This Code Simple addresses a new provision in the 2009 International Building Code (IBC), which became available in March 2009. One of the most significant revisions to the structural provisions in this edition of the IBC is the introduction of the Alternate All-Heights Method of wind design in Section 1609.6.
The Alternate All-Heights Method is a welcome simplification to the American Society of Civil Engineers’ Minimum Design Loads for Buildings and Other Structures’ (ASCE 7) Method 2-Analytical Procedure. The title of this new method implies that it applies to structures of all heights; however, that is not exactly so. The method applies only to structures with height not exceeding 75 feet and the height-to-least-width ratio not exceeding 4. Alternatively for a structure to be eligible to be designed by this method, its fundamental natural frequency must be no less than 1 Hz, making it a rigid structure.
There is a rather lengthy checklist of other conditions that must be met in order to use this new method. See Table 1 on the opposite page for an explanation of this checklist in order to facilitate the use of this method whenever possible.
Answers to FAQs
One may question why this new method is in the IBC and not in ASCE 7, especially since the other design methods (Method 1—Simplified Procedure, Method 2—Analytical Procedure, and Method 3—Wind Tunnel Procedure) are found in ASCE 7-05.
The reason for this is really an issue of timing. The 2009 IBC, just like the 2006 IBC, references ASCE 7-05, which does not include this method. The Alternate All-Heights Method was only developed during the last couple of years and incorporated into the 2009 IBC as a modification to ASCE 7-05, through the ICC code-development process.
It is expected that the next edition of ASCE 7, scheduled to be published in 2010, will include a simplified version of Method 2 in Chapter 6, and at that time the Alternate All-Heights Method will be deleted from the next issue of the IBC, which is expected to be the 2012 IBC.
S.K. Ghosh Associates Inc., is a structural seismic, and code consulting firm located in Palatine, Ill., and Aliso Viejo, Calif. President S.K. Ghosh, Ph.D., and Susan Dowty, S.E., are active in the development and interpretation of national structural code provisions. They can be contacted at email@example.com and firstname.lastname@example.org, respectively, or at www.skghoshassociates.com.