The American Society of Civil Engineers’ 2005 Minimum Design Loads for Buildings and Other Structures (ASCE 7-05), as it is now printed with the blue cover, was first made available in November 2005 and includes Supplement No. 1. The reason that the standard already had a supplement with its release is in itself rather interesting. A white-cover version of ASCE 7-05 was released at the end of 2004 for the purpose of the International Code Council (ICC) code development cycle. However, it was always the intent that ASCE 7-05 reference the latest materials standards, which weren’t, for the most part being completed until 2005. The ICC Structural Subcommittee needed a copy of ASCE 7-05 in 2004 so that they could review and approve it through the code development process. In order to achieve both acceptance by the ICC Structural Subcommittee and have the final version reference the latest materials standards, a white-cover version was released while Supplement No. 1 was being simultaneously worked on. Supplement No. 1 simply updated ASCE 7-05 to the latest (2005) editions of a number of materials standards such as the American Concrete Institute’s ACI 318-05.
More recently, Supplement No. 2 has been approved by the ASCE 7 Committee. This Code Simple will present what Supplement No. 2 contains and the reasons for its approval.
Supplement No. 2 modifies equations12.8-5, 15.4-2 and 15.4-3 in ASCE 7-05, as shown below:
0.01 0.044SDSI ≥ 0.01 (Eq. 12.8-5) [applicable to buildings]
0.03 0.044SDSI ≥ 0.03 (Eq. 15.4-1) [applicable to non-building structures not similar to buildings]
0.01 0.044SDSI ≥ 0.01 (Eq. 15.4-3) [applicable to an exception for non-building structures not similar to buildings]
In the previous edition, ASCE 7-02, the corresponding minimum Cs value for buildings (Item 1 above) was 0.044SDSI. We think it would be most helpful if we first explained why 0.044SDSI changed to 0.01 in ASCE 7-05 and then why it changed back to 0.044SDSI in Supplement No. 2.
Why 0.044SDSI changed to 0.01 for buildings in ASCE 7-05
The reason for the removal of the minimum base shear (0.044SDSIW) that was a function of SDS was that there is now the constant-displacement branch to the design spectrum, which means no arbitrary minimum should be necessary. The proponents of the removal did not even want the 0.01 minimum, but had to relent when it was pointed out that it was a structural integrity minimum, required even for buildings assigned to SDC A (see ASCE 7-05 Section 11.7.2). This resulted in changing the seismic design force levels for some tall buildings rather drastically. For buildings where S1 ≥ 0.60g, a minimum base shear of 0.5S1W/(R/I) is required (see ASCE 7-05 Equation 12.8-6). This equation produces results that exceed those associated with the former equation 0.044SDSIW, which is consistent with near-source effects. See Figure 1 for illustration of minimum base shears as set forth in ASCE 7-05 Section 22.214.171.124.
Why 0.01 changed back to 0.044SDSI for buildings in Supplement No.2
The need for this change was indicated by the results from the 75 percent Draft of ATC-63, Quantification of Building System Performance and Response Parameters, which indicate that tall buildings, designed without imposing the minimum design base shear requirement of 0.044SDSIW, may fail at an unacceptably low level of seismic excitation.
As for non-building structures not similar to buildings, they have low R-values compared to the special reinforced concrete moment frames studied in ATC-63, and the ASCE 7 standards committee chose not to restore the high minimum base shears (such as 0.14SDSI) for non-building structures not similar to buildings found in ASCE 7-02. In many cases, these previous minimum base shears gave many non-building structures not similar to buildings effective R-values less than 1.0. Therefore, the ASCE Seismic Subcommittee believes that the minimum base shear equation of 0.044SDSI used for buildings should also be applied to non-building structures not similar to buildings.
At the 2008 ICC Public Hearing held in late February, the International Building Code (IBC) Structural Subcommittee recommended approval of a code change that adopts Supplement No. 2 to ASCE 7-05 into the 2009 IBC. While this recommendation is subject to public comment, it is a virtual certainty that the recommendation will stand. Thus when the 2009 IBC is adopted by a local jurisdiction, Supplement No. 2 will be enforced by the jurisdiction. When the 2006 IBC was adopted by the State of California as the basis of the 2007 California Building Code, which went into effect on January 1, 2008, Supplement No. 2 had not yet been developed. Many California jurisdictions, such as the Cities of Los Angeles, San Diego, and San Francisco, have since adopted Supplement No. 2 changes by way of local amendments to the 2007 California Building Code.
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 firstname.lastname@example.org and email@example.com, respectively, or at www.skghoshassociates.com.