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. 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.
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.
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.
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