Validation of a coupled FE-BE model of a masonry building with in-situ measurements

Earthquakes induced by the gas extraction is a problem of serious concern in the northern part of The Netherlands. The earthquakes recorded to date can be classified as minor based on their maximum local magnitude (ML=3.6). However, (i) their shallow focus, (ii) the special in-situ soft soil conditions and (iii) the fact the building stock in the region consists primarily of unreinforced masonry, requires some special attention. For this reason, several studies are initiated to investigate the vulnerability of the masonry structures to withstand earthquakes of minor to moderate magnitudes. This paper discusses a detailed study of a masonry school in the region. A coupled finite element-boundary element (FE-BE) model is developed to study the linear dynamic response of the structure to induced seismicity. The structural part, i.e. the masonry building, is modelled using finite elements whereas the soil is described by boundary elements. The modelling of the layered soil medium using boundary elements reduces the computational demands and avoids the need to incorporate non-reflecting boundaries since the radiation condition at infinity is satisfied in an exact manner. This is particularly important for the relatively long wavelengths associated with the low seismic frequencies. The coupled FE-BE model is validated with a full scale in-situ experiment in which the structure is set into motion by a vibratory device (shaker) which is placed close to the building. To serve this purpose, a special shaker was chosen able to extract significant amplitudes in the frequency range between 2-10Hz, which is considered to be relevant for the shallow-focus earthquakes in the region. The dynamic behaviour of the structure, i.e. natural frequencies and modal shapes, is first identified based on ambient vibration measurements. Subsequently, model predictions based on monochromatic ground excitation were compared with in-situ measurements for validation purposes. It is shown that the coupled FE-BE model is capable of predicting the dynamic response of the actual system for a wide range of frequencies. Finally, the effects of soft soil-structure interaction and the vibrational characteristics of the structure are investigated for a wide range of frequencies.