**11. Conclusions**

174 Earthquake Engineering

the height of the floor considered.

**Enveloped and broadened spectra with 2% damping along Ox, g**

0 5 10 15 20 25 30 **Frequency, Hz**

(a) and vertical axis Oz (b)

system placed on the platform.

to the surface of the rigid and flexible soil. In the first case this very motion became the response motion of the base mat; in the second case, the response motion of the base mat was modified by the SSI effects. The comparison of the floor response spectra (enveloped over 8 corner nodes of the floor slab, smoothed and broadened 15% each side in the

We see the considerable difference in the spectral shape: SSI effects form the main spectral peak, but for high frequency range (here - after 5 Hz) spectral accelerations with SSI are less than without SSI. As to the maximal accelerations (we see them in the right part of the spectral curves), SSI effects may decrease them (see Fig.14b) or not. It sometimes depends on

**Enveloped and broadened acceleration spectra with 2% damping along Oz, g**

0 5 10 15 20 25 30 **Frequency, Hz**

RG1.60\_flex

RG1.60\_ rigid

**Figure 14.** Comparison of enveloped broadened spectra with 2% damping at the level +21.5 m for excitation RG1.60 for rigid and flexible soil; spectral accelerations are shown along horizontal axis Ox

RG1.60\_flex

RG1.60\_ rigid

spectra is shown in Fig.15 in the same format as in Fig.14.

The second comparison is for surface and embedded base mats. Site-specific threecomponent seismic excitation is "applied" at the free surface of two flexible soil foundations: first, real soil foundation; second, the same soil foundation without upper 10.4 meters of soil (corresponding to the embedment depth of the structure). The comparison of the response

0 0,5 1 1,5 2 2,5 3 3,5 4 4,5

We see that the embedment considerably impacts the first spectral peaks, decreasing spectral accelerations. The physical reason is that the mass of the "outcropped soil" for the embedded basement in fact is subtracted from the mass of the basement when inertial loads are developed for the platform model of "inertial interaction" in the moving coordinate

frequency range) at the structural level +21.5 m is shown in Fig. 14.

Concluding the chapter about soil-structure interaction (SSI), the author should like to give several recommendations to engineers.


3. SSI effects are frequency-dependent. Most of effects are valid in a certain frequency range. Out of this range they may lead to the opposite changes.

Soil-Structure Interaction 177

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Nowadays, the research goes forward. The current goal is to combine non-linearity inside Vint (including contact non-linearity and structural nonlinearity) with linearity of infinite soil in Vext.
