N-year demand-to-capacity indexes and the design of high-rise buildings for wind

The purpose of this work is to assess the global load effects method, commonly used in current commercial practice for determining design wind effects on high-rise buildings. The assessment is performed by comparing results based on the global load effects method to their counterparts obtained by a conventional, well established structural engineering approach. That approach yields time series of the demand-to-capacity index (DCI) induced in structural members by time series of effective (aerodynamic + inertial) wind loads. The DCI time series so obtained depend upon member-specific influence coefficients, meaning that the time series of the DCIs, and therefore the peak DCIs and their times of occurrence, differ from member to member. In contrast, the global load effects method assumes that all the structural members' peak DCIs occur at the same time, i.e., at the time of occurrence of the largest combination of base moments produced by the wind forces acting on the structure. The use of the global wind effects method in commercial practice is due to the fact that wind engineering laboratories do not have access to the structure's influence coefficients. This work reports the novel finding that the global load effects method results in the underestimation of the DCIs of nearly all the members of the building's structural system. Numerical examples are presented which show that the underestimation can be substantial. Therefore, the use of the global load effects method needs to be discontinued, and the estimation of the peak DCIs needs to be performed by accounting for their dependence upon the structure's influence coefficients, a task that can only be performed by the structural engineering office.