GROUND ACCELERATION & AREA COEFFICIENT

GROUND ACCELERATION & AREA COEFFICIENT

1. PGA(Peak Ground Acceleration)
: The amplitude of the ground acceleration time-history caused by earthquake waves.

2. EPA(Effective Peak Ground Acceleration)
: The EPA was defined as proportional to the spectral ordinates corresponding to periods within the range of 0.1 to 0.5 sec. The constant of proportionality (for a 5 percent damping spectrum) was set at a standard value of 2.5 which is a spectral amplification factor. The EPA is determined by dividing by 2.5 the corresponding short period(around 0.1~0.5sec, c.f.> long period: not less than 1sec) spectral acceleration value.
'Concept of Effective Acceleration- It is that acceleration which is most closely related to structural response and to damage potential of an earthquake. It differs from and is less than the peak free-field ground acceleration. It is a function of the size of the loaded area, the frequency content of the excitation, which in turn depends on the closeness to the source of the earthquake, and to the weight, embedment, damping characteristic, and stiffness of the structure and its foundation.’ Earthquake Spectral and Design(N.M. Newmak, 1982)

Schematic representation showing how effective peak acceleration and effective peak velocity are obtained from a response spectrum.

3. A(Area Coefficient, F =M x Sa = (Mg) x (Sa/g) = AW): Area coefficient is based on the EPA related to structural responses and on 10% probability of exceedance in 50 years that is about 500 years return period. Area coefficients (A=0.07, 0.11) in Korea are equivalent to EPA=0.07g & 0.11g.

4. Equations between PGA & Intensity
1) Eastern US: Log_10 Acc= 0.30MMI + 0.014 (Trifunac & Brady, 1975)
2) Western US: Log_10 Acc = 0.33MMI - 0.500(Gutenberg & Richter, 1956)
Here, Acc: PGA(unit: gal(=cm/sec2)), MMI: Grade Number from MMI