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RE: ASCE-7-05 Table 6-1

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I believe Scott Maxwell provided an explanation a few months back.


If I remember correctly, it is a fudge to account for the way the basic winds speeds are statistically derived for the different regions, due to differences in how the raw wind speed data is collected and measured: gusts, averages, mile-run etc.., and length of statistical record.


The importance factor is a fudge in any case. If do more probabilistic based limit state design: then select wind speed with return period which has a defined risk of being exceeded for a given life expectancy. For example wind speed for a mean return period of 500 years has approximately 10% probability of being exceeded for a 50 life expectancy. The probabilistic approach is part explained in the commentary to ASCE7-05, with some explanation of the importance factors and load factors applied to the loads derived from  50 year mean return period wind speeds.



Conrad Harrison

B.Tech (mfg & mech), MIIE, gradTIEAust



South Australia