Keywords: seismic design, seismic norms, EC8 provisions, local site conditions, dynamic soil-structure interaction, retaining systems.

It is evident that the dynamic response of various structures and infrastructures that are founded on the ground surface may be influenced by the local site conditions, the dynamic soil–foundation–structure interaction, and the potential existence of nearby structures, such as retaining systems or underground structures. By recognising the importance of the aforementioned phenomena, the contemporary seismic norms worldwide, such as Eurocode 8 [1], try to cover most of these topics. However, the rather simplistic provisions of the seismic norms make the seismic design rather conservative, but not always safe.

A representative case of complex dynamic soil-structure interaction in urban environments is the retaining systems that are used to support, apart from soil layers, structures founded on the retained soil layers. It is evident that during a seismic event the dynamic response of each component of this complex system (retaining system, soil, and structure) substantially may affect the response of the rest and vice versa. Therefore, to illustrate more clearly the above issues, one of the most complex cases that geotechnical/structural engineers have to confront in urban environments is the phenomenon that could be adequately described as "dynamic wall–soil–structure interaction" (DWSSI). This is a very complicated problem that combines: (a) the dynamic interaction between the wall and the retained soil layer(s), and (b) the standard dynamic interaction of a structure with its underlying soil layer(s). It is most probable that a standard design will not deal with this multiple dynamic interaction, but it will contain the following (according to EC8 provisions) [2]: (a) selection of the proper design acceleration spectrum for the given local soil conditions, ignoring the presence of the retaining wall; (b) examination of the standard dynamic interaction of the structure with its underlying soil layer(s), ignoring again the presence of the retaining system; (c) design of the retaining wall using the standard pseudo-static approaches proposed by EC8.

From this perspective, the current study briefly highlights these aspects by: (a) describing the significant characteristics of the two most important topics (e.g., local site conditions and dynamic soil-structure interaction) and the way they are separately tackled in EC8, (b) presenting an illustrative case of DWSSI in which EC8 is incapable of covering the multiple interaction phenomena, and (c) suggesting guidelines to deal with complex phenomena in a more realistic and accurate manner.

[1]

Eurocode 8 (EC8), "Design of structures for earthquake resistance, Part 1: General rules, seismic actions and rules for buildings", CEN-ENV, European Committee for Standarization, Brussels, 2004.

[2]

Eurocode 8 (EC8), "Design of structures for earthquake resistance, Part 5: Foundations, retaining structures and geotechnical aspects," CEN-ENV, European Committee for Standarization, Brussels, 2004.

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