ASCE/SEI’s Minimum Design Loads for Buildings and Other Structures (ASCE 7-05) Section 126.96.36.199 states the following regarding special moment frames in areas of high seismicity:
188.8.131.52 Special Moment Frames in Structures Assigned to Seismic Design Categories D through F. For structures assigned to Seismic Design Category D, E, or F, a special moment frame that is used but not required by Table 12.2-1 shall not be discontinued and supported by a more rigid system with a lower response modification coefficient, R, unless the requirements of Sections 184.108.40.206 and 220.127.116.11 are met. Where a special moment frame is required by Table 12.2-1, the frame shall be continuous to the foundation.
Answers to FAQs
The last sentence of this section is being questioned. Depending on how it is interpreted, it can have a dramatic impact on tall buildings requiring special moment frames. It has been common design practice to discontinue moment frames in buildings with subterranean floors to the building foundations. In these designs, the superstructure base shear is generally transferred to the perimeter walls through diaphragms at or below the street level, which is typically assumed to be the base. The base is defined in ASCE 7-05 Section 11.2 as the level at which the horizontal seismic ground motions are considered to be imparted to the structure.
Tracing this provision back to the inception of the International Building Code (IBC), we find that it was part of the 1997 NEHRP Provisions, remained part of the 2000 NEHRP Provisions, and has remained part of the 2003 NEHRP provisions. From the 1997 NEHRP Provisions, it found its way into the 2000 IBC. From the 2000 NEHRP Provisions, it became adopted into ASCE 7-02 and thus became part of the 2003 IBC. Similarly, from the 2003 NEHRP Provisions, it has become adopted into ASCE 7-05 and thus has become part of the 2006 IBC. There is no commentary provided on this item in any of the three editions of the NEHRP Provisions. This requirement is not in the 1997 Uniform Building Code, which for California designers, is the last code they used before switching over to the 2006 IBC in January 2008.
The crux of the interpretation issue is how the phrase "required by Table 12.2-1" should be interpreted. There are basically the following two interpretations, with Interpretation 1 being less restrictive than Interpretation 2:
Interpretation 1—"Required by Table 12.2-1" means special moment frame is the only option based on building height. For example, if a building is located in SDC E with a height of 200 feet, then special moment frames (as part of a moment-resisting frame system or a dual system) are the only choice since other types of systems only allow up to 160 feet of height. Using this interpretation, Section 18.104.22.168 will require special moment frame columns to continue down to foundation for buildings taller than 160 feet.
Interpretation 2—"Required by Table 12.2.-1" means wherever special moment frames are chosen to be used for the seismic force-resisting system and designed per design parameters in Table 12.2-1.
From feedback that we have received from various sources, Interpretation 1 is the interpretation being used by some of the most prominent design offices and apparently accepted by the jurisdictions they are dealing with.
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 firstname.lastname@example.org and email@example.com, respectively, or at www.skghoshassociates.com.
Focus On: Codes
The following is a partial list of "Code Simple" columns previously published in Structural Engineer relating to codes. To access the full list online, visit www.gostructural.com/codes.
- ASCE 7-05 Section 12.6: Analysis Procedure Selection, December 2008
- Section 12.12.3 Building Separation and Section 12.8.7 P-Delta Effects, October 2008