Numerical study of the effect of seismic in-plane damage on out-of-plane performance of unreinforced Masonry walls

Abstract

Previous earthquake reports have highlighted the high vulnerability of masonry walls to failure when subjected to the out-of-plane (OOP) loading. Particularly, this OOP failure is a significant cause of collapse in unreinforced masonry (URM) buildings, especially those with slender walls and large openings. Moreover, in the past they were designed with minor regard to seismic design concepts. In Groningen, the Netherlands, where the majority of houses are constructed using unreinforced masonry, there have been frequent occurrences of low-intensity seismic activity, causing pre-damage to the walls. Typically, the walls get pre-damaged in the in-plane (IP) direction due to low-intensity shakes and settlements, since the box-like behaviour is established when the forces are low. Box-behaviour refers to the case where there is a strong connection between orthogonal walls and a stiff rigid floor diaphragm. As a consequence, the walls experience the desired IP shear failure during seismic loading. However, research regarding the OOP performance of pre-damaged URM walls is limited, and existing studies have shown that IP pre-damage decreases the ultimate OOP load-bearing capacity of the walls. To address this gap, this report aims to conduct a preliminary investigation on the OOP strength of pre-damaged URM walls, considering both walls without and with openings. Notably, there is no existing evidence in the literature regarding walls with openings on this specific topic. The primary research question can be stated as follows:
How does the in-plane pre-damage affect the out-of-plane load-bearing strength of URM walls?

This thesis employs a numerical modelling approach to address the research question. The software package DIANA 10.5 is used, adopting a simplified micro modelling method with shell elements to simulate the behaviour of masonry. Bricks are modelled with continuum shell elements, while the mortar joints are modelled using zero-thickness interface elements. Also, the potential vertical crack in the bricks is considered using the interface elements. The validation of the numerical modelling approach is performed in two steps. Firstly, the mechanical properties of the numerical model are calibrated using small-scale material tests. In the second step, the calibrated parameters are directly applied to the numerical monotonic analyses of full-scale walls, and the results are then compared to experimental test results. The calcium silicate (CS) brick masonry tests, from material to structural levels, conducted at the Delft University of Technology are selected as benchmarks for the calibration and validation of the numerical model. A good agreement is observed between the numerical and experimental results for the IP walls in terms of initial stiffness, peak shear force, and crack patterns. However, the results for the OOP walls are significantly overestimated, with an average overestimation of 35% for the peak force. This is attributed to the differences in boundary conditions (full or partial rotation restriction), type of tests (cyclic or monotonic) and loading conditions (displacement control or load control) between the numerical simulation and experimental tests. Nevertheless, the crack patterns are in good agreement with the experiments.

The validated model is used to investigate the OOP performance of pre-damaged walls, including both solid wall (without opening) and wall with an asymmetric opening. Two distinct approaches are considered to simulate the response of the pre-damaged walls. In the first approach, known as the reduced-parameters approach, a model is created with varying properties: reduced stiffness and strengths are assigned to locations where cracks were observed during the IP tests, while regular properties are maintained in other locations. Subsequently, the model is subjected to OOP analyses. In the second approach, known as the sequential loading approach, the wall is initially monotonically loaded in the IP direction that represents the pre-damage of the wall. Then, the OOP load is applied while maintaining the IP pre-deformation until the wall experiences failure. The study considers four different states of pre-damage, ranging from minor to extensive damage. For the solid wall, the damage levels are based on the observed damage during the IP tests up to the maximum drift of 0.2%. On the other hand, the damage levels for the wall with opening are derived from the damage observed during the IP monotonic pushover numerical simulation up to the maximum drift of 0.14%.

For the solid wall, it is found that both approaches give same results for low pre-applied IP damages, up to 0.06% drift. The reduction of the OOP peak strength is almost negligible until this drift level. As the damage increases, the reduction of the strength also sharply increases. At the maximum of 0.2% IP-drift, 40% reduction of the OOP peak strength of the wall is observed in the reduced-parameters approach, while in sequential loading approach, the wall immediately failed resulting in negligible OOP strength because of the severe IP damage. For the wall with opening, similar to the solid wall, no measurable influence of the OOP strength due to minor IP damages, up to 0.06% drift, could be observed in both approaches. The maximum reduction that could be observed is approximately 15% at maximum of 0.14% pre-applied IP drift. In the sequential loading approach, as the level of pre-damage is increased, the pre-peak stiffness of the force-displacement curve decreases, as expected for a pre-damaged wall. However, the reduced-parameters approach does not show this reduction, which could be attributed to the pre-damage applied only at specific locations. For both types of walls, there is no significant difference in the crack pattern of the undamaged and pre-damaged wall. The well-known envelope crack pattern is obtained in both cases.

To conclude, the out-of-plane load-bearing strength of URM walls is significantly affected by the presence of in-plane damages. The impact is minimal under minor in-plane damages but increases rapidly as the damage becomes more severe.

This thesis limits the numerical analyses to monotonic loading. However, for future research, it would be beneficial to extend the analyses to include cyclic or dynamic loads, as they provide a more realistic representation of seismic loading conditions. Moreover, it is recommended to incorporate various boundary conditions for the OOP loading. For instance, the research could be expanded to include a C-shaped wall configuration, where the lateral edges are supported by return walls, a scenario commonly encountered in practice.