The seismic safety of an existing building may be assessed for several reasons, for instance with the aim to retrofit and thus preserve structures in future earthquakes or to reduce human risk. For the latter reason, existing structures in Switzerland are usually assessed when a renovation or transformation is planned. Assessing here means that the behaviour of a structure under earthquake excitation is evaluated in relation to the requirements of the current building codes. If the earthquake safety is insufficient, retrofitting measures are required to reduce the risk for the people in the building to an acceptable level. These tasks may be particularly challenging for historical structures, because of the unknowns concerning the structural properties, e.g. the fact that construction drawings in the proper sense of the term often do not exist, and because potential retrofitting measures need to be carefully adapted to the historical structure.

For the assessment, the existing documentation of the building, such as drawings, photos and information on previous renovations, is gathered and studied first. Based on this information, potential weaknesses are identified and suitable techniques to model the building are defined. The initial models, often relatively simple mechanical models which aim at capturing the expected failure modes, serve to obtain a first indication of the resistance of a building in terms of strength and / or deformation capacity. They also help to identify where more information on the structural characteristics and hence probes or other measurements may be necessary. With this additional information the modelling is refined to obtain a more accurate estimate of the earthquake resistance. In case the latter is insufficient, retrofitting concepts for improving the resistance are developed. For historical structures, these concepts are developed not only keeping in mind their technical feasibility and efficiency but also the preservation of the structure. Therefore, retrofitting measures that are removable and as little invasive as possible are typically preferred.

In this presentation, the described assessment procedure is illustrated mainly with two projects in Basel. Both buildings were partially constructed before the earthquake in 1356 and underwent many transformations and renovations throughout the centuries. In one of the buildings, retrofitting measures which shall be implemented during a renovation were planned.