2006 IBC Section 1605.4: Special Seismic Load Combinations

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This "Code Simple" is about the special seismic load combinations set forth in the 2006 International Building Code (IBC) and how they work with the provisions of Minimum Design Loads for Building and Other Structures (ASCE 7-05) Section 12.4. First let us take a look at what the 2006 IBC Section 1605.4 says:
1605.4 Special seismic load combinations—For both allowable stress design and strength design methods where specifically required by Section 1605.1 or by Chapters 18 through 23, elements and components shall be designed to resist the forces calculated using Equation 16-22 when the effects of the seismic ground motion are additive to gravity forces and those calculated using Equation 16-23 when the effects of the seismic ground motion counteract gravity forces. The load combinations are as follows:
* 1.2D + f1L+ Em, (Equation 16-22)
* 0.9D + Em (Equation 16-23)
where Em = the maximum effect of horizontal and vertical forces as set forth in Section 12.4.3 of ASCE 7; f1 = 1 for floors in places of public assembly, for live loads in excess of 100 psf (4.79 kN/m2) and for parking garage live load, or = 0.5 for other live loads.

The reference to Section 12.4.3 of ASCE 7 in the definition of Em above identifies the source of a major disconnect that exists between the special seismic load combinations of the 2006 IBC and the "seismic load effect including overstrength factor" equations of ASCE 7-05 Section 12.4.3.

The problem did not exist in the 2000 and the 2003 IBC, where the maximum earthquake effect, Em, was defined in Section 1617.1 of the code itself. The problem is a direct result of the wholesale adoption of ASCE 7-05 in the 2006 IBC.

ASCE 7-05 no longer uses the phraseology "special seismic load combinations." In fact, ASCE 7-05 does not have separate, unique load combinations that are to be applied where specifically required, as is done by 2006 IBC Section 1605.4. Instead, ASCE 7-05 prescribes an equation for Em that is to be used in ASCE 7-05 Chapter 2 load combinations (both allowable stress design and strength design load combinations). This has always been the approach of ASCE 7, and it has always been different than the IBC approach.

The big differences between the code (2006 IBC) and the standard (ASCE 7-05) are as follows:
(1) The code does not include the design snow load, S, in the special seismic load combinations, the standard does; and
(2) The code specifically says Equations 16-22 and 16-23 shall be used "for both allowable stress design and strength design methods…," the standard explicitly includes ASD load combinations with overstrength factor, in which Em is multiplied by 0.7.

These two provisions of ASCE 7-05 are in direct conflict with those of the 2006 IBC. And the general rule is that in all such cases, the code governs over the standard.

Unfortunately, in this case, the situation is not as simple as that. Section 1617.1 of the 2000 and 2003 IBC included a provision that read: "Where allowable stress design methodologies are used with the special load combinations of Section 1605.4, design strengths are permitted to be determined using an allowable stress increase of 1.7…." This provision is not to be found in the 2006 IBC, since Section 1617.1 is no longer in the code. The corresponding provision is found in ASCE 7-05 Section 12.4.3.3. However, the allowable stress increase factor is now 1.2 because Em has been multiplied by 0.7 (1.2/0.7 = 1.7).

Also, take a look at ASCE 7-05 Sections 12.2.5.2, 12.3.3.3, 12.10.2.1, 12.13.6.4, and 12.13.6.5; all of these sections specifically require compliance with provisions of Section 12.4.3. And all of these sections are adopted by reference by the 2006 IBC.

So what is the code user to do in this quagmire? The good news is that the problem has been solved for the user in the 2007 Supplement to the 2006 IBC. The Supplement has deleted the special seismic load combinations in 2006 IBC Section 1605.1. The section now simply directs the users to use Section 12.4.3 of ASCE 7-05, except that it clarifies: "With the simplified procedure of ASCE 7 Section 12.14, the overstrength factor load combinations of Section 12.14.3.2 of ASCE 7 shall be used." Although supplements are typically not adopted by local jurisdictions, what the 2007 Supplement has done would be an advisable way to resolve the problem that exists with the 2006 IBC.

S.K. Ghosh Associates Inc., is a seismic and building code consulting firm located in Palatine, Ill., and Laguna Niguel, 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 skghosh@aol.com and dowtyskga@cox.net, respectively, or at www.skghoshassociates.com.