Current Seismic Code Basis 1997 NEHRP or IBC 2000
Objective: Reduction of likelihood of collapse during earthquakes and physical survival
No explicit design for post-earthquake functionality and economic survival
Practicality dictates a linear elastic analysis-based procedure based on reduced ground motion
- System Response Modification Coefficient R ranges from 1.5 to 8
Net return period for elastic design varies depending on site location in the U.S.A.
- 500-1500 year events are reduced down to net 50-500 year elastic events
Notes:
The current U.S. seismic code is limited in delivering predictable building performance. The code is only intended to preserve life safety by reducing the likelihood of collapse, and does not include explicit design to maintain post-earthquake building function or economic survival of the structure after a design event.
System Response Modification Factors are highly significant in reducing the design seismic force for practical elastic analysis, but in fact are calibrated to past consensus judgement on building performance. Performance is therefore an inferred indirect result of current design procedures.
Return periods for design events vary from 500 years on the west coast to 1500 years on the east coast of the U.S. The net return period of current code for strength level design is 50-500 years, depending on the site and structural system.