Soil-Structure-Interaction: Analysis through measurements

In common buildings the soil-structure-interaction (SSI) is governed by the elastic behavior of the building. The best conditions to see the soil effect predominate would be a dynamic rigid body movement of a massive building. For this reason measurements were carried out on an old anti-aircraft tower from World War II on typical Viennese soil conditions. The dynamic properties of the soil could be determined in two different ways. The first one is the common way using the dispersion of the Rayleigh waves on the soil surface and the back calculation to shear wave profiles. The second approach is the back calculation of the soil stiffness based on the soil-structure-interaction. With the simultaneous measurement of the dynamic movement of the building the kinematic behavior of the SSI could be determined. The tilting oscillations of the tower in both directions with different frequencies transmitted to the soil could be measured in the surroundings. The decay of these vibrations and their influence on the H/V-method results was studied. The results of the dynamic measurements were compared with different methods of numerical simulation. The benefits and the disadvantages of the methods can be compared. The calculation values of the numerical models could be calibrated with the real dynamic properties of the measured SSI-system. er. To measure this effect, two artificial excitation types: an impulse (for the high frequencies) and a harmonic excitation of the soil (for the low frequencies) were applied. The ambient vibrations were recorded in several triangular setups with different dimensions. By analyzing the measured vibrations the dispersion curve could be evaluated. Figure 2. Surface-wave dispersion test setup with artificial impulse excitation using a light falling weight device. Figure 3. Surface-wave dispersion test setup with artificial harmonic excitation using a “shaker” as a mass-spring-system. Figure 4. Triangle setup: dispersion of best fitting profiles (left) and shear wave velocities of best fitting profiles (right). Since the natural soil is no homogeneous infinite half-space but consists of different layers of soil types with diverse soil properties, the dispersion curve of the surface waves is influenced by this. By means of numerical simulations of different layered soil profiles the corresponding dispersion curves can be evaluated. The artificial dispersion curve best fitting to the measured one can be found by a special optimization algorithm. This inverse analysis method does not give a unique result but a most likely soil profile can be found. The results of these tests are the shear wave velocity profiles of the soil. The shear wave velocity is the leading parameter for assessment of the dynamic soil properties. The shear wave velocity is a real material parameter and it is constant for all frequencies (shear waves do not disperse). 3 MEASUREMENTS ON THE TOWER The ambient vibrations of the Leitturm-structure were measured with three very sensitive geophones (Lennartz LE-3D-5s) on different levels. The Fast Fourier Transformation (FFT) analysis of the measured data of vibrations on the 11 th floor shows the eigenfrequency of 1.6 Hz for the tilting motion in the “weak”, y-direction (blue) and 2.0 Hz in the “stiffer”, x-direction (red). The peak of the vertical oscillation is not as significant but can be found at 3.9 Hz. Because of the tower’s rigid structure the soil-structure interaction behavior is governed by the mass of the tower and the stiffness of the soil. Figure 5. Sensor setup for the measurement on the gallery level