Damage Assessment of Stone Masonry Piers Using Imaged Surface Cracks

After an earthquake, the structural safety of all buildings in the affected region needs to be assessed. This is currently done by a visual inspection of each building. This approach is very time-consuming and requires a significant amount of expertise from the inspectors. A lack of the latter can lead to inaccurate assessments, which are often overly conservative. This is of course, on the safe side but negatively affects the resilience of societies. In the future, the post-earthquake assessment process will be accelerated and objectified by replacing the visual inspection with an automated image-based evaluation of the buildings. One research line of the Earthquake Engineering and Structural Dynamics Laboratory is to develop such a process for stone masonry buildings, which are among the most vulnerable buildings during earthquake loading. The objective of this thesis is to contribute to this vision.Earthquake damage to stone masonry structures manifests itself mainly in cracks. Surface cracks have been shown to contain important information to build empirical and physics-based models to understand the activated mechanisms and estimate the level of damage to a structure. This Ph.D. thesis aims at developing methods for detecting, segmenting, quantifying, and interpreting surface cracks using optical measurement and machine learning techniques. The methods are applied to a set of six large-scale quasi-static cyclic shear-compression tests on plastered rubble stone masonry piers, which have been conducted as part of the thesis and provide a unique data set of high-resolution images and displacement-field measurements. The output is used (i) to investigate the influence of shear span ratio, axial load ratio, and the number of cycles on the crack width and (ii) to build empirical models for the stiffness loss, the residual force, and the drift capacity of rubble stone masonry piers as a function of crack features.

Investigating the cracking of plastered stone masonry walls under shear–compression loading

Katrin Beyer, Michele Godio, Amir Rezaie

Cracks are the most important source of information about the damage that occurs to unreinforced masonry piers under seismic actions. To predict the structural state of unreinforced masonry piers after an earthquake, research models have been developed to quantify important features of crack patterns. One of the most used crack features is the width, but this can be influenced by several parameters such as the axial load ratio, the shear span ratio, and the loading protocol, which have not been fully studied in previous research studies. In this study, we use experimental data to investigate the evolution of cracking in stone masonry piers during the application of cyclic shear–compression loading. The data consists of gray-scale images taken during quasi-static shear–compression tests performed on six plastered rubble-stone masonry walls subjected to constant axial force and cycles of increasing drift demand. Through the combined use of digital image correlation and a pre-trained deep learning model, crack pixels are identified, post-processed, and quantified based on their width. The dependency of the crack width on the axial load ratio, the shear span ratio, and the loading protocol at the peak force and ultimate drift limit states of the piers is clarified by a displacement vector field analysis, histogram of the crack width, and the concentration of deformation in the cracks. We show that, as opposed to flexural cracks, diagonal shear cracks do not fully close when moving from the applied drift demand to the residual drift measured upon removal of the lateral load. Furthermore, we provide the maximum residual crack width at peak force and ultimate drift limit states. This study will improve the decision making abilities of future models used to quantify earthquake-induced damage to stone masonry buildings.

2021

Seismic design of torsionally eccentric buildings with U-shaped RC walls

Katrin Beyer, Alessandro Dazio

The objective of the present work is that of making a contribution to the performance-based seismic design of RC buildings with U-shaped walls. Since U-shaped walls are often placed at the perimeter of a building and are typically stiffer and stronger than other lateral strength providing structural elements, many of these buildings possess in-plan strength and stiffness eccentricities and are therefore prone to damage due to twist-induced displacements. The work focuses therefore on two aspects of the design of such buildings for which the knowledge state was judged unsatisfactory. These are: (i) the inelastic behaviour of U-shaped walls under different directions of seismic loading and (ii) the estimation of displacement demands on structural elements in torsionally eccentric buildings. Within the scope of this endeavour two U-shaped walls were tested under a quasi-static cyclic, bi-directional loading regime. The two test units were analysed using plastic hinge models and wide-column models. From the comparison with the experimental results, modelling guidelines for U-shaped walls were developed. In the second part of the report the torsional response of inelastic structural wall buildings was studied and a semi-empirical method for estimating the twist-induced displacements was developed. The method can be used to extend the application of the direct displacement-based design approach to the class of torsionally restrained structures that are asymmetric in-plan but regular over the height.

IUSS Press2008

Performance limits of brick masonry spandrels

Katrin Beyer

Spandrel elements in unreinforced brick masonry buildings with timber floors consist of a masonry spandrel supported by either a timber lintel or a masonry arch. When subjected to seismic loading, the force-deformation relationship of such spandrel elements can be described by a piecewise linear relationship which distinguishes two principal regimes: The first regime describes the behaviour up to peak strength of a largely uncracked spandrel. The second regime is associated with a residual strength mechanism after the formation of major cracks in the spandrel. The residual strength of brick masonry spandrels is often less than 80% of their peak strength. Hence, according to established rules in seismic engineering for estimating the ultimate drift capacity of structural members, the residual strength would typically be neglected when assessing the seismic behaviour of existing buildings. However, the residual strength mechanism is typically associated with a rather large deformation capacity and it is therefore argued that it should be considered. Moreover, small cracks due to, for example, previous earthquakes or differential foundation settlements might reduce the peak strength of the spandrel but will have little influence on its residual strength. This paper discusses on the basis of experimental and numerical results the different limit states of brick masonry spandrels subjected to seismic loading, which characterise the two regimes and the ultimate rotation capacity of the spandrel.

2013