RPRTD THAT AFTER 3 LNDGS 'OUT OF A HOVER,' HE RETURNED TO THE DEPARTURE HELIPORT & LNDD AGAIN FROM A HOVER. DRG TOUCHDOWN, THE HELICOPTER BGN TO ROCK FM SIDE TO SIDE & ENTEREDGROUND RESONANCE. THE PLT RPRTD HE LOWERED THE COLLECTIVE PITCH TO THE BOTTOM STOP, BUT STATED THE SIDEWARD BOUNCING FM SKID TO SKID WORSENED. HE REACHED FOR THE OVERHEAD THROTTLES, BUT WAS BEING FLAILED ABOUT THE COCKPIT & COULD NOT HOLD ONTO THEM. SUBSEQUENTLY, THE HELICOPTER WAS EXTENSIVELY DAMAGED. WITNESSES VERIFIED THE SIDE TO SIDE BOUNCING & RPRTD HEARING A LOUD METALLIC SOUND BEFORE THE HELICOPTER TOUCHED DOWN. A POST-CRASH EXAM REVEALED THE L FRONT, L REAR & RGT REAR SUPPORT TUBES (GEARBOX BARS) WERE BUCKLED IN A 'D' SHAPE & FRACTURED AT MIDSPAN. METALLURGICAL EXAM OF THE FRACTUREAREAS DISCLOSED EVIDENCE OF HIGH STRESS, LOW AMPLITUDE FATIGUE. NO HISTORY OF HARD LNDGS OR MAINTAINANCE/MANUFACTURING DISCREPANCIES WERE FND CONCERNING THE GEARBOX BARS; L FRONT BAR HAD 1522 HRS OF SVC, REAR BARS HAD 3396 HRS OF SVC.