We are frequently asked what spectral response accelerations should be used for areas outside the United States where there are no mapped values in the International Code Council’s (ICC) International Building Code (IBC) or the American Society of Civil Engineers’ Minimum Design Loads For Buildings and Other Structures (ASCE 7).

Prior to the IBC, it was customary for code users to use the Uniform Building Code (UBC) Appendix Chapter 16 Seismic Zone table for areas outside the United States. This table, introduced in 1976, was completely revised and expanded in the 1997 edition of the UBC. With the 1997 UBC Appendix Chapter 16, a paragraph was added indicating that the compilation was only a partial listing of seismic zones outside the United States and may not, in all cases, reflect local ordinances or current scientific information. The seismic zones tabulated in UBC Appendix Chapter 16 were based on the Tri Services Manual, Seismic Design for Buildings, Departments of the Army [TM 5-809-10], Navy [P-355], and Air Force [AFM 88-3], Chapter 13.

In answer to the question what spectral accelerations should be used for areas outside the United States where there are no mapped values in the IBC or ASCE 7, it is possible to use the Ca and Cv values of the 1997 UBC corresponding to the Seismic Zone listed in Appendix Chapter 16 to determine SDS and SD1, using the following equivalency relationships (this is referred to hereinafter as Method 1): SDS = 2.5Ca and SD1 = Cv.

An example of entries in the 1997 UBC Seismic Zone table with corresponding Ca and Cv and converted SDS and SD1 values is given in Table 1.

However, the values obtained using Method 1 are considered crude. In the absence of maps developed for the local foreign region, a source of SS and S1 values for locations outside of the United States may be the U.S. military document, UFC (Unified Facilities Criteria) 3-310-01, Structural Load Data, dated May 25, 2005, including Change 2, dated Dec. 5, 2007. Tables D-2 and E-1 of this document should be used. These values (hereinafter referred to as Method 2 values) are used by the United States Department of Defense. Of course, the same statement made in the 1997 UBC Appendix Chapter 16 applies here as follows: The values may not, in all cases, reflect local ordinances or current scientific information. Entries in Table E-1 of UFC 3-310-01 for the same locations as in the above table are given in Table 2.

The 10/50 entry represents the design earthquake ground motion prior to the 2000 IBC, reflecting 10 percent probability of being exceeded in 50 years (approximately a 475-year return period). The data in Table 3 compares the SDS and SD1 values using the two methods presented.

S.K. Ghosh Associates, Inc., (SKGA) was recently contracted by ICC to develop maps of SS and S1 for the United Arab Emirates (UAE) because the Emirate of Abu Dhabi is scheduled to adopt the 2009 IBC this year. E. V. Leyendecker, Ph.D., principal seismological consultant with SKGA and scientist emeritus at the U.S. Geological Survey, conducted the study. Time was not available to conduct a probabilistic seismic hazard assessment, which would have been the preferred way to proceed. The Global Seismic Hazard Assessment Program (GSHAP) data were used to replace the Tri Services (TI 809-04) Seismic Zone tables in UFC 3-310-01. The GSHAP data did not form the basis of the maps developed by SKGA because of a number of limitations including the fact that the values of GSHAP ground motion were several times higher than unpublished proprietary data for a number of sites in the region. Abdalla and Al-Homoud of the Civil Engineering Department, American Univerity of Sharjah, have published maps of the UAE region in the Journal of Earthquake Engineering, Imperial College Press, 2004. These maps are more in line with the unpublished data mentioned previously and form the basis of the maps developed by SKGA. This one instance serves to underscore the fact that the GSHAP data and UFC 3-310-01 may not reflect current scientific information for local regions.

Although not originally scheduled for this issue, we wanted to share this recent development; therefore, our next Code Simple column 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 skghosh@aol.com and susandowty@gmail.com, respectively, or at www.skghoshassociates.com

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