Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 238

motions depends on the earthquake magnitude. For magnitudes larger than M 7,
pulses are present in ground motions with natural periods of 1 to 2 seconds, while
for magnitudes smaller than M 7, these pulses practically disappear (see Figure
4.9)
To consider the local site profile only by recorded ground motions is a very
difficult task. Therefore, numerical methods were introduced in design, considering
one dimensional site response. For a refined solution, two or three dimensional site
analysis may be performed by means of finite elements , but this methodology is
rarely used in the design practice.
The one-dimensional method has now been introduced in the Building Code of
Japan (Midorikawa et al, 2000, Otani, 2004). The verification procedure for
developing the seismic design spectra includes (Fig. 7.17):
(i) The basic design spectra defined at the engineering bedrock;
(ii) The evaluation of the site response from the geotechnical data of
surface soil layers.
The verification procedure considers the soil as a multi-degree-of freedom system,
composed by the soil layers and transforms it into a single-degree-of freedom for
which the spectrum is determined. Using this analytical model, the amplification
factor for surface soil layers is performed and the design acceleration response at
ground surface is determined (Fig.7.18).
The computer program SHAKE (in the new versions SHAKE-91 and SHAKE
2000) is available for the evaluation of the response of a horizontally layered soil
system, by using an equivalent soil traveled by seismic waves (Schnabel et al,
1972, Idriss and Sun, 1993, SHAKE 2000, 2007). This program provides an
approximation of the dynamic response of the soil deposit.
(a) (b)
Figure 7.17 Analytical models: (a) Basic spectrum at engineering bedrock;
(b) Soil model (Gioncu, 2006, after Otani, 2004)