Cross-comparison of Non-Linear Seismic Assessment Methods for Unreinforced Masonry Structures in Groningen

A large amount of low-rise unreinforced masonry structures (URM) can be foundin Groningen, the Netherlands. More and more induced earthquakes with shortduration have been detected in this region due to gas exploitation. Local unreinforcedmasonry (URM) buildings were initially not designed for withstanding seismicactions, so that unexpected damage may occur due to their vulnerability, raising insecurityamong residents. Existing low-rise masonry buildings in Groningen can bedivided into different categories based on their characteristics. Two types of residentialmasonry buildings that fulfil the prerequisites for performing non-linear seismicassessment are chosen to be studied in this thesis project, including the terracedhouse and the detached house.The seismic assessment of structures requires the use of both a discretization methodand a seismic assessment method. The discretization method is used to translate themechanical model into a finite element model used for the numerical analysis. Severalmethods have previously shown to be applicable for seismic assessment, but thiswork investigates the implications of using a continuum model (CM) and an equivalentframe model (EFM) approach to discretization in the general-purpose finiteelement package described in DIANA-FEA-BV (2017). The continuum model approachadopted was in a previous work by Schreppers et al. (2017) validated againstexperimental results and is as such deemed representative of the physical behaviourof the mechanical models investigated. An equivalent frame model approach to beused with DIANA is proposed in the work by Nobel (2017). The continuum modelapproach uses continuum elements with a constitutive model developed for the seismicassessment of masonry structures. This constitutive model captures both shearand flexural failure mechanisms. The equivalent frame model approach uses a combinationof numerically integrated beam elements and nodal interfaces, each witha distinct constitutive model, thus decoupling the description of the flexural andshear behaviour. This approach aims to capture the macro-behaviour at the structurallevel. The applicability of the proposed equivalent frame model approach isevaluated by how well it replicates the validated continuum model approach results.The two discretization methods described are evaluated using two types of seismicassessment methods. The first seismic assessment method used consists of first performinga quasi-static non-linear pushover analysis (NLPO) on the model. Thisresults in the pushover curve, which describes the global behaviour of the modelunder an equivalent lateral load based on the fundamental mode shape of the structure.The pushover curve is then used with the N2-method described in EN1998-1iii(2004) to assess at which peak ground acceleration (PGA) that the model reachesthe near-collapse (NC) limit state. The second seismic assessment method consistsof performing dynamic non-linear time-history analyses (NLTH). This method usesrecorded accelerograms to impose the inertial forces. The PGA for the accelerogramwhere the near-collapse limit state is reached is compared to the PGA fromthe use of the N2-method. The applicability of the pushover analysis in conjunctionwith the N2-method is evaluated by how well it replicates the PGA found from thetime-history analyses and by how well it replicates local failure mechanisms.Therefore, the main objectives of this project can be described by the following twoquestions:i. To what extent can the equivalent frame method be applicable as a properdiscretization method for pushover analyses and time-history analyses oflow-rise unreinforced masonry residential buildings in the Groningen region?ii. To what extent can the non-linear pushover method be adopted toassess the seismic behaviour of low-rise unreinforced masonry residentialbuildings in the Groningen region?The applicability of the equivalent frame model showed to vary. For describing localfailure mechanisms its applicability is poor. Further work on connecting the edgepiers to transverse walls is needed. For seismic assessment using the N2-method theapplicability of the equivalent frame model approach is sensible. The conservativedisplacement capacity counteracts the fact that it is worse at describing local unloading,which produced a larger initial equivalent stiffness of the bi-linear curvesin comparison to the continuum model. For seismic assessment using the timehistorysignals, its applicability is possible. While it could show different behaviourin terms of displacement and damping forces, it still showed a similar PGA at thenear-collapse limit state for the cases at hand.The seismic assessment of the terraced and detached houses by the N2-method issimilar to the seismic prediction by applying time-history analyses. However, thereare still some variations in the initial stiffness, force capacity and displacement capacitybetween these two assessment methods due to the assumptions and limitationsin this study. Overall, considering the pros and cons of the quasi-static pushovermethod, it is deemed applicable during the seismic assessment of the unreinforcedmasonry structures in the Groningen area