Applicability of UBC-97 Eqn. 30-7 to Drift Calculations

Design of buildings in California currently falls under the provisions of the 1997 Uniform Building Code (UBC-97). Among its provisions, UBC-97 prescribes minimum base-shear strength requirements (PDF file - 105.5 KB). In addition to satisfying strength requirements, UBC-97 establishes maximum drift limits. For tall building designs satisfying the prescriptive provisions of UBC-97, an important question is whether Equation 30-7 should be used to calculate the drift demand. This subject has been addressed in a SEAOC Seismology Committee Background and Position Regarding 1997 UBC Eq. 30-7 and Drift (September 2001) (PDF file - 146.5 KB), where it is stated that the Seismology Committee position is that Eq. 30-7 should be used for drift demands.

Some jurisdictions currently permit performance-based seismic designs of tall buildings in which exceptions are taken to some of the prescriptive provisions of the code. A question that arises in these designs is whether it is acceptable for one of the exceptions to be an exemption to the drift checks using Equation 30-7. To help develop a position on this question, PEER conducted a two-hour WebEx meeting on 23 February 2007. The agenda (PDF file - 63.5 KB) provided opportunities for prepared presentations and open discussion by the meeting participants.

The meeting resulted in development of a consensus on the following position:

For performance-based design, it is acceptable for the design to not apply Equation 30-7 to the calculation of drift demands, provided there is a peer-reviewed performance check at MCE level that demonstrates, using a nonlinear dynamic analysis with appropriate ground motions, that the drift demand does not exceed 0.03. This position is intended to be used in conjunction with a performance-based approach such as the one being developed by an ad hoc task group of SEAONC for the City and County of San Francisco. See draft document (PDF file - 293.3 KB). Note that this is a draft document, not an official position of SEAONC or any other organization.

During the discussion it also was noted (PDF file - 36 KB) that we only have one recording close to a magnitude ~8 earthquake - the Pump Station 10 recording of the Denali, Alaska earthquake - so we do not know much about such ground motions. From a theoretical point of view, we would expect a magnitude ~8 earthquake to have quite large demands. Ground motion selection and scaling should include these effects if warranted.

Supporting Documents

Agenda (PDF file 144 KB)
Seismology Committee Background and Position Regarding 1997 UBC Eq. 30-7 and Drift September 2001 (PDF file - 60 KB)
Near-Source Factors - History - Kircher (PDF file - 316 KB)
Design Drift Requirements For Long-Period Structures - Searer and Freeman (PDF file - 100 KB)
Unintended Consequences Of Code Modification - Searer and Freeman (PDF file - 80 KB)
Poorly Worded, Ill-Conceived, And Unnecessary Code Provisions - Searer (PDF file - 180 KB)
P-Delta and Minimum Base Shear a - Krawinkler (PDF file - 224 KB)
P-Delta and Minimum Base Shear b - Krawinkler (PDF file - 480 KB)
Code Minimum Base Shear Requirements - Maffei (PDF file - 7.4 MB)
Minimum Base Shear - Graphics - Maffei (PDF file - 36 KB)
Selected Near Field Motions - Hamburger (PDF file - 128 KB)
Thoughts On Minimum Strength & Stiffness Requirements For Seismic Design - Deierlein (PDF file - 140 KB)
Expected characteristics of near fault magnitude 8 ground motions - Somerville (PDF file - 36 KB)
Draft SEAONC Recommended Version 13 February 2007 (PDF file - 212 KB)

Meeting Participants:

- Norm Abrahamson
- Yousef Bozorgnia
- Greg Deierlein
- Dave Fields
- Sigmund Freeman
- Michael Gemill
- Ron Hamburger
- Helmut Krawinkler
- Marshall Lew
- Joe Maffei
- Steve Mahin
- Neville Mathias
- Jack Moehle
- Mark Moore
- Farzad Naeim
- Paul Somerville
- Jeff Taner