04-08-14 Building Codes Appeals Board

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Summary of [low the Codes Changed from 2009 to 2012 for Wind The 2009 IRC and IBC both reference and utilize ASCE 7-05 "Minimum Design Loads for Buildings and Other Structures" for structural loads and requirements. The 2012 iRC and IBC both reference and utilize ASCE 7-10 "Minimum Design Loads to] - Buildings and Other Structures" for structural loads and requirements, The ASCE 7- 10 manual made significant changes to the wind load section which went from I chapter in ASCE 7-05 to 6 chapters in ASCE 7- 10. These changes were intended to better organize the various wind design approaches and building types into separate chapters and to normalize wind design on the Strength Design approach. Previously, the wind design was normalized on the Allowable Stress Design approach. In addition, the new ASCE 7-10 manual eliminated using importance factors with only one local wind speed map to using the three win(] speed maps which are based on the risk category of the building or structure. The wind speeds used for determining the design wind pressure are higher in the 2012 codes. For example, a 120 mph wind speed in the 2009 code is equivalent to a 152 mph wind speed in the 2012 code. In the 2009 codes and in the 2012 codes, the design wind pressure is essentially calculated the same way except for using different wind speeds for the different risk categories instead of the importance factors used in the 2009 codes. The main difference between the codes is how the design wind pressure is applied as wind loading on a structure using the different load factors for either Strength Design or Allowable Stress Design. In the 2009 codes, the Strength Design approach multiplied the design wind pressure by a load factor of 1.6 and the Allowable Stress Design approach multiplied the design wind pressure by a load factor of 1.0. In the 2012 codes, the Strength Design approach multiplies the design wind pressure by a load factor of LO and the Allowable Stress Design approach multiplies the design wind pressure by a load factor of 0.6. So while the design wind pressures are higher using the new wind speed maps in the 2012 codes, the actual wind loadings used to design a structure are less. The attached tables show the calculated wind loadings utilizing the Allowable Stress Design for different building types and with different exposures,