The left main landing gear collapsed moments after touchdown. Winds as reported by the tower were from 350 degrees at 19 knots gusting to 29 knots. The captain and copilot reported the touchdown was normal. The three flight attendants said the aircraft landed hard and started shaking very badly. The flight attendants also reported that the previous two landings had brought a great deal of vibrating and shaking to the aircraft. The left main landing gear tee bolt was found separated. Metallurgical examination of the tee bolt showed the fracture emanated from a shallow preexisting crack in the drag brace bearing surface radius that the metallurgist found to be sharper than desired. The collapse of the landing gear is considered related to the fractured tee bolt. The Safety Board is unable to determine whether other events, circumstances, or conditions were present that contributed to the tee bolt failure in this instance. The operator has amended their repair specifications for the tee bolts.