Modern reassessment of the Citicorp Building design wind loads.

Following structural engineering practice of the 1970s, engineers designed the Citicorp Building for the action of wind in each of the structure's principal axes. One problem they faced was how to determine design values by combining simultaneous wind loads from the means and root mean squares of these loads. Given the technology available at the time, the simplifying assumption was made of decomposing the effects of a corner wind into the sum of the effects on two adjacent faces, as if these wind loads were static and perfectly correlated, which they are not. As a result of this assumption, the connections in the diagonal braces were deemed overstressed, and emergency repairs had to be performed on the just completed building. Nowadays, the Database-Assisted Design method and modern computer technology are capable of accounting for simultaneous dynamic loads properly, performing structural analyses for all time steps where measurements are available, and extrapolating to longer duration windstorms using extreme value distributions. Modern analysis thus determines design loads on a more rational basis and shows that the combinations of wind loads that caused such concern in 1978 do not need to be considered for mean recurrence intervals of practical interest.