Wind Speeds in ASCE 7 Standard Peak-Gust Map: Assessment
Emil Simiu, R Wilcox, Fahim Sadek, James J. Filliben
The ASCE 7 peak-gust map division of the conterminous United States into two adjacent wind speed zones does not reflect correctly the country's differentiated extreme wind climate. The methodology used to develop the map averages out real climatological differences for the following reasons: (a) The estimation of the speeds specified in the map was originally based on the use of superstations so composed that, in 80 percent of the cases, component stations belong to more than one superstation; (b) superstations were in many instances composed of stations with different physical geography and meterological features; (c) legitimate wind speed data were omitted from data records; (d) off-the-shelf smoothing software was used that lacks the capability to account for physical geography and meteorological differences. We present case studies invalidating the map authors' view that extreme speed estimates do not depend significantly upon superstation composition. We conclude that the ASCE 7 peak-gust map causes, on a national scale, waste of material due to overestimated wind loads, and losses due to underestimated wind loads. A wind map to be included in future versions of the ASCE 7 Standard needs to be improved substantially with respect to the current ASCE 7 peak-gust map. We comment on how such improvements can be achieved and how the large bias errors inherent in the map can be avoided.