Structural Damage to Masonry Housing due to Earthquake-Flood Multi-Hazards

The possibility of multi-hazards, the occurrence of two or more (natural) hazards at once or shortly after another in the same location, can lead to an increased risk of structural damage and life loss. In the Netherlands, seismically-unprepared houses are especially vulnerable to newly expected earthquakes in the region of Groningen. These seismic events have the potential to compromise levees in the area, in a way that can lead to flooding. The flood, in combination with its triggering earthquake, are the multihazard that is closely studied in this report; damage is assessed to typical unreinforced masonry houses standing close to the aforementioned levees in the Groningen area. In the study, a framework is introduced to analyse damage and its probability for a study case, which consists of a house with longitudinally-oriented precast hollow concrete slabs resting on 100mm-thick calcium-silicate load-bearing masonry walls, and longitudinal in-filled masonry walls. This house is located behind a levee, for which the failure probability due to earthquakes has been characterised by the study of Zuada (2014). First, earthquake damage is assessed by obtaining the lateral displacement of the house in response to the earthquake’s peak ground acceleration via a finite element model and a non-linear time history analysis. And, relating the computed displacement to the amount of cracking and cracked walls as observed from a generated pushover curve that depicts loss of lateral resistance against lateral displacement. For each possible damage state of the house, a set of parameters that describe its weakening is defined. Then, the impact of various flood actions are analysed on the potentially weakened structural cases. Firstly, the flood resulting from a levee breach is studied with a FEM hydraulic model in order to obtain the probable flood depth and flow velocity, related to initial probabilistic parameters for the water level and the characteristics of the breach. Secondly, the interaction of debris (such as trees and vehicles) with the flood and the structure is studied. Then, the loads generated by the flood and the potential impact of debris are evaluated on a non-linear FEM of a wall of the house for an appraisal of damage. Finally, the probability for the house to be in a particular damage state as a result of the earthquake is computed and joined to the probability of the flood further damaging the house and the failure probability of the levee. From this, fragility curves are elaborated detailing the probability of damage as a function of earthquake ground acceleration. For a period of fifty years, the risk of structural collapse for the case study house was computed at 1/25. The impact of debris was observed to a be an often overlooked flood action with a paramount influence on structural damage. For the medially intense flood due to a levee breach with a water level difference of around two meters, all collapse cases due to flood actions were due to debris impact. Severe earthquake damage was observed to be a result of the infilled walls that contribute little to the structural resistance of the house, as well as the lack of redundancy and ductility of the structure. The influence of the multi-hazard on the risk of individual loss of life was also investigated and seen to increase from 4.2·10-4 for earthquake-only analyses to 9.1·10-4 per year for an earthquake-flood multihazard. Clearly, multi-hazards are both structurally damaging and dangerously threatening to human life.Civil Engineering and GeosciencesStructural EngineeringHydraulic Structure