Section 2.0 - Loads

ID #1171

I am a structural engineer in Oregon, and I have a question concerning the interplay between the requirements for wind loading from the 2012 IBC, and designs based upon the TIA-222-G standard. We have a tower that supports 911 services and therefore is considered a category IV structure. Using the 2012 IBC Section 1609.1.1, Exception 5 allows for designs using the TIA-222-G standard. Language at the end of this section notes that for designs using the TIA the ultimate design wind speeds determined from the Figures should be converted to nominal wind speeds, following Section 1609.3.1. With this tower being an essential facility, the Figure for Category III and IV structures would be used to determine the wind speed, and then converted to ASD following the Code’s provisions.From this point forward the design would be per the TIA-222-G (with Addendum 2 updates). It is our understanding that the VASD determined in the IBC serves as the basic speed to be used for the TIA analysis. Our question concerns where the importance factor comes into effect. Following the TIA as it is currently written, an analysis using this standard would apply an Iw value of 1.15 for wind without ice conditions, as required from Table 2-3, regardless of how the basic wind speed was determined. We want to validate that this is correct, as following 2012 IBC, based upon the ASCE 7-10, wind design has been revised to include the importance factor in the wind charts and removes this factor from the equations. We would appreciate direction on if the importance factor should now be considered included in the wind speed provided by the 2012 IBC and not applied when designing per the TIA, or the changes to the IBC do not affect the TIA, and the Iw values should be applied as written in the current TIA standard.

See response to similar previous FAQ question within the link (and repeated) below. Per TIA-222-G Section the wind maps per ASCE 7-10 may be used to determine the basic wind speeds. The ASCE 7-10 wind maps for Category I, II and III structures include the appropriate load and importance factors based upon the structure classification; therefore, the load factor and importance factor for use with TIA-222-G must be equal to 1.0. In addition, when ASCE 7-10 basic wind speeds are used, hurricane coastal areas must be considered as Exposure D as an exception to TIA-222-G Section which allows the use of exposure C for hurricane coastal areas. ASCE 7-10 wind speeds have been reduced in the hurricane coastal areas based on new research. In addition, hurricane coastal areas are now considered as exposure D based on research which has indicated that the roughness of the ocean is more representative of exposure D conditions. In the current version of revision G and previous versions of ASCE 7 the roughness of the ocean was believed to be more representative of exposure C conditions.

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Last update: 2015-04-17 19:23
Author: John Erichsen
Revision: 1.0

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