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According to AASHTO regulations:
•Bridges shall be designed for the life safety performance objective considering a seismic hazard corresponding to a seven percent probability of exceedance in 75 yr.
•The precise definition used in the development of the ground shaking hazards maps and the ground motion design tool is five percent in 50 yr.
•Thus, the return period used in development of the hazard maps and in the design tool is actually 975 yr compared to that of seven percent PE in 75 yr of 1,033 yr.
The probability that a coefficient will not be exceeded at a given location during a 75-yr period is estimated to be about 93 percent, i.e., seven percent probability of exceedance.
The use of a 75-yr interval matches the design life prescribed by the AASHTO LRFD Bridge Design Specifications.
It can be shown that an event with a seven percent probability of exceedance in 75 yr has a return period of about 1,000 yr. This earthquake is called the design earthquake.
The figure presents the spectrum development according to AASHTO code:
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Figure: Spectra Development
Table: Spectral Accelerations at different period range
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Period Range
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T ≤ T0
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T0 ≤ T ≤ Ts
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T > Ts
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Sa
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[(SDS – As)*(T/T0)] + As
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Sa = SDS
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Sa = SD1 / T
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Conditions
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T0 = 0.2 Ts
Ts = SD1 / SDS
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Where:
•As: acceleration coefficient
•SDS: short period acceleration coefficient
•SD1: 1-sec period acceleration coefficient
•Fpga: site coefficient for peak ground acceleration
•PGA: peak horizontal ground acceleration coefficient on Class B rock
•SS: 0.2-sec period spectral acceleration coefficient
•S1: 1.0-sec period spectral acceleration coefficient on Class B rock
•Fa: site coefficient for 0.2-sec period spectral acceleration
•Fv: site coefficient for 1.0-sec period spectral acceleration
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