Recent, induced earthquakes in the north of the Netherlands have led to a large number of damage claims. Many claims can be considered to fall into the category of ‘light damage’ to the ubiquitous, unreinforced masonry structures in the region. To evaluate and predict the behaviour of cracks, characteristic of light masonry damage, caused by seismic or other actions, an experimental campaign, linked to the validation of computational models, has been pursued. To accurately capture the initiation of visible cracks, wider than 0.1 mm, Digital Image Correlation (DIC) was applied to monitor the entire surface of full-scale wall panels and smaller specimens. Moreover, an optimised speckle pattern and solving algorithm was developed to be able to monitor not only the initiation, but also the propagation of the cracks during subsequent (repeating) loading cycles. In this approach, the crack data is then used to characterise the intensity of damage with a single scalar; the parameter, denoted Ψ and comprising the number, width and length of the cracks, is used to evaluate the progression of light damage in experiments and finite element models. A description of the DIC technique applied and of the development and usage of the damage parameter for masonry is presented herein.

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Recent, light earthquakes induced by the extraction of gas in the north of the Netherlands have been linked to light, mostly aesthetic damage of the traditional masonry structures in the region; this is also connected to economic losses and societal unrest. To be able to accurately assess the light damage, detailed finite element models are necessary and need to include realistic soil movement, wave propagation, and soil-structure interaction boundaries. Moreover, the minute deformation of the soil, including the rocking and translational components of seismic ground motion, has shown to be influential to light damage. Consequently, this study has pursued the definition of efficient soil-structure interaction boundaries to implement in finite element models of buildings. A methodology, following the sub-structure method for the seismic Soil-Structure-Interaction (SSI) is defined and presented. The soil-structure-system is divided into three sub-systems: the far-field soil, the near-field soil and the superstructure. First, a 3 km deep and 8 km wide, plane-strain model of the soil is employed to study the behaviour of the soil at the surface due to deep, simplified seismic events. The soil model is linear-elastic since only light seismic excitations are considered. Next, a smaller, 30 × 300 m (shallow) soil model with a building on top, is given boundary elements calibrated to replicate the behaviour observed at the surface in the larger model. Finally, 2D models of masonry façades set on the intermediate soil model are used to reduce the soil-structure interaction to representative interface elements. The models are matched in terms of dynamic behaviour, strains, cracking, and displacements, and the behaviour is compared to existing ground motion data for the Zeerijp and Westerwijtwerd earthquakes. It is demonstrated that the equivalent interface allows efficient modelling of seismic excitations considering a detailed soil-structure interaction for complex, smeared non-linear, time-history analyses of wall models to assess (light) damage in probabilistic studies. Models with this equivalent interface show greater damage than comparison models without it.

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To pragmatically answer the question of whether masonry walls can withstand a hydrostatic water pressure, a clay-brick wall was built and tested at Flood Proof Holland. The specimen was exposed to various hydrostatic loads, also in combination with debris impact loads, with the goal of providing experimental data for the calibration of structural models and later development of fragility models to study and assess (economic) damage and life loss from potential Dutch floods. The 2.7x2.7 m2 wall, 100 mm thick, was constructed on a steel rig and subjected to a water level difference of up to 125 cm, where the level, at what was considered the back of the wall, was kept low (0 cm). The wall, which was restrained on all four boundaries by being glued at the bottom and top steel beams and constrained against the lateral steel columns of the rig, was subjected to various combinations of water level in front and behind it with the most unfavourable combinations leading to minor cracking. The out-of-plane deformation, perpendicular to the face of the wall, reached approximately 2 mm for the hydrostatic pressure produced by 95 cm of water. The masonry structure, which was also subjected to a vertical overburden by the steel rig mimicking the vertical stress produced by gravity loads on buildings, deformed in two-way bending, meaning that the horizontal supports also prevented the wall from moving. Non-linear, finite element models calibrated against the experimental results offered additional insight into the behaviour of the wall. It was observed that the wall behaved linear-elastically up to a water level of about 90 cm. At higher water levels, the deformation of wall increased significantly which is associated, according to the models, to the initiation of cracking in the wall and the redistribution of the bending stresses from the vertical (stiffer yet weaker) direction to the stronger horizontal direction. At 125 cm, the wall deformation reached up to 5 mm. From hereon, damage in the wall would have progressed, further reducing its stiffness up until collapse at approximately 150 cm of water depth. This latter value could not be achieved experimentally but is inferred from the trend of the experimental results and the understanding provided from the non-linear models. The models explored comprised analytical and numerical models with linear-elastic and non-linear material models, in one- or two-way bending. The behaviour of the wall up until the initiation of cracking could be predicted well with the simpler, one-way bending analytical or FE linear-elastic models, but more complex, two-way bending models were required to output the same fidelity at higher water levels, with only the non-linear, two-way bending FE model capable of representing the experimental behaviour at the highest water levels.
These calibrated models were also used to predict that, if the wall had not been constrained on its sides and thus only at the top and bottom, more alike walls in buildings will long end walls, it would have failed much earlier, at a water depth of about 90 cm depending on the vertical overburden applied. These values for water depth consider that no water was present behind the wall. During the test, water did not infiltrate through the wall, with the highest water pressures leading only to wetness at the back of the wall. In reality, the inside of a building may be subjected to a water level because of water entering the building through windows or doors. The lower differences of water level between the front (outside) and back of the masonry are also associated with lower out-of-plane displacements and lower internal bending moments in the wall. However, in terms of hydraulic head, if the head was located higher and closer to the middle of the wall, it was observed to be more detrimental than an identical head where the inside of the building is dry. The masonry wall was also subjected to a combination of hydrostatic pressure and impact of debris. Two types of debris were considered: a floating tree log representing soft debris, and a suspended steel cube, mimicking hard debris. Impacts of the soft debris of up to 1.8 m/s (90 Joules) did not lead to any additional deformations of the wall, while collisions with the steel cube (up to 180 J) in combination with water depth of about 90 cm, did generate accumulated out-of-plane deformations, visible cracking in a diagonal pattern with leakage of water through the cracks, and ultimately local failure of the masonry. The experiments conducted herein prove that traditional, single-wythe masonry walls in regular buildings can safely withstand water depths of up to 90 cm when the inside of the building is dry. This corresponds roughly to a hydraulic head of 90 cm when the level inside is low. At larger water depths damage in the form of cracks is expected and the boundaries of the wall and its overburden become relevant in determining whether the wall has sufficient capacity to avert collapse. For a square wall, restrained on all four boundaries, the maximum water level was determined to be about 150 cm; but, wider walls or walls without lateral supports are much more vulnerable having fewer possibilities to redistribute stresses, especially to the horizontal direction and thus exhibiting more brittle failure after the onset of cracking. Future work should hence focus on testing the difference between one-way and two-way bending in masonry walls, assessing the effect of leakage through walls with openings, and observing the influence of cavity walls where two walls are built in front of each other with a space in-between, which are common in Dutch buildings, and where water outside and inside the building may exert pressure against the empty cavity. The rate at which such a cavity is filled with water would also provide additional insight into the flood resilience of dutch buildings.@en

Failure of residential buildings during floods is an important cause of damage and loss of life. In the case of the Netherlands, the collapse of buildings is implicitly included in current damage and mortality curves since these are generated from historical data. However, the Netherlands has not experienced destructive flooding since 1953, so damage functions for modern buildings do not exist. Therefore, this paper assesses the effect of floods on modern Dutch residences with laboratory tests and structural models in order to formulate physically-based fragility curves. The results gathered are also applicable to similarly-built masonry and cavity-wall rowhouses elsewhere. Almost half of the Dutch population live in terraced houses (also known as townhouses or rowhouses), of which the critical failure mechanism during a flood is out-of-plane bending of the load-bearing walls. Failure of these structural elements should be analysed with the pressure coefficient, Cp, instead of the currently used drag coefficient, CD, because wall collapse is more likely than displacement of the entire structure. This paper describes the quantification of both coefficients by conducting flume experiments on rectangular boxes with different geometries and orientations. Higher drag coefficients are derived from the experiments than provided by FEMA, resulting in higher hydrodynamic loads on the residences. The physical approach to evaluate the collapse of residences is exemplified with a case study of the three most common type of residences in the Netherlands. Structural analyses of their load-bearing walls subjected to a hydrostatic and hydrodynamic load perpendicular to the wall show failure due to milder flood conditions than the current damage curves do for all case study residences. A sensitivity analysis shows an important influence of wall thickness, initial axial loading of the wall, and the flood water level inside the residence.@en

Induced seismicity in the north of the Netherlands has recently exposed unprepared, unreinforced masonry structures to considerable earthquake hazard. While the ultimate-limit state capacity of the structures is vital to assess the individual's risk, their behaviour during more frequent, lighter earthquakes leading to ‘light damage’, has shown to be strongly linked to economical losses and societal unrest. To study the effect of these repeated light earthquakes, the behaviour of masonry structures, typical of the region, needs to be investigated when subjected to multiple small cycles. In particular, insight into the potential aggravation of the light damage is sought. Hence, shear walls of replicated clay brick masonry have been tested in the laboratory and exposed to a high number of low in-plane drift cycles. The experiments show, under inspection with high-resolution Digital Image Correlation, how stair-case, diagonal cracks initially distribute in multiple cracks running in joints at the centre of the walls, but later localise into a single, visible, wider crack. The tests are used to establish an interval over which light damage can be expected for these types of walls. Furthermore, this initial distribution of strains in ‘bands’ and subsequent localisation, previously observed only with computational finite element models and which could not be verified experimentally, is compared and reproduced with improved models both at the composite continuum scale (macro-model) and the brick-to-brick micro scale. The models are calibrated based on the quasi-static, cyclic experimental results and later used for extrapolation in dynamic nonlinear settings to assess the influence of natural ground motion excitations.

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The north of the Netherlands is prone to frequent, light earthquakes linked to economical losses and societal unrest due to the induced seismicity in the region. These light earthquakes produce correspondingly low values of in-plane drift on the typical masonry structures of the region, many of which are built with cavity walls composed of an inner, load-bearing calcium-silicate masonry leaf, and an outer, exposed fired-clay masonry veneer. To assess the resulting damage from the lighter earthquakes, it is thus necessary to understand the difference in behaviour of the inner and the outer masonry leaves when exposed to the same drift values. Experimental tests of replicated, full-scale calcium-silicate brick walls and spandrels are detailed herein and compared to previously tested clay masonry samples. A purposely developed, scalar damage parameter is used to assess the width, number and length of the cracks to objectively quantify damage. High resolution digital image correlation is used to accurately monitor the initiation and propagation of cracks. The experiments reveal that calcium-silicate samples exhibit slightly greater damage than clay samples when subjected to the same in-plane drift. From the tests, drift values for light damage or ‘damage state one’ are set between 0.15 and 0.65‰ for the type of wall tested. Moreover, in these tests, cracks in calcium-silicate samples were significantly more likely to split brick units, whereas cracks in the type of clay samples employed, always followed the masonry joints. This fundamental difference in the light-damage behaviour between the two materials is of importance when considering the perception of damage, the strategies and cost of the repairs, and the strategies for strengthening of masonry structures with cavity walls resembling the type of masonry tested herein.

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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, 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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Evaluating and predicting structural damage from multi-hazards is a complex task mainly due to the varying ways in which hazards affect structures. Also, different damage scales that employ different parameters and criteria are used for evaluating the hazards, making a connection between the damage assessment of two or more hazards difficult. Attempting to compute the cumulated structural damage from various hazards becomes very difficult with these limitations. This paper describes the implementation of a probabilistic framework that includes effects such as structural weakening due to a first-acting hazard in the analysis of structural damage when contemplating subsequent hazards. It also proposes the formulation of damage scales tailored to assess cumulative structural damage from all the hazards involved in the analysis. This allows for the computation of probabilities for final damage states, which can be used in multi-hazard risk analysis or in design with performance objectives. The article explores the application of the proposed framework on the case of structural damage to masonry housing due to earthquakes and earthquake-triggered floods. The particular case concerns an unreinforced masonry house located behind a levee.

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In recent years, induced seismicity in the north of the Netherlands significantly increased. As a consequence, the seismic assessment of the built environment, which mainly consists of unreinforced masonry (URM) structures not designed for seismic loads, became of high relevance. Within this context, an extensive multiscale testing program has been performed at the laboratory of Delft University of Technology since 2014 to characterize the behavior of URM buildings from structural down to material level and provide benchmarks for the validation of numerical and analytical models. The paper presents an overview on the experimental campaign, which was structured in three phases: characterization of existing buildings; study of the structural response up to near collapse on replicated specimens; study of light damage state, also on replicated URM walls. The experimental campaign was characterized by a multiscale approach, with tests at structural, component, connection, and material level. At structural level, the campaign comprehended two quasi-static cyclic tests on full-scale two-story high assembled structures and a large number of both in-plane and out-of-plane tests performed either on single piers or on walls with openings. The in-plane stiffness and capacity of as-built and retrofitted timber floors was also assessed. At material level, destructive and slightly-destructive laboratory tests were performed on both existing and replicated masonry and timber specimens. Existing and retrofitting connections between the leaves of cavity walls and between concrete slabs and masonry veneers were studied. To study the initiation and propagation of cracking in URM structures, Digital Image Correlation (DIC) was used during dedicated in-plane tests.@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