Seismic Analysis of Zagreb Cathedral - Current State

The neo-Gothic cathedral in Zagreb, the largest Croatian sacral building, was severely damaged by the 5.4 magnitude earthquake on March 22, 2020. Three years after the main seismic event, aftershocks are still causing displacements on the stone blocks of the towers. The assessment of the current condition of the Cathedral has just been completed by the authors. The distinct element method was chosen for the static and dynamic analysis of the Cathedral since it can better describe the behavior of the structure, a clearly discontinuous medium. The numerical model consists of deformable blocks and their contacts (a series of nonlinear springs). With such a model, discretized and connected blocks can deform and move (slip, separate, break, rotate, fall) practically without restrictions. The paper discusses the choice of input data for modelling materials and load-bearing elements. The basic principle is to adjust the input parameters of the elements and springs so that the characteristic points on the response curve (forces with associated displacements) correspond to those obtained from numerous well-documented experiments. The problem of the size of the elements with respect to the size of the model is addressed. Three benchmarks are made: block on block, and a shear cyclic wall test for a very wide wall (squat) and a wide wall.