Assessment of ASCE 7 Standard Wind Load Factors for Tall Building Response Estimates

Wind load factors incorporated in the ASCE 7-05 standard are based on rough approximations of wind effects and the uncertainties inherent in them. These factors are routinely applied to tall building design, even though the original calculations on which they are based disregard two important facts characterizing tall building response to wind. First, wind effects on flexible structures are proportional to the wind speeds raised to powers larger than two, instead of the power two, as is the case for rigid buildings. Second, dynamic response parameters, that is, the natural frequencies of vibration and the damping ratios affecting the response, exhibit significant uncertainties. It is shown that, for these reasons, the use of ASCE 7-05 wind load factors for the design of tall flexible buildings results in safety levels that can be significantly lower than safety levels typical of common, rigid structures.