In a significant revision to the 2009 International Building Code (IBC), the requirement that wall anchorage needs to be designed to resist a minimum horizontal out-of-plane force of 280 pounds per linear foot (plf) in IBC Section 1604.8.2 is deleted. Instead, the section now simply refers to ASCE 7-05 Section 11.7.3 for the determination of the minimum anchorage force. ASCE 7-05 Section 11.7.3 provides minimum connection forces based on the consideration of a minimum level of structural integrity, and in the context of 2009 IBC Section 1604.8.2, it means that wall anchorage needs to be adequate to resist a horizontal force equal to 5 percent of the wall weight tributary to the anchor. This change resulted because it has been demonstrated by considering a number of typical scenarios that the old minimum of 280 plf was conservative and not consistent with the minimum level of forces for which some other structural elements are designed.
We would like to point out somewhat of an anomaly with this provision. This provision is found in 2009 IBC Section 1604 General Design Requirements, which is completely independent of the seismic provisions of the 2009 IBC found in Section 1613. Yet, it references ASCE 7-05 Section 11.7.3, which is found in Chapter 11, Seismic Design Criteria. It is important that the code user not get confused and understand that the requirement in ASCE 7-05 Section 11.7.3 is simply referenced in 2009 IBC for convenience and applies across the board to all walls regardless of seismic design category (SDC) or whether wind or seismic forces govern.
Further, the title of 2009 IBC Section 1604.8.2 is changed to refer generally to walls of all materials, rather than only concrete and masonry walls, as was the case in the 2006 IBC.
Lastly, the revision to 2009 IBC Section 1604.8.2 would not have been too meaningful if ASCE 7-05 continued to contain the same 280-plf requirement. Therefore, through a code change, the 2009 IBC has effectively eliminated the old minimum 280-plf requirement, as follows. Currently, the 2009 IBC Section 1613.1 adopts by reference ASCE 7-05 Chapter 11, which contains the 280-plf minimum requirement in Section 11.7.5, applicable to structures assigned to SDC A. Consequently, a revision has been made that adds a new section to the 2009 IBC — Section 1613.7 — which removes the minimum 280-plf requirement from the referenced ASCE 7-05 Section 11.7.5.
In our next Code Simple, we will address the anchorage requirements for structural walls as it relates to seismic design and will include a thorough discussion of ASCE 7-05 Section 12.11 for structural walls in structures assigned to SDC B through F.
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