An error in calculating the weight of steel beams likely caused a fatal accident at a construction site near JR Tokyo Station last year, an investigative source said Sunday. Two workers died and three were injured on Sept. 19 when steel beams fell on the seventh floor of a building being constructed by Obayashi Corp., which informed police of the calculation error. Police are investigating the accident on suspicion of professional negligence resulting in injury and death, as the error may have resulted in an excessive weight load on a temporary foundation supporting the beams, the source said….