MS7.11 - Dynamic Soil-Structure Interaction and Wave Propagation

In the seismic design of building structures, the natural period and damping ratio are important parameters that greatly affect the response of the building. However, most of the seismic observation data for actual buildings are affected by the dynamic soil structure interactions (SSIs). Therefore, a method for estimating the effect associated with the SSI from measurement data in actual buildings is required. In this study, as one of the methods for estimating the vibration characteristics of the SSI system and those of the building itself from the actual data, we estimate the vibration characteristics of the SSI system by applying the modal error iterative correction (MIEC) method, which is a general purpose inverse analysis method for input output (IO) systems, to the sway rocking (SR) model. The parameters identified by the MIEC method are collectively identified as the story stiffness and damping ratio of the building and dynamic soil springs. In addition, by identifying each parameter, we evaluate how the horizontal and rocking interactions in the SSI system affect the vibration characteristics of the building itself. The novelties of this study are that the parameters of each story of the super structure and the dynamic soil spring are estimated collectively. Moreover, the parameters are estimated considering not only the first mode but also the higher modes. First, an overview of the IO systems to estimate the vibration characteristics using the MIEC method is presented. Next, a seismic response analyses are conducted using a numerical analysis model simulating a three story reinforced concrete (RC) building supported by pile foundations. The proposed identification method is applied to the seismic response analysis data. Then, estimation accuracy of the parameters based on this method is verified for the parameters set in the analysis model. Finally, the proposed method is applied to the large shaking test data of the RC structure specimen supported by the pile foundation performed at E Defense which is the largest shaking table facility in the world. In the experiment, vibrations were applied from small amplitude to large amplitude level for seismic excitations that greatly progressed the plasticization of the specimen. By comparing the results of the parameters estimated using the numerical analysis of the SR model and the MIEC method, it was confirmed that each parameter could be estimated with generally good accuracy. By applying this method to a large shaking table test of the SSI system and identifying their parameters according to the amplitude level experienced, the variation characteristics of the SSI system and their changes can be investigated. As a result, it was found that the natural period and damping ratios of the first and second modes tended to increase due to the influence of SSI. On the other hand, the variation of these effects according to the amplitude level was found to be relatively large in the damping ratio than in the natural period. In the future, we plan to study the vibration characteristics of an actual building by applying this proposed method.

This study presents a semi-analytical solution for the 3D problem of a cylindrical tunnel embedded in an elastic half-space subject to plane harmonic compressional and shear waves. Both the tunnel and soil are modelled as an elastic continuum. Conformal mapping is employed to transform the original physical domain with boundary surfaces of two different types onto an image domain with surfaces of the same type, which makes the problem easier to solve. The total wave field in the half-space consists of incident and reflected (from the half-space surface) plane waves, as well as directly and secondary scattered cylindrical waves, while the total wave field in the tunnel consists of refracted cylindrical waves. The secondary scattered waves, generated when the cylindrical waves directly scattered from the tunnel meet the half-space surface, are represented by cylindrical waves that originate from an image source, which is in line with the spirit of the method of images. The unknown amplitude coefficients of the cylindrical waves are determined from the boundary and continuity conditions of the tunnel-soil system by projecting those onto the set of circumferential modes, which results in a set of algebraic equations. Results show that the present method converges for a small number of circumferential modes. We observe very good agreement between the obtained results and those in literature. In a systematic evaluation, we demonstrate that the method works well for the frequency band of seismic waves, as well as for the complete considered ranges of the tunnel/soil stiffness ratio, the embedded depth of the tunnel, the vertical incident angle and the tunnel thickness. However, the results obtained for a moderate tunnel/soil stiffness contrast under the incident compressional wave are inaccurate when using Hankel functions to represent the cylindrical waves in the tunnel, which is due to the refracted shear waves in the tunnel transitioning from propagating to evanescent. These inaccuracies can be perfectly overcome by representing the waves in the tunnel by Bessel functions. We also find that the present method generally works better for the incident compressional wave than for the incident shear wave, as the condition number of the matrix (related to the mentioned algebraic equations) is often larger in the latter case. In view of engineering practice, we conclude that the tunnel is safer when the surrounding soil is stiffer, the tunnel is thicker and the vertical incident angle is larger. Finally, the present method, which is in general fast, elegant and accurate, can be used in preliminary design so as to avoid pronounced resonances, and to assess stress distributions and ground vibrations.

The design of structures to seismic excitation in which soil-structure interaction (SSI) is not considered with enough detail can lead to overdesigned solutions and, in the case of large structures, lead to significant increases in the overall budget of the project. For such large structures, spending more effort in modelling SSI to derive less conservative solutions (yet, solutions that satisfy the seismic demand) can be advantageous. In this work, we report on the method recently adopted at Royal Haskoning DHV (RHDHV) to assess the seismic response of a large building, which is partly deeply embedded in soil and partly founded on piles. Due to the critical nature of the project, SSI is not just of financial interest, but also a requirement from the relevant authorities. The method described in this work consists of a substructuring technique, where the structure is decoupled from soil. This enables each part to be analyzed with the most suitable domain: frequency for soil and time for the structure. The two parts are thereafter re-coupled in a time-implicit FEM solver. The division between substructures is made at the edges of the floors and walls in contact with soil (for the embedded parts of the structure) and at the foundation beams for the more superficial parts of the buildings. Piles are included in the part containing soil. For the soil part (and piles), a frequency-domain FEM strategy is adopted. This limits the analysis to linear elasticity. Nevertheless, the soil properties can be made linearly equivalent based on the non-linear ‘free-field’ response (i.e., without the presence of the building), as specified in some standards, e.g., ASCE-4-16. In order to consider appropriate reflection-free boundaries and radiation of waves, perfectly matched layers (PML’s) are used at the edges of the FEM model. A Python program has been developed for this purpose, and parallel computing was used to take advantage of the multiple cores available at calculation servers. For the upper structure, an implicit time stepping scheme is used, in which structural non-linearities (e.g., sliding between components) can be modelled. The structural model is created and solved using ABAQUS software. The coupling between the upper structure and soil is done after transforming the soil response to the time-domain. This results in time convolutions at the interaction surfaces, which in turn can lead to an unstable stepping algorithm. Therefore, one must be critical and careful when deciding the time-step to use in the simulation. In this work, we further present our approach to analyze the stability of the system (without having to solve it in its entirety, which is a time consuming task), and present a strategy/simplification which showed to improve the convergence of the coupled algorithm.

Earthquakes are the most intensive sources of free-field vibration. The buildings are also threatened by vibrations caused by other reasons, such as the travel of trucks, passenger cars, trams, and trains, including metro trains, but also mining rockbursts, resulting from underground and open-cast exploitation in mining areas. However, mine-induced rockbursts, as a result of underground and open-cast exploitations are also a hazard to buildings. This problem occurs in many countries all over the world. Additionally, as in the case of earthquake-induced vibrations, the soil-structure interaction (SSI) phenomenon is observed in the case of mining tremors. In this paper, an investigation of one of the SSI consequences – differences between the simultaneously recorded free-field and building foundation vibration, is performed. The research was carried out on the example of an actual, typical, masonry, low-rise office building situated in a coal basin in Poland – the Upper Silesian Coalfield (USC). Two methods of evaluation were used in the analysis: experimental with the use of full-scale tests and numerical with the use of finite element method (FEM) modelling. A rich set of experimental data (including several hundred strong mining tremors) was collected from the free-field and foundation acceleration stations equipped with vibration monitoring. Using the measurement data, very important parameters that affect the SSI phenomenon were considered, e.g. magnitude of mining tremor energy, epicentre distance of mining rockburst, and peak ground value of vibration. Besides the full free-field and building foundation vibration simultaneously registered in the time domain, corresponding response spectra (dimensional and dimensionless) and the ratio of those response spectra were used in the analysis. Numerical analysis of the transmission of the free-field wave to the building foundation is based on three-dimensional (3D) FEM models. The models considered both fixed and flexible supports of buildings with parameters resulting from the properties of the subsoil. Various conceptions of modelling the ground vibration transfer to the foundation were discussed. Additionally, the influence of changes in site conditions on the SSI phenomenon was investigated in the case of low-rise office building vibration induced by mining rockbursts. The collected experimental data allowed for the verification of the proposed models.