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Coupled Site and Soil-Structure Interaction Effects with Application to Seismic Risk Mitigation

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The purpose of NATO ARW 983188Coupled Site and Soil-Structure Interaction Effects with Application to Seismic Risk Mitigation, held in Borovets, Bulgaria, 30 August—3 September 2008, was to present state-of-the-art, onsite, soil-structure interaction effects (SSSI), as manifested in the broader area of south and south-eastern Europe, which is the most seismically prone region of the European continent.

 Another objective was to attempt to define the seismic risk posed to the built environment in this area and to present modern methods for seismic risk mitigation. 

The ARW was very successful and generated an interdisciplinary-type information exchange between the three main groups of participants: geophysicists, geotechnical engineers, and structural engineers.

 The presentations during the workshop can be grouped into four subject areas: (1) site conditions and their role in seismic hazard analyses, (2) soil-structure interaction, (3) the role of site effects and of soil-structure interaction in the design of structures, and (4) general and related subjects.

 The following fields were addressed during the presentations and the discussions: strong ground motion (near-field effects, seismic-wave propagation, free-field motion); geotechnical engineering (slopes, foundations, lifelines, dams, and retaining walls); and structural engineering (buildings, bridges, field measurements, and protective systems). 

The work presented in this volume includes contributions from engineers and scientists, mainly from south-eastern Europe and the neighbouring regions of the Near East. 

The arrangement of contributions in different chapters is not rigorous, and many papers present similar material, which includes broad coverage and different disciplines, since earthquake engineering is by its nature an interdisciplinary subject.

The conclusions reached by the workshop participants can be summarized as follows.

 1. It is important to create an extensive strong-motion database for major urban areas in the seismically prone regions of Europe, and to document local soil and geological site conditions. These data are essential for all aspects of earthquake engineering research and applications and should be made available to the research community through Web sites. 

2. Development of hybrid methods for computer simulations of free-field strong ground motion are of paramount importance if reliable artificial time histories are to be produced “on demand” for the aforementioned regions. 

3. It is important to develop and implement protective systems for special classes of structures in the earthquake-prone regions of Europe

4. It is hoped that in the future the cost of protective systems and the placement of technology will become economically feasible to the point that they can be implemented in a routine fashion in the large groups of conventional structural systems. 

5. The ultimate goal is a high level of protection of the built environment to earthquakes and the availability of low-cost insurance.

 The roundtable discussions during the final day of the workshop addressed a large number of topics. The following represents a summary of the principal and most important observations and recommendations.

New characterizations of site conditions in the near field should be developed that include all relevant components of the forces acting on a structure. With large amplitudes of strong motion, surface soil experiences large, nonlinear response, and ultimately soil failure and liquefaction can lead to large transient and permanent motions.

 Examples of ground failure that can follow liquefaction are lateral spreading, ground oscillations, flow failure, and loss of bearing strength.Lateral spreadsinvolve displacements of surface blocks of sediment facilitated by liquefaction in a subsurface layer.

 This type of failure mayoccuronslopesupto3◦ and is particularly destructive to pipelines, bridge piers, and other long and shallow structures situated in flood plain areas adjacent to rivers.Ground oscillationsoccur when the slopes are too small to result in lateral spreads following liquefaction at depth.

 The overlying surface blocks break, one from another, and then oscillate on liquefied substrate.Flow failuresare a more catastrophic form of material transport and usually occur on slopes greater than 3 ◦ . 

The flow consists of liquefied soil and blocks of intact material riding on and with liquefied substrate, on land or under the sea. Loss of bearing strengthcan occur when the soil liquefies under a structure. 

The building can settle, tip, or float upward if the structure is buoyant. The accompanying motions can lead to large transient and permanent displacements and rotations, which so far have been neither evaluated through simulation nor recorded by strong-motion instruments.