Monitoring and quantifying crack-based light damage in masonry walls with digital image correlation

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.Accepted Author ManuscriptApplied MechanicsEmerging Material

High-resolution monitoring of the initial development of cracks in experimental masonry shear walls and their reproduction in finite element models

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.Applied Mechanic

Testing, Characterisation, And Modelling Of Nehobo Floors For Groningen Seismic Assessment: Intermediate Report: Laboratory Tests and Numerical Models

Applied Mechanic

Calcium silicate against clay brick masonry: an experimental comparison of the in-plane behaviour during light damage

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.Applied Mechanic

Fragility curves for light damage of clay masonry walls subjected to seismic vibrations

The probability of light damage to unprepared, unreinforced masonry structures exposed to induced seismicity due to gas extraction in the north of the Netherlands is still under investigation. Repeated light seismic excitations caused by frequent, light and nearby earthquakes have been linked to economical losses and societal unrest in particular, with extensive damage claims. Moreover, the damaging potential of the seismic events has been related to the condition of the structure, especially if damage corresponding to settlement causes is already present. A comprehensive testing campaign oriented towards the initiation and progression of light damage of replicated clay brick masonry has been conducted at Delft University of Technology. Based on these tests, calibrated finite element models have been produced. This article uses the calibrated non-linear time-history models to simulate the effect of earthquake ground motion on a variety of initial conditions, wall geometry, material properties, and number, type and intensity of earthquakes. The results are then used to regress a relationship between damage and these parameters. This is subsequently employed to run a MonteCarlo simulation and produce fragility curves where the probability of exceeding specific damage values for various initial damage levels is presented against the seismic hazard. The vulnerability or fragility curves show that visible damage, with cracks wider than 0.1 mm, appears, with a 10% exceedance probability, at 13 mm/s of peak ground velocity; but, if the masonry had already undergone some light, yet imperceptible damage, a PGV of 6 mm/s was sufficient to aggravate it into visible cracks. To attain a 1% probability of exceeding light damage however, for which the masonry would need more invasive repair, it was observed that PGVs larger than 15 mm/s were required. These fragility curves were finally compared to graphs from other authors and found to capture well the variability in the range assigned to light damage.Applied Mechanic

Probabilistic assessment of structural damage from coupled multi-hazards

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.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Applied MechanicsHydraulic Structures and Flood Ris

Experimental and computational study of the influence of pre-damage patterns in unreinforced masonry crack propagation due to induced, repeated earthquakes

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.Accepted Author ManuscriptApplied Mechanic

Post-flood field survey of the Ahr Valley (Germany): Building damages and hydraulic aspects

Applied MechanicsHydraulic Structures and Flood Ris

Crack initiation and propagation in unreinforced masonry specimens subjected to repeated in-plane loading during light damage

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.Applied MechanicsStructural Design & Mechanic

In-Plane Behaviour of Unreinforced Masonry Strengthened with a Structural Glass Window: A Proof of Concept: Buildings

Innovative solutions for seismic-retrofitting existing structures are currently required, as often traditional strategies are expensive, non-reversible, highly invasive, and/or fail to address both serviceability and ultimate limit states together. The present paper describes a preliminary experimental campaign performed at TU Delft to investigate an innovative structural glass window for strengthening masonry buildings. To this purpose, a prototype composed of a timber frame, a semi-rigid adhesive, and a 20 mm thick structural glazing layer was designed. The prototype aimed to improve the structure’s behavior against minor but more frequent service vibrations (SLS), as well as against ultimate ones (ULS). Specifically, an increase in the structure’s in-plane capacity and stiffness was targeted to reduce cracking at low drifts/displacements, while at larger drifts, the adhesive’s tearing and timber crushing were used to activate damping. To evaluate the prototype’s performance, a quasi-static, cyclic, in-plane test on a strengthened full-scale wall was performed and compared with available data on a similar, yet unstrengthened, wall. Although the benefits were not pronounced in terms of cracking and energy dissipation, the implementation of the proposed strategy provided an increase in terms of initial stiffness (18%), force capacity (8%, 36%), and ductility (220%, 135%). This outcome provides the ground for numerical studies that will help better delineate the proposed strategy and improve the current design.Applied Mechanic