The economic losses left by large recent earthquakes are still considerable, and modern society is wanting not only life protection; it is also demanding that buildings can be immediately occupied after a strong earthquake. The performance-based seismic design allows engineers to design structures with a desired seismic performance for a specified level of hazard. This requires a high standard in the different items involved in the seismic design. One of the key factors is associated with the seismic loads, which are strongly dependent on the local ground conditions. Accordingly, an alternative seismic site classification is proposed, which is based on two dynamic parameters of the ground: the equivalent shear wave velocity, VS30-E, that reproduces the dynamic lateral stiffness of the upper 30 m of the ground, and the predominant period of the site, which is proposed to be estimated applying the H/V spectral ratio of ambient vibration measurements. All the details of this site classification are explained in the paper.

Introduction

In spite of the tremendous advances in the field of earthquake engineering, economic losses generated by recent earthquakes are still considerable, far from any socio-economically satisfactory standard. An important part of these losses is attributed to the severe damages suffered by buildings (residential, commercial, industrial, governmental, educational, cultural, hospital, etc.), infrastructure and structures of the production sector. In Table 1 the estimated direct economic losses of the latest earthquakes are presented (Data from USGS[1], Kajitani et al. [2], Horspool et al. [3], Aon Benfield [4] and Senplades [5]). It can be observed that in the particular case of Tohoku Earthquake, the cost is substantially high due to the damages caused by the tsunami. In any case, modern society is wanting not only life protection; it demands that buildings can be occupied and function following a strong earthquake. This also means that water, electricity, gas, and other services have to be operational as well. Therefore, the challenge is to reduce the tremendous economic impact that earthquakes still have on society, and accordingly, resilience and reliability of structures is an important issue [6]. According to FEMA [7], one of the most promising tools that can be used to reduce the damage and losses resulting from an earthquake, or other similar disaster, is the performance-based seismic design (PBSD). The philosophy of this design methodology is to accomplish a reliable structure design meeting performance objectives [8]. Historically, seismic design has focused on providing resistance to the structural components of the structures. However, it is well recognized that this approach by itself does not guarantee successful seismic behavior of the structure.