The private pilot was landing in calm wind conditions when the left main gear collapsed. The airplane veered to the right and, although the pilot attempted to keep the airplane on the runway by applying engine power and right aileron, it went off the runway and spun, resulting in structural damage to the fuselage. Examination revealed the left main landing gear strut failed due to a pre-existing crack in the weld joint. The crack would not have been readily visible to the pilot during a pre-flight inspection.
