IRC 2012 / IBC 2012 / ASCE 7 10 - Why did wind speed increase 30mph?

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%.  For example, in the typical risk cat II, wind speed in Providence is now 133 mph (instead of 100 mph before).  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.   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).   It also depends a little on type of load combinations.  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.   So for Boston wind pressure went down about 8% +/-, but in Providence it went up about 10% +/-.  Not a big deal but here is the problem... Believe it or not, our brilliant code writers are using different wind speeds now for the IRC and IBC codes.    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).   So it is Vult for IBC and Vasd for IRC - but both use ASCE-7 2010 as the reference standard.     Per IBC 2012 section 1609.3.1 you can get back to ASD values by multiplying the ult values by the square root of 0.6 (or 0.775). So, for Providence 133mph x 0.775 = 103 approx= 100mph which is what can be used in IRC/SBC-2 (this conversion is also necessary when comparing to others standards/triggers that are not updated).

Yes, this is going to lead to much confusion because load cases are now mixed up with load combinations.  Here is another problem - if I have a house and I choose to use the IRC/SBC-2 wind speeds (say 100mph based on Vasd), I better not use the ASCE-7 10 combinations - that is double dipping since there is already a 0.6W factor within the new ASD combos (instead of 1.0W).   So if you are using IRC wind speeds, you should use the old ASCE 7 05 load combinations - but that is no longer a reference standard!   Therefore, one option is if you are using IRC wind speeds, you can factor them up by 1/0.775 to get pressures, and then use the ASCE-7 2010 combinations.   What a mess!   I am planning on using Vult from ASCE 7 10 for both IBC and IRC for wind to avoid load combo confusion.

These code people are obviously not designers because this new code will lead to more confusion and more errors (and how would a building official understand this?  they don't see combos used,  only mph on drawings).   It was already unnecessary to go to Vult in my opinion, but then for IRC to not follow IBC is really strange.   Good luck with this!   My understanding is we are in a transition, and IRC will eventually adopt the same methods as IBC.

• Settings >> Advanced >> Code Injection >> Footer