The intent of each Code Simple article is to analyze a code requirement and shed some light on related issues that code users find difficult to interpret. As we set out to do the same with 2003 International Code Council’s International Building Code (IBC) Section 1801.2.1, we found ourselves in a “code maze” as we referred back to different code sections and compared the intent and meaning with what the code actually said. Although not as simple and clear as previous Code Simples, we hope you find this article enlightening and of assistance in understanding the historical perspective of allowable reductions in foundation design.
2003 IBC Section 1801.2.1 allows the seismic overturning moment (determined by either equivalent lateral force method or modal analysis method) to be reduced by 25 percent when the foundation is designed using the strength design load combinations of Section 1605.2. This provision recognizes that there is a reduction of response by the non-linear effects of uplift at the foundation-soil interface.
What is interesting to note is that the American Society of Civil Engineers’ 2002 Minimum Design Loads for Buildings and Other Structures (ASCE 7-02), unlike the 2003 IBC, allows the 25 percent reduction of overturning moment not just when the strength design load combinations are used in design, but also when the basic allowable stress design (ASD) load combinations are used (see ASCE 7-02 Section 188.8.131.52, last paragraph). Note that ASCE 7 does not contain the alternative basic load combinations found in 2003 IBC Section 1605.3.2. The reason that ASCE 7-02 allows this reduction for basic ASD load combinations can be understood when the following load combinations that govern overturning are compared:
• 2003 IBC Equation 16-6 for strength design: 0.9D + 1.0E
• 2003 IBC Equation 16-12 for basic ASD: 0.6D + 0.7E
• 2003 IBC Equation 16-18 for alternative basic ASD: 0.9D + 0.7E
It is apparent that the first two equations are compatible with each other, with the former representing strength level forces and the latter representing the corresponding service level forces. The third equation, however, includes the earthquake forces at service level (0.7E) while still having the dead load at strength level (0.9D), which counteracts the overturning effect of lateral earthquake forces. As a result, the alternative basic ASD load combination provides a lesser margin of safety against overturning, compared to those provided by the basic load combinations for strength design and allowable stress design. Therefore, a reduction in foundation overturning moment is not permitted with the alternative basic ASD load combinations.
Looking at the 2006 IBC, Section 1801.2.1 is revised somewhat, although the 25 percent reduction is still only permitted when using the strength design load combinations. 2006 IBC Section 1801.2.1 directly references Section 12.13.4 of ASCE 7-05, which does not allow the 25 percent reduction for inverted pendulum or cantilevered column-type structures, and the reduction is only 10 percent for foundations designed in accordance with the modal analysis procedure. A lower reduction in the overturning moment obtained using the modal analysis procedure is justified because modal analysis results in a more realistic distribution of lateral forces along the height of a building, which in turn results in a more realistic estimate of overturning moment at the base.
There is new language in 2006 IBC Section 1605.3.2 that specifically addresses how to handle the design of foundations when using alternative ASD load combinations. First it states, “The reduction of foundation overturning from Section 12.13.4 in ASCE 7 shall not be used.” And second, it states that the vertical seismic load effect, Ev, can be neglected in calculating the total seismic load effect E, which is consistent with similar provisions in the 1997 Uniform Building Code. That code allowed Ev to be neglected in the allowable stress design of any structural member, whereas the 2006 IBC only allows Ev to be neglected for the design of foundations. Ev is not allowed to be neglected when using the basic ASD load combinations so as to maintain consistency with strength design. Please note that for the determination of demands on the soil-structure interface, Ev can be taken equal to zero irrespective of the load combinations in accordance with ASCE 7-05 Section 184.108.40.206, Exception 2.
The fact that the 2006 IBC does not allow a reduction in the overturning moment other than in conjunction with strength design load combinations may be inadvertent rather than deliberate. This is because the code specifically precludes such reduction to go with the alternative basic ASD load combinations, whereas it does not do so for the basic ASD load combinations.
S.K. Ghosh Associates, Inc., is a structural, seismic, and 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 email@example.com and firstname.lastname@example.org, respectively.