Matt S Dietz

Model Container Design for Soil-Structure Interaction Studies

Physical modelling of scaled models is an established method for understanding failure mechanisms and verifying design hypothesis in earthquake geotechnical engineering practice. One of the requirements of physical modelling for these classes of problems is the replication of semi-infinite extent of the ground in a finite dimension model soil container. This chapter is aimed at summarizing the requirements for a model container for carrying out seismic soil-structure interactions (SSI) at 1-g (shaking table) and N-g (geotechnical centrifuge at N times earth’s gravity). A literature review has identified six types of soil container which are summarised and critically reviewed herein. The specialised modelling techniques entailed by the application of these containers are also discussed.

Seismic Behavior of Thin-Bed Layered Unreinforced Clay Masonry Shear Walls Including Soundproofing Elements

According to the current standards, unreinforced masonry may only be used in regions of low seismicity as the material for the lateral-load resisting system. This requirement may be too safe-sided and leading to not cost-effective solutions for moderately seismic regions. This chapter presents overview of experimental results from shake table tests on unreinforced masonry shear walls carried out in the EQUALS Laboratory of Bristol University, in order to assess, and possibly enhance, the current seismic design rules. The study also includes as additional parameter the presence of soundproofing devices required in buildings with numerous dwellings, in order to achieve the acoustic isolation recommended by recent standards. In practice the required level of acoustic isolation is obtained by locating horizontal rubber layers in the wall. These layers are likely to influence significantly the dynamic response of the wall and hence of the whole structure under seismic actions. Tests are performed on walls realized with masonry units and construction methods typical of North-Western Europe.

An overview of control schemes for hydraulic shaking tables

Shaking table testing is a common experimental method in earthquake engineering for performance assessment of structures subjected to dynamic excitations. As most shaking tables are driven by servo hydraulic actuators to meet the potentially significant force stroke demand, the review is restricted to hydraulic shaking tables. The purpose of the control systems of hydraulic shaking tables is to reproduce reference signals with low distortion. Accurate control of actuators is vital to the effectiveness of such apparatus. However, the system dynamics of a shaking table and the specimens to be tested on the shaking table are usually very complex and nonlinear. Achieving the control goal can prove to be challenging. A variety of closed- and open-loop control algorithms has been developed to solve different control problems. With the focus placed on the control schemes for hydraulic shaking tables, the paper reviews algorithms that are currently used in the testing industry, as well as those which are the subject of academic and industrial research. It is by no means a complete survey but provides key reference for further development.

Elastodynamic metasurface: Depolarization of mechanical waves and time effects

We report the concept of microstructured surfaces with inner resonance in the field of elastodynamics, so-called elastodynamic metasurfaces. Such metasurfaces allow for wavefield manipulation of mechanical waves by tuning the boundary conditions at specific frequencies. In particular, they can be used to depolarize elastic waves without introducing heterogeneities in the medium itself; the physical means to do so in homogeneous elastic media used to remain, surprisingly, an open question while depolarization is commonplace in electromagnetism. The principle relies on the anisotropic behaviour of a subwavelength array of resonators: Their subwavelength configuration confines the Bragg interferences scattered by resonators into a boundary layer. The effective behaviour of the resonating array is expressed with homogenization as an unconventional impedance, the frequency-dependence, and anisotropy of which lead to depolarization and time effects. The concept of the elastodynamic metasurface is tested experimentally and results bear testament to its efficacy and robustness. Elastodynamic metasurfaces are easily realized and analytically predictable, opening new possibilities in tomography techniques, ultrasonics, geophysics, vibration control, materials and structure design.

Experimental Investigation of Dynamic Behavior of Cantilever Retaining Walls

The dynamic behaviour of cantilever retaining walls under earthquake action is explored by means of 1-g shaking table testing, carried out on scaled models at the Bristol Laboratory for Advanced Dynamics Engineering (BLADE), University of Bristol, UK. The experimental program encompasses different combinations of retaining wall geometries, soil configurations and input ground motions. The response analysis of the systems at hand aimed at shedding light onto the salient features of the problem, such as: (1) the magnitude of the soil thrust and its point of application; (2) the relative sliding as opposed to rocking of the wall base and the corresponding failure mode; (3) the importance/interplay between soil stiffness, wall dimensions, and excitation characteristics, as affecting the above. The results of the experimental investigations were in good agreement with the theoretical models used for the analysis and are expected to be useful for the better understanding and the optimization of earthquake design of this particular type of retaining structure.

Dynamic Behaviour of Reinforced Soils – Theoretical Modelling and Shaking Table Experiments

The dynamic response of soil-pile-group systems are modelled both analytically, using homogenisation theory, and physically, using a shaking table to excite a soft elastic material periodically reinforced by vertical slender inclusions. A large soil/pile stiffness contrast is shown to lead to full coupling in the transverse direction of the bending behaviour from the piles and the shear behaviour from the soil. Analytically derived performance predictions capture important characteristics of the experimentally observed response that are missed when using alternative analytical modelling approaches. The homogenisation theory approach to modelling of generalised media is valid.

Acceleration harmonic estimation for a hydraulic shaking table by using particle swarm optimization

The acceleration response of a servo-hydraulic shaking table to a sinusoidal input motion is inevitably distorted by parasitic harmonic content caused by inherent non-linearities within the system. Herein, an algorithm is developed to characterize these parasitic motions in order to facilitate harmonic cancellation. The proposed algorithm is based on particle swarm optimization (PSO). Optimization is achieved by each particle’s movement, updated by its local best-known position and global best-known position according to a fitness function, which itself is a function of the estimation error between the identified acceleration and the original acceleration response. The estimation scheme is validated by experiments on a servo-hydraulic shaking table and results are compared with a more traditional harmonic analysis.

Multi-Building Interactions and Site-City Effect: An Idealized Experimental Model

This chapter aims at identifying, describing and quantifying multi-building interactions and site-city effects through experimental, theoretical and numerical crossed-analysis. Multiple Structure-Soil-Structure interactions are investigated through an idealized experimental model of a city on a soft layer the properties of which are simple enough to be reproduced in numerical and theoretical models. The experimental set-up is designed to provide a good matching between the fundamental frequencies of the city and of the layer. Experimental results show (i) drastic changes in the layer’s response with two low amplitude resonance peaks favorable to longer coda and beatings and (ii) unconventional depolarization effects. The resulting data is compared with the theoretical city-impedance model derived by homogenization methods (Boutin and Roussillon, Bull Seismol Soc Am 94(1):251–268, 2004) and with a 2D hybrid BEM-FEM numerical model (Padron et al., Calculo de estructuras de barras incluyendo efectos dinamicos de interaccion suelo-estructura. Master thesis, Universidad de Las Palmas de G.C., Spain. http://hdl.handle.net/10553/10472, 2004). The specific features of multiple interactions are successfully reproduced by both models providing a qualitative and quantitative agreement with experimental results and with one another.

Seismic Behaviour of Thin-Bed Layered Unreinforced Clay Masonry Frames with T- or L-Shaped Piers

According to current standards, the use of unreinforced masonry is only recommended in regions of low to moderate seismicity as a resisting system to carry earthquake-induced horizontal forces. This requirement is however felt as rather conservative and leading to uneconomical constructive solutions, in particular for low seismic regions. Moreover, the seismic analysis of masonry structures has to be performed along two main perpendicular directions, usually neglecting the contribution of wall elements perpendicular to the seismic action. Horizontal elements (spandrel, etc.) are also commonly disregarded. In such a context, the present contribution provides an overview of experimental results obtained from shake table tests on unreinforced masonry frames carried out in the EQUALS Laboratory of Bristol University in order to assess, and possibly enhance, current design rules. The study is focused on the contribution of walls perpendicular to the seismic action and on the influence of the frame effect induced by the coupling of the walls through horizontal reinforced concrete elements, such as lintels and floor slabs. Another point of interest of the study is the influence of the gravity loading situation, comparing a floor slab supported by the shear walls as well as by the perpendicular elements, with a floor slab supported by the perpendicular walls only. Tests are performed on walls constructed with units and construction methods typical of the North-Western European region.

Experimental Assessment of Seismic Pile-Soil Interaction

Physical modeling has long been established as a powerful tool for studying seismic pile-soil-superstructure interaction. This chapter presents a series of 1-g shaking table tests aiming at clarifying fundamental aspects of kinematic and inertial interaction effects on pile-supported systems. Pile models in layered sand deposits were built in the laboratory and subjected to a wide set of earthquake motions. The piles were densely instrumented with accelerometers and strain gauges; therefore, earthquake response, including bending strains along their length, could be measured directly. Certain broad conclusions on kinematic and inertial SSI effects on this type of systems are drawn.