In recent years, gas extraction in the northern part of the Netherlands has been causing low-magnitude, induced, shallow earthquakes. Besides safety, the prediction and evaluation of ‘light’ damage due to these induced ground motions is important, as it is related to economic and serviceability losses, and societal unrest. An experimental and numerical campaign is ongoing at Delft University of Technology, aiming to improve the knowledge of the underlying physics of crack initiation and propagation in unreinforced masonry (URM) structures typical in the Netherlands. A damage scale and damage parameter are defined herein in order to objectively quantify cracking damage as a function of the number, length, and width of cracks in masonry walls. The cracking mechanisms are studied for URM walls and spandrels subjected to in-plane loading. Displacements, strains, and loads under which cracking starts and propagates are evaluated and correlations are sought. The Digital Image Correlation measuring system is used to accurately detect crack formation and the evolution of the cracking pattern. This is also utilised to validate and calibrate non-linear finite element models. From the experiments, drift values are obtained for the light damage state of the masonry walls. A range between 0.3‰ and 1.1‰ is set as belonging to light damage. Moreover, a damage accumulation or material degradation was observed during cyclic testing. Additionally, fracture-mechanics based, micro and macro finite element models are capable of reproducing the repetitive behaviour of the tests.

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Induced seismicity in the north of the Netherlands has recently exposed unprepared, unreinforced masonry structures to considerable earthquake risk. While the ultimate-limit state capacity of the structures is vital to assess the individual’s risk, their behavior during more frequent, lighter earthquakes, leading to ‘lighter damage’, has shown to be strongly linked to economic losses and societal unrest. When observing the light damage caused by minor earthquakes, the existing state of the structure appears to be highly relevant for the final damage intensity and configuration: earthquakes that may have otherwise caused no apparent damage, may intensify existing damage. In particular, incipient damage due to settlements is common in the baked-clay and calcium-silicate brick masonry structures of the region. This paper details the study of full-scale laboratory walls, pre-damaged following typical (crack) patterns caused by settlements and tested with quasi-static lateral loads. The aggravation of the damage during a relevant number of load cycles is monitored using full-field digital image correlation. The damage is quantified objectively using a purposely-developed damage parameter. The tests are used (together with previous studies) to further calibrate computational finite element models, which coupled with detailed soil-structure interaction boundary conditions, are then employed to assess a larger number of structural geometries and pre-damaged configurations exposed to (repeated) induced earthquake acceleration histories. Both experimental and computational approaches show that settlement pre-damage in masonry structures increases the likelihood and the amount of further damage. This is more easily observed when some initial, yet limited damage exists and the masonry wall is exposed to moderate earthquake vibrations in the order of 30 millimeters per second.@en

In recent years, the gas extraction in the northern part of the Netherlands is causing induced, shallow earthquakes. Besides safety, the prediction and evaluation of ‘light’ damage due to induced ground motions is important, being related to economic and serviceability losses, and societal unrest. Hence, the initiation and propagation of cracking (related to light damage) need to be investigated. An experimental and numerical campaign is ongoing at the Delft University of Technology, aiming to improve the knowledge of the underlying physics of crack initiation and crack propagation in unreinforced masonry (URM) structures typical in the Netherlands. A damage scale and damage parameter with a look at cracking in the light damage stages are defined in order to objectively quantify the cracks in a URM structure. Different causes, among which dynamic loads (earthquake vibrations), imposed deformations, and repeated loading are investigated. The cracking mechanisms are studied for URM walls and spandrels subjected to in-plane loading. Displacements, strains and loads under which cracking starts and propagates are evaluated and correlations are sought. The Digital Image Correlation (DIC) measuring system is used to accurately detect the crack formation and the evolution of the crack pattern. This is used also to validate and calibrate FEM models. The models are used to extrapolate to situations of combined loading, difficult to accomplish experimentally. These situations include walls pre-damaged by settlement or shrinkage stresses, which also exhibit comparatively more damage when then subjected to earthquake loads. With there results, (fragility-like) indicative resistivity curves are produced.@en