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The code provisions for earthquake-resistant design have been substantially revised in the development of the International Building Code (IBC) and other relatively new standards. One of the most significant changes, which is likely to have significant repercussions nationwide, is the switch from the use of a 475-year mean return period earthquake (an earthquake with a 10% chance of exceedance in 50-year) to a 2475-year mean return period earthquake (an earthquake with a 2% chance of exceedance in 50-year) in design of new buildings and in the evaluation of existing buildings. According to the Federal Emergency Management Agency (FEMA), the design life span (or average life) of a normal structure is generally assumed to be ∼50 years. Consequently, wind and snow loads are based on a 50-year mean return period, and flood hazard maps are generally based on 100-year and 500-year return periods. Prior to the 2000 IBC, seismic design loads generally matched the assumption of a 50-year design life and were designed with the intent to withstand a 475-year earthquake. This paper addresses the switch to a 2475-year earthquake from engineering and economic perspectives. Based on engineering and economic research, the switch does not appear to be fully justified and may have significant negative repercussions, particularly on existing building stock. Using the annualized estimated loss (AEL), the annualized estimated loss ratio (AELR), and the per capita annualized estimated loss—all based on results from the FEMA-developed Hazards U.S. (HAZUS) analysis program—the use of the 2475-year earthquake in the design of new structures appears to be questionable from both economic and engineering standpoints.

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