Why are Risk Category I and II structures not required to be designed for tornado loads?
A reliability analysis found that utilizing the same return periods for tornado loads as are used for straight line wind loads provides reasonably consistent reliability. While tornado loads for Risk Category III and IV buildings, with return periods of 1,700 and 3,000 years, will often control portions of the building design, tornado speeds at Risk Category I and II return periods of 300 and 700 years are generally so low that tornado loads would not control over straight line wind loads.