3/14/2024 0 Comments 2012 ircSo it is Vult for IBC and Vasd for IRC - but both use ASCE-7 2010 as the reference standard. The IBC has gone to an ultimate strength value to be used (LRFD) while the IRC has the old 3-sec gust data (based on ASD). Not a big deal but here is the problem.īelieve it or not, our brilliant code writers are using different wind speeds now for the IRC and IBC codes. So for Boston wind pressure went down about 8% +/-, but in Providence it went up about 10% +/. For Boston, the wind in Cat II went from 105mph to 128mph, and 128mph^2/105mph^2 = 1.49 which is less than 1.6. It also depends a little on type of load combinations. Since wind pressure is proportional to velocity squared, it turns out that in Providence, wind pressure did increase a little overall, since wind speed increased 133mph^2/100mph^2 = 1.77, but the load factor in the ASCE 7 10 combos only went down from 1.6 to 1 (so wind pressure after doing the load combinations did still increase about 10% or 1.77/1.6 in Providence). That increase is counter balanced by new load combinations that reduce the wind load factor within the ASCE 7 USD/LRFD combos from 1.6 to 1.0 (or from 0.8 to 0.5) depending on the combination used. For example, in the typical risk cat II, wind speed in Providence is now 133 mph (instead of 100 mph before). One big change from IBC 2009 / ASCE 7 05 is that wind speeds in IBC 2012 / ASCE 7 10 are now "ultimate" values and associated with risk category and have increased about 30%.
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