It may be counterintuitive to think that the administrative provisions in the 2009 International Building Code (IBC) can be of major help to structural engineers, but they are. There are a number of administrative provisions that structural engineers need to be aware of; familiarity with these can be quite useful. This Code Simple will identify those sections and provide examples of their application.

Section 102.1 General. This section states that where there is a conflict between a general requirement and a specific requirement, the specific requirement is applicable. It also states that where different sections of the code specify different materials, methods of construction, or other requirements, the most restrictive governs. An example of this can be drawn from Sections 1806.3.3 (increase in lateral bearing pressure due to depth) and Section 1807.3.2.1 (maximum depth of embedment for the purpose of computing lateral bearing pressure), as they apply to embedded posts and poles. The question of what depth can be used for determining prescriptive lateral bearing pressure for a non-constrained embedded post arises when applying these two sections. Section 102.1 answers this question; the more specific criteria and limits in Section 1807.3.2 govern the design of an embedded pole foundation over the more general requirements of Section 1806.3.

Section 102.4. Referenced codes and standards. Structural engineers are all too familiar with the practice of referencing multiple standards within the IBC. The question of what to do if a referenced standard conflicts with the IBC is something that is asked frequently. The second sentence of Section 102.4 reads as follows: “Where differences occur between provisions of this code and referenced codes and standards, the provisions of this code shall apply.” Below are three examples that illustrate the application of this provision:

  1. 2009 IBC Table 1604.5 (Occupancy Category of Buildings and Other Structures) is not the same as ASCE 7-05 Table 1-1 (Occupancy Category of Buildings and Other Structures for Flood, Wind, Snow, Earthquake and Ice Loads), yet both tables assign Occupancy Category. Per 2009 IBC Section 102.4, 2009 IBC Table 1604.5 governs and should be used, rather than ASCE 7-05 Table 1-1.
  2. There is a conflict between 2009 IBC Section 1803.5.12 and ASCE 7-05 Section 11.8.3 Item 2. The former requires evaluation of liquefaction potential under design earthquake-level PGA (approximated as SDS/2.5), while the latter requires evaluation of liquefaction potential under MCE-level PGA (approximated as SS/2.5). If your design is by the 2009 IBC, it governs over ASCE 7-05 per 2009 IBC Section 102.4.
  3. There is a conflict between 2009 IBC Section 1602 and ASCE 7-05 Section 12.3.1 when it comes to rigid diaphragm assumption. ASCE 7-05 Section 12.3.1 requires that unless a diaphragm can be idealized as either flexible or rigid in accordance with Sections 12.3.1.1 (“prescriptively flexible”), 12.3.1.2 (“prescriptively rigid”), or 12.3.1.3 (“flexible by calculation”), the structural analysis must explicitly include consideration of the stiffness of the diaphragm (i.e., semirigid modeling assumption). However, 2009 IBC Section 1602 definitely implies that unless a diaphragm is flexible by calculation, lateral load distribution can be on the basis of analysis that assumes rigid diaphragm behavior. 2009 IBC Section 1602 defines a rigid diaphragm as follows: “A diaphragm is rigid for the purpose of distribution of story shear and torsional moment when the lateral deformation of the diaphragm is less than or equal to two times the average story drift.” In other words, a diaphragm that is not flexible is rigid. This is a clear case where the 2009 IBC and ASCE 7-05 are in conflict. 2009 IBC Section 102.4 definitely can be invoked in this particular case.

Section 104.11 Alternative materials, design, and methods of construction and equipment. This section is provided for the building official to use when presented with new ideas or technological advances. For example, if a newly introduced building material is proposed to be utilized in the construction of a building and this material is not specifically addressed by the code, either in material performance or its installation, the building official has the authority to accept its use under Section 104.11. The building official also has the authority to require special inspection for items approved under Section 104.11, in accordance with Section 1704.15. Prior to the 2009 IBC, Section 104.11 was used to allow the design and construction of log structures. This section no longer needs to be used for that purpose because language has been added in Section 2301.2, Item 4, specifically addressing the design and construction of log structures.

S.K. Ghosh Associates Inc., is a structural seismic and code consulting firm located in Palatine, Ill., and Aliso Viejo, Calif. Presidents S.K. Ghosh, PhD., and Susan Dowty, S.E., are active in the development and interpretation of national structural code provisions. They can be contacted at skghosh@aol.com and susandowty@gmail.com, respectively, or at www.skghoshassociates.com.

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