Special facilities, such as nuclear power plants or liquefied natural gas tanks, require specific evaluations of the seismic hazard prior to designing the facility. For other more conventional installations, the direct application of the current seismic provisions is usually sufficient. The seismic hazard at a site is the annual probability of experiencing each level of seismic motions at the site; its determination is frequently accompanied by the generation of uniform hazard spectra.

Principia has evaluated the seismic hazard at many sites around the world. Sometimes the evaluation is carried out using Cornell's traditional method, which requires the construction of seismogenetic zones. More often, we prefer the newer and more objective methodology developped by Woo, with whom we collaborate often, which does away with the requirement of a more or less artificial zonation.

The figures show a map with the historical distribution of epicentres in Spain and the seismic hazard curve at a Spanish site, obtained using the two methodologies mentioned above.

The dynamic effects of wind action on structures with large dimensions may be sufficiently important that the static approximations become clearly inapplicable. For large structures, the use of gust factors may be too crude as a methodology; instead, spectral methods are required, which provide a better description of the random characteristics of the wind.

Principia has applied these methods in bridges of various types. The example shown here corresponds to a concrete viaduct, with long spans and very high piers; its response under wind loading was studied both in the service conditions and in various construction phases.

The figures show the pressure spectrum obtained, the spatial cross-correlations between stations and the spectral distribution of displacements.

Society is gradually adopting more rational and objective approaches to establish the risk levels that it considers acceptable. Also the methodologies provided by risk analyses are increasingly being employed in the process of deciding between various available alternatives.

Principia frequently contributes in projects of this type, some times with hazard evaluations, others with consequence analyses, etc. In such projects, Principia normally collaborates with a risk analysis specialist, thus providing a more global perspective than Principia alone could generate. In particular, Risk Support Ltd is a frequent partner of Principia in these projects.

The new port planned for A Coruña in Spain is one of the projects in which Principia and Risk Support collaborated for conducting the risk analysis. The figure shows a typical loss-frequency curve; the comparison of curves of this type, obtained for the various options, is usually an essential ingredient of the decission making process.