2009 IBC Section 1613.6.7: Minimum distance for building separation

December 2010 » Columns » CODE SIMPLE

By Susan Dowty, S.E., S.K. Ghosh, Ph.D.

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The 2009 International Building Code (IBC) Section 1613.6 includes eight alternatives to the 2005 American Society for Civil Engineers’ Minimum Design Loads for Buildings and Other Structures (ASCE 7-05) seismic provisions. In an important alternative found in Section 1613.6.7, provisions regarding minimum building separation have been added back into the 2009 IBC.

Minimum separation Similar provisions were included in the 1997 Uniform Building Code (UBC) as well as the 2000 and 2003 editions of the IBC. However, when the 2006 IBC was being developed, it was decided to delete much of the seismic design provisions (along with much of snow and wind load provisions) from the code itself and incorporate them only through reference to ASCE 7-05. The objective was to eliminate possible error and confusion arising from the IBC’s practice of adopting and transcribing, sometimes with modifications, the ASCE 7 provisions. The building separation provisions also were deleted, overlooking the fact that ASCE 7 did not include such requirements. This error is now rectified by having the building separation provisions included in the 2009 IBC.

Figure 1: Required separation between two adjacent buildings

Separation between two adjacent buildings — The provisions are very similar to those included in the 2003 and the 2000 IBC. The separation between two adjacent buildings assigned to SDC D, E, or F needs to be adequate to accommodate the maximum inelastic displacements of the two buildings (see Figure 1). The maximum inelastic displacement is computed as the following:

In this equation, δmax is the maximum elastic displacement that occurs anywhere in a floor or roof from the application of the design base shear to the structure. The Deflection Amplification Factor,Cd, is obtained from ASCE 7-05 Table 12.2-1 and the Importance Factor for seismic loading, I, is obtained from ASCE 7-05 Table 11.5-1. The maximum elastic displacement, δmax, includes the effects of translation as well as rotation due to torsion (inherent and accidental), but not the amplification of the accidental torsion. It must be noted that δmax is different from the elastic displacement, δxe, which is determined at the center of mass of floor x or the roof and is used in ASCE 7-05 Equation 12.8-15 to compute inelastic deflection.

The maximum inelastic displacements from adjacent buildings are combined by the SRSS (Square Root of the Sum of the Squares) method to compute the minimum required building separation as follows:

Separation from property line not common to a public way — Where a building assigned to SDC D, E, or F adjoins a property line not common to a public way, the building also needs to be set back from the property line by at least δM (see Figure 2). The distance between two buildings on the same property is concerned with damaging contact from pounding of buildings belonging to presumably the same owner. The property line setback requirement is based on consideration that one owner should not encroach onto another’s property. In some cases, there is an existing building involved, and the engineer needs to compute or assume maximum inelastic displacements of that building, which is not always easy. In cases where enough information is not known to calculate the maximum inelastic displacement, it may be appropriate to take it as corresponding to the maximum allowable drift.

Figure 2: Required separation from property line not common to a public way

Building occupancy It also is worth noting that the maximum inelastic displacement is computed independently of the occupancy of the building, as the expression for the quantity includes the Importance Factor, I, which depends on the occupancy, in the denominator. The importance factor gets included in the maximum elastic displacement, δmax, because it is computed under the design base shear, V, the expressions for which include the Importance Factor, I, in the numerator. Thus, when the Cdδmax is divided by I, the effect of building occupancy is eliminated.

2009 IBC and ASCE 7-10 As an additional note, ASCE 7-10 includes the same building separation requirement; however, it is different from the 2009 IBC in its applicability; it applies to all SDCs, not just to SDCs D, E, and F.

S.K. Ghosh Associates Inc., is a structural seismic and code consulting firm located in Palatine, Ill., and Aliso Viejo, Calif. Presidents 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 atskghosh@aol.com and susandowty@gmail.com, respectively, or at www.skghoshassociates.com.