The seismic assessment of unreinforced masonry structures in Groningen is still ongoing. The assessment is vital to determine whether a building must be strengthened or not. Different assessment approaches have been followed in recent years. First, the Non Linear Time-History analyses (NLTHA) were initially the only approach used for the seismic assessment. They are still overall the most accurate type of assessment, but they are also the most time-consuming one. Nowadays, NLPO analyses are more frequently used. This assessment procedure presents some limitations of application and may be less accurate for complex structures but it requires less computational time. The NLPO analyses can be performed by means of different tools, such as analyses based on the finite element method (FEM), equivalent frame (EF) or macro-element based analyses and, eventually, also analytical mechanism based analyses. The SLaMA method belongs to this last category: this method is an analytical approach already tested and validated in New Zealand for RC structures.
This research aims to answer the following research question:
• How is the in-plane behaviour of single-storey URM wall facades affected in simplified calculation methods compared to FEM when geometrical irregularities are present?
The walls have been modelled in 2D with three different methods: FEM, EF and SLaMA. Material properties and modelling assumptions were maintained as consistent as possible within the three different methods. For researching the influence of the geometrical irregularities on the accuracy of EF and SLaMA when compared to FEM, the variation of geometrical irregularities, each quantified by an index value, have been studied. The influence of these indices on the accuracy of the calculation methods has been researched with a sensitivity analysis.
The objective has been pursued by looking into single-floor URM façades, and the conclusions of this research can be applied to this typology of walls in Groningen made of solid clay brick masonry (pre 1945). The study focuses specifically on the base shear capacity of the walls.
The differences observed when comparing the in-plane behaviour of a wall analysed with 3MURI and DIANA are not significantly affected by the presence of geometrical irregularities. The ratio between the base shear capacity computed with the two approaches and the predicted failure mechanisms remains consistent for all geometrical irregularities defined in this report.
Similarly, the differences observed when comparing the in-plane behaviour of a wall analysed with SLaMA and DIANA are not largely affected by the presence of geometrical irregularities, since the base shear computed according to SLaMA is consistently lower than that obtained with DIANA. However, the base shear capacity obtained with SLaMA showed large variations between 0.34 and 0.75 with respect to DIANA when implementing geometrical irregularities. The largest variation is obtained when more than a single pier is considered, due to the inability of SLaMA to define the re-distribution of the vertical axial forces in the piers, nor correct boundary conditions at the top of the piers since the constraining action of the spandrel appear underestimate. This affected also the prediction of the failure modes, which differed for the two methods. However, in most of cases flexural failure mode was obtained, and the study should be extended to consider also geometries and loading conditions that cause also the shear failure of the walls.

Each year, the province of Groningen experiences many induced earthquakes due gas extraction, which has been ongoing since 1963. The earthquakes cause damage to the buildings situated in the Groningen area, and they constitute a potential danger for the safety of the residents. These buildings are typically unreinforced masonry structures which are designed without knowledge of the presence of seismic activity in this area.

It is therefore essential develop and use assessment methods that are on one end reliable and accurate, but on the other hand allow to perform a large number of assessments of the vulnerability of the buildings in a short time. In other words, the assessment of all the buildings requires a quick and reliable assessment method. Such an assessment method should offer a strong understanding of the occurring failure mechanism during an earthquake, an acceptable prediction of the ground acceleration at which the collapse of the building may occur (maximum base shear force) and the displacement capacity of unreinforced masonry (URM) building.

The NPR9998 recommends four seismic assessment approaches, which differ in complexity and assessment time needed to be performed. The most comprehensive and time-consuming assessment method is the NLTHA (nonlinear time history analysis), which includes both the dynamic and nonlinear effects. In practice, this method is used only in special cases, such as in the case of monumental buildings. A simpler approach is the NLPO (nonlinear pushover) analysis, which is static and considers the nonlinear properties of the structure. An NLPO is less time consuming than an NLTHA, even when the finite element method (FEM) is considered.

A more simplified approach is the Simple Lateral Mechanism Analysis (SLaMA). This method is a simplified mechanism-based analytical approach. If the SLaMA method predicts realistically conservative global capacities, it could serve as an effective alternative assessment method for URM buildings, and especially to the NLPO FEM analysis. This study focusses on the comparison between the SLaMA method and the NLPO FEM analysis. Therefore, this study aims to answer the following research question:

Could the SLaMA method be a realistically conservative and effective alternative to the NLPO FEM analysis in making a seismic assessment for two-storey unreinforced masonry buildings?

In conclusion, the SLaMA method could be a realistically conservative and effective alternative to the NLPO FEM analysis in predicting the maximum base shear force. The displacement capacity predicted using the SLaMA method is validated only for buildings with RC floors. This predicted SLaMA method was realistically conservative compared with the ultimate displacement achieved using the NLPO FEM analysis. The SLaMA method is overall suitable for obtaining a quick understanding of the behaviour of an URM building. However, it requires a proper evaluation of the analyses to identify properly the type and the location of the failure mechanisms. For this reason, this method could be valuable to be applied before using a more complex assessment method.