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Section 1620.4.5 of the International Code Council’s 2003 International Building Code (IBC)—which only applies to structures assigned to Seismic Design Categories (SDC) D, E, and F—gives the minimum separation between adjacent buildings on the same property, and minimum setback distance from the property line when a structure adjoins a property line not common to a public way. The purpose of the separation is to permit adjacent buildings to respond to earthquake ground motion independently without significantly impacting (or pounding on) each other. The effects of pounding have been observed in all recent major and moderate earthquakes.

Seismic separation is critical for the design of new buildings adjacent to existing ones. Since the specific dynamic behavior of the existing building is usually unknown, the engineer needs to be conservative in estimating its motion.

Pounding is particularly important in cases where adjacent structures have diaphragms at different levels.

The separation distance is the statistical sum (square root of the sum of the squares) of the estimated design earthquake displacements (Cd times the elastically computed displacements under the code-specified design seismic forces) of the adjacent buildings. In earlier codes, the arithmetic rather than the statistical sum was used. However, the design earthquake displacement was underestimated in those codes. Table 1 shows the building separation provisions of various recent codes and standards.

Answers to FAQs

Q: Where is the "minimum separation between adjacent buildings" requirement in the 2006 IBC?

A: The reason you cannot find this requirement is because it is not in the 2006 IBC or in the American Society of Civil Engineer’s 2005 Minimum Design Loads for Buildings and Other Structures (ASCE 7-05). ASCE 7-05 Section 12.12.3 (which applies to structures designed in all SDCs using the 2006 IBC; see 2006 IBC Section 1613.1) reads as follows: 12.12.3 Building Separation. All portions of the structure shall be designed and constructed to act as an integral unit in resisting seismic forces unless separated structurally by a distance sufficient to avoid damaging contact under total deflection (dx) as determined in Section 12.8.6.

In other words, if there are two portions of a building that are designed independently from each other, they need to be separated from each other so that each can experience its design earthquake deflection without damaging the other. If Portion A has a total design deflection at roof level of 6 inches and the taller Portion B has a total design deflection of 8 inches at the same level, then the portions of the building need to be separated by at least 14 inches. The 2006 IBC and ASCE 7- 05 are silent with respect to separation between adjacent buildings and the distance by which a structure needs to be set back from the property line.

Q: The above suggests that for purposes of separation, the IBC and ASCE 7 treat portions of a structure differently than adjacent structures on the same property or on adjoining properties. Is this correct?

A: Yes. It also means that, if in the above example, the two portions were adjacent buildings instead, the minimum separation distance required would be 10 inches, not 14 inches, under the 2003 IBC (as already noted, the 2006 IBC does not address separation of adjacent buildings). Note that it is possible to reduce this gap in accordance with the exception to 2003 IBC Section 1620.4.5. Also, strictly speaking, the title of ASCE 7-05 Section 12.12.3 is not quite correct. A more appropriate title would be "Portions of the Same Structure."

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 skghosh@aol.com and dowtyskga@cox.net, respectively, or via the firm’s website at www.skghoshassociates.com.

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