Unreinforced Masonry Buildings Research Articles

Effect of flanges on the in-plane behavior of the masonry walls

In unreinforced masonry buildings, load bearing walls are not of only rectangular shape in the plan, but the cross walls connected to them make the walls of either I shape, T shape, C shape, etc. The presence of the cross walls can significantly change the behavior of the masonry walls in terms of strength and the failure modes of the walls. In this paper, a parametric study has been performed to investigate the effect of different parameters, namely pre-compressions, material properties, aspect ratios, and boundary conditions on the in-plane behavior of the masonry walls. The masonry walls have been modeled using the simplified micro-modeling approach in the standard finite element software Abaqus without any user-defined subroutine. It has been found that the strength and damage pattern of the flanged walls are different from the rectangular walls, which needs to be considered in the analysis of masonry walls using a numerical approach or while conducting experiments.

Post-earthquake damage assessments of unreinforced masonry (URM) buildings by shake table test and numerical visualization

Masonry structures are one of the structures most affected by earthquakes. In addition, since masonry structures constitute a large part of the building inventory, it is important to determine the dynamic characteristics of these structures. The present study is aimed to evaluate the seismic performance of a half-scale hollow brick unreinforced masonry (URM) building considering different damage states. Shake table test was conducted with seven loading conditions. In order to obtain different damage states, seismic excitations were applied incrementally to structure. After each loading, the damage conditions and the corresponding damage patterns were examined comparatively, and the structure collapsed in the last loading condition. To experimentally observe the damage, the modal parameters for un-damaged and damaged situations of the building were identified by ambient vibration tests performed before and after the seismic loadings. The experimental results obtained were then compared with the analyses made in numerical softwares, and similar results were obtained.

An expeditious tool for the vulnerability assessment of masonry structures in post-earthquake reconstruction

AbstractPost-earthquake reconstruction activities need tools for assessing the vulnerability of structures, based on visual inspection and quick survey. So far, the public administrations in charge of granting the funds and monitoring reconstruction have adopted different methods taken from scientific literature or developed ad hoc for specific events. The time is ripe for a standard method to be developed, treasuring from the lessons learned in the field. This paper proposes an expeditious tool based on visual survey for the assessment of the seismic vulnerability of unreinforced masonry buildings, starting from available methods, whose parameters were reassessed and recalibrated, and scientific evidence from recent literature. The influence of the main structural features was considered to calculate a vulnerability index in the 0–100 range. The effects of materials and arrangement of masonry were quantitatively evaluated through the masonry quality index, which represents an advancement with respect to current practice. The method was validated on a sample of 50 masonry aggregates and nearly 200 structural units of L’Aquila, Italy, where reconstruction is underway after a destructive earthquake in 2009. The sample is representative of the built heritage of other earthquake prone areas in Italy and many European Countries and the proposed method is a step forward the development of a standard tool to be used in future post-earthquake reconstruction scenarios.

The continuous Airy-based for stress-singularities (CASS) method: an energy-based numerical formulation for unilateral materials

Unreinforced masonry buildings represent a large part of the world's architectural heritage. Despite their significant diffusion, their mechanical behaviour is still subjected to increasing scientific investigations as modelling difficulties arise from the negligible tensile strength in tension. Because of this unilateral response, masonry constructions can flexibly accommodate different loading and displacement-like boundary conditions in a new deformed configuration through the formation of crack patterns. The internal stress state changes accordingly and can be represented through stress singularities. Defining the internal stress states compatible with general boundary conditions is primarily essential from an engineering perspective.This paper presents the Continuous Airy-based for Stress-Singularities (CASS) method, a new numerical strategy to model the internal stress state in unilateral materials, whose main peculiarity is the ability in describing singular stress fields independently of the discretisation. The internal equilibrium is guaranteed using the Airy stress potential discretising the domain with plate-type finite elements. The compatibility with boundary displacements is satisfied through a variational formulation based on the minimum of the Total Complementary Energy. The boundary value problem is then reduced to a second-order cone programming where the linear objective function represents the work of the emerging stress for the boundary displacements, and the constraints enforce the boundary equilibrium and the material compatibility. The CASS method is benchmarked against analytical solutions and other numerical strategies on several 2D problems, also used to illustrate and discuss its main features. Lastly, a real masonry façade is considered to show the method's effectiveness clearly.

Material aging effects on the in-plane lateral capacity of tuff stone masonry walls: a numerical investigation

Recent studies have shown how the variability of material properties affects the nonlinear behaviour of unreinforced masonry (URM) walls. To preserve the historical built heritage, variations in structural capacity of URM buildings associated to aging and deterioration of masonry should be quickly predicted, by integrating with structural health monitoring and risk management. In this study, relationships between structural capacity features and material properties are numerically investigated for single walls, based on a structural modelling strategy that was experimentally validated on full-scale URM walls. The paper proposes an evaluation of the effects of degradation of material properties on the macroscopic descriptors of single masonry walls, such as peak strength and stiffness, also considering the uncertainties in the estimate of those properties. The authors do not attempt to model the physical processes of material aging with time, but assume certain levels of material degradation and investigate their effects on the structural response and capacity. Force–displacement curves and failure modes are associated with the overall nonlinear response of masonry walls due to progressive deterioration of material properties. Regression models are then proposed to predict variations in the peak load-bearing capacity and in -plane lateral stiffness when the mechanical properties of the constituents changed.

Multiscale procedure to assign structural damage levels in masonry buildings from observed or numerically simulated seismic performance

The damage level (DL) is a fundamental metric in earthquake engineering and is particularly effective for multiple purposes, such as correlating structural damage to consequences and losses (e.g. economic, casualties, etc.) as well as to physical and mechanical variables quantifiable from both experimental and numerical tests. Despite the relevance of the topic, the conversion of complex information on the damage spreading over a 3D building into a synthetic DL remains an open issue for which there are currently no agreed-upon scientific criteria in the literature. Within this context, the study focuses on unreinforced masonry (URM) buildings by providing an analytical multi-scale approach that integrates structural damage at the panel-scale and macro-element size (where the macro-element is intended as an assemblage of components, like vertical walls or diaphragms). The approach is specifically conceived to be applied consistently to both observed and numerically simulated damage. As a result, the method is particularly effective for supporting the validation of numerical models or synthetically interpreting the huge amount of results from nonlinear analyses as much as feasible using an objective basis. The paper proposes a comparison of DL interpretation arising from other approaches in the current literature, highlighting their pros and cons. The effectiveness of the proposed procedure is then assessed using four URM case studies, all of which exhibited primarily a ‘box-like behaviour’. They were specifically chosen based on the extremely detailed documentation available for both the damage and geometric/mechanical features of the buildings as well as aiming to reduce the uncertainty in the seismic input, making them ideal candidates also for validation purposes. To this end, the applicability of the procedure also to the damage simulated by nonlinear dynamic analyses was proven by adopting equivalent frame models of the case studies, developed and calibrated in previous researches. Definitely, the results encourage the adoption of the proposed procedure to interpret as analytically as possible the real damage that occurred on URM building, when accurate damage observations from field surveys are available, as well as that coming from numerical simulations.

Using heritage building registers to characterise unreinforced masonry buildings of Brisbane, Australia

ABSTRACT Worldwide, unreinforced masonry (URM) buildings are present in abundance with many holding cultural and heritage value. URM buildings have a known history of poor performance during past earthquakes because of the weak tensile properties and lack of mechanical connections between elements. Information about the construction of URM buildings for a region is important because it can help engineers and policy makers to perform seismic assessment, risk and loss studies. In this regard, this study includes a characterisation of the heritage URM buildings in Brisbane City, Australia, and its surrounding suburbs using the Queensland Heritage Register (QHR). The QHR includes a total of 1775 heritage sites, of which 323 are identified as URM buildings in Brisbane City and surrounding suburbs. The studied building characteristics obtained from QHR included the architectural styles, construction year, function/use, number of storeys, roof material and load-bearing wall materials. Other features such as roof types (shapes), plan geometry, isolated or interconnected buildings, verandas and openings, and finally the presence of seismic hazards such as gable-end walls, chimneys, and parapets have been obtained using online 3D maps. It was concluded that heritage registers in combination with online digital resources can be successfully utilised to characterise URM buildings.

Assessment of a Full-Scale Unreinforced Stone Masonry Building Tested on a Shaking Table by Inverse Engineering

The material deterioration of an unreinforced stone masonry (URSM) building, due to subsequent dynamic loadings of increasing intensity on a shaking table, is investigated by means of inverse engineering, i.e. calibrating a finite element (FE) model to the experimental response data. The mechanical properties of the structure were initially estimated by preliminary characterisation tests. A two-storey full scale URSM building was tested on a shaking table using a sequential testing procedure of stationary and strong motion vibrations. The building was submitted to five uniaxial time-histories with gradually increasing intensity on a shaking table at the EUCENTRE laboratory (Pavia, Italy) up to a near collapse damage state, each one followed by a stationary vibration test. A frequency domain calibration was carried out to extract the mechanical properties of the equivalent elastic model. To this end, the stationary measurements were used to build up the state-space model. On the other hand, a recognition model was employed using the finite element method (FEM), whose stiffness and mass matrices were used to derive the corresponding analytical state-space model, which was compared to the experimental one. The calibration of the model against the experimental dynamic results includes increased complexity and high computational effort. Through an iterative optimisation trial and error procedure, the mechanical properties of masonry and the shear modulus of the flexible diaphragm of the structure for each test phase were derived. It is shown that the deterioration is more intense for the shear modulus of the walls compared to their elastic modulus. The ratio of the in-plane shear to the elastic modulus decreases substantially. The deterioration of the shear modulus of the timber floors is comparable with those of masonry walls.

Could building replacements significantly contribute to seismic vulnerability mitigation at urban scale? Case studies of two typical swiss cities

This paper contains an investigation on the potential decrease of seismic vulnerability related to building replacements for the cities of Sion and Martigny in Switzerland. Those two cities were identified for the present research based on the accuracy of the corresponding available information. Seismic assessment at urban scale was recently achieved for both cities and new buildings are systematically indexed according to the construction regulation of the canton of Wallis (Valais). Sion and Martigny belong to the largest cities of the canton of Wallis and this region is characterized by the highest seismicity within Switzerland. The investigation focuses on a 4-year period between 2016 and 2019. Cases for which an existing building is demolished and replaced by a new one were identified and checked. Those cases do not represent general practice. The majority of new buildings are built on free land. Consequently, the building replacement rate is too low to lead to a significant decrease of the seismic vulnerability at urban scale. In addition to the systematic appropriate seismic design of new buildings, retrofitting of existing buildings therefore remains the adequate strategy for seismic vulnerability mitigation. In case of replacement, low-rise buildings are generally replaced by mid-rise buildings. Moreover, unreinforced masonry buildings are usually replaced by reinforced concrete shear-wall buildings. This slightly impacts the building stock distributions but the seismic vulnerability is not significantly changed, since the demolished buildings are not the most vulnerable ones. Nevertheless, few obvious isolated vulnerability decreases occurred with respect to the demolition of soft-story buildings for instance. By contrast, seismic vulnerability may also be increased in the case where a new building is built against an existing one with a different story level and without an adequate seismic separation joint.

Natural Fibers for Out-of-Plane Strengthening Interventions of Unreinforced Masonry Buildings in Aggregate Configuration

Most of the Italian historical centers are composed of unreinforced masonry (URM) buildings arranged in aggregate configurations. Past and recent seismic events have underlined the high vulnerability of these buildings especially towards out-of-plane mechanisms. In order to reduce their vulnerability, the use of strengthening interventions based on fiber reinforced composite materials has become widespread in the last years. More recently, strengthening systems using natural fibers have been the object of experimental tests since they represent an innovative environmentally sustainable solution. The aim of this paper is to numerically analyze the feasibility of strengthening systems made of natural fibers embedded into cementitious matrices to prevent the out-of-plane mechanisms of perimeter façades belonging to masonry buildings in aggregate configurations. For this purpose, numerical analyses based on a macro-modeling approach for out-of-plane mechanisms are performed by considering the influence of adjacent structural units and the presence of strengthening systems made of natural fibers. Both aspects have been analyzed in detail and taken into account by introducing in the equation governing the problem both the friction acting between adjacent walls of building units, when in aggregate, and the contribution of the strengthening system. A building case study forming part of an aggregate of an Italian historical center has been considered for the development of the numerical analyses.

Historical Development and Seismic Performance of Unreinforced Masonry Buildings with Vertical Extensions in the City Centre of Barcelona

ABSTRACT This paper presents the historical development of the vertical extensions of unreinforced masonry buildings in the Eixample district of Barcelona and their impact on the seismic behaviour. Existing masonry buildings of Eixample present significant seismic risk because, despite the low to moderate seismic hazard of Barcelona, they show a marked vulnerability given their average height of six storeys and the substantial slenderness of the walls. Between the end of the 19th century and the beginning of the 20th century, the vertical extensions, known as remuntes in Catalan, were a common solution to the increasing demographic demand of the city. In their majority, these vertical extensions add a structural irregularity that represents a potential vulnerability factor not yet investigated. The present study focuses on Eixample’s remuntes, including their historical development, the description of their structural features, and the identification of their most representative configurations in the existing building stock. A seismic analysis based on the Finite Element Method is presented on a reference model without extensions and three others with the most recurrent configurations of remuntes. The seismic response is investigated through pushover analysis. The results provide a better understanding of the negative impact of the vertical extensions on the seismic performance.

Damage survey and seismic vulnerability assessment of unreinforced masonry structures in low-intensity Ambasa earthquake of northeast India

Unreinforced masonry (URM) buildings in many developing countries are susceptible to severe damage during earthquakes as these are designed and constructed without considering the earthquake risk of that region. Many low-rise URM buildings in Tripura, a northeast state of India, have suffered substantial damage during the recent low-intensity Ambasa earthquake (Mw 5.7). In this context, a damage survey was conducted at first to assess the cause behind the damaged URM buildings. Thereafter, a publicly funded three-storied URM building in Kumarghat, Tripura, has been chosen as a typical representative case for the vulnerability assessment. The Equivalent Frame Modelling approach has been used to model the building in SAP2000 with three different sets of material properties. The Incremental Dynamic Analysis is performed considering seven ground motions to obtain the damage measures for fragility analysis. The visual observations and numerical results match well for the damaged building. In fact, the modelling and analysis technique seems to predict the reasonably accurate damage scenario within a limited computational effect. This opened up a possibility for assessing seismic of masonry buildings on a regular basis which was otherwise a challenge. It has been found that the use of poor quality materials is the primary reason behind the damage to the buildings in such earthquakes. The derived fragility curves also show that with the deterioration of material quality, the probability of suffering moderate to extensive damage increased significantly at the PGA level of 0.052 g that was recorded during the earthquake. Hence, this paper may help the practising engineer to understand several cruxes of vulnerability involved in the design and construction of structures. Further, the method summarized in this study may be extended for the vulnerability assessment of all the building typologies of the earthquake-prone areas of developing countries.

Numerical Assessment of the Seismic Performance of a Timber Retrofit Solution for Unreinforced Masonry Buildings

ABSTRACT This paper presents the results of a numerical study about the effectiveness of a new multi-component timber retrofit solution for unreinforced masonry buildings in seismic regions. The proposed retrofit consists of timber frames fastened to the internal surface of masonry piers, with oriented strand boards eventually nailed to the frames. This technique aims at improving both in-plane and out-of-plane capacities of masonry piers as well as wall-to-diaphragm connections. Numerical models were built and calibrated against experimental data from quasi-static cyclic shear-compression tests on two isolated piers and dynamic shake-table tests on two full-scale buildings, in bare and retrofitted conditions. The building prototype represented the end-unit of a cavity-wall terraced house. A strategy to include the retrofit contribution in equivalent-frame masonry models is proposed. Starting from the calibrated models, two additional strengthening schemes were numerically generated to investigate the influence of increasing retrofit levels on the building performance. For each model, a cloud nonlinear time-history analysis was performed using 250 ground-motion records to derive damage-state fragility functions. The numerical outcomes can assist in the selection of an optimal level of the proposed retrofit solution, balancing structural and economic goals.

Nonlinear Seismic Assessment of a Historic Rubble Masonry Building via Simplified and Advanced Computational Approaches

This research presents a comprehensive nonlinear quasi-static seismic assessment of an unreinforced rubble masonry building, Bytown Museum in Ottawa, Canada, using discontinuum-based analyses. In the proposed modeling approach, non-uniform geometrical properties of rubble masonry walls are replicated via a group of rigid polyhedral blocks interacting along their boundaries based on the discrete element method (DEM). Once the adopted modeling strategy is validated, the nonlinear quasi-static analysis of the South and North façades of the Bytown Museum is performed. Special attention is given to the irregular block generation within the discontinuum analysis framework, where discrete element models are generated from high-resolution site recording data, representing the masonry morphology at a high level of detail. Then, the predicted collapse mechanisms from advanced computational models are further utilized to generate pre-defined macro-blocks in kinematic limit analyses, providing a simple alternative solution for seismic assessment. The results reveal the significant effect of openings and the construction technique (morphology) in unreinforced rubble masonry buildings that can play an important role in the structural capacity and behavior. Moreover, it is noted that DEM-based solutions provide lower seismic capacity compared to kinematic limit analyses. Finally, a noticeable sensitivity to the input parameters in the discrete element models is noted; therefore, characterization of material properties is necessary for reliable predictions.

Optimization of Synergetic Seismic and Energy Retrofitting Based on Timber Beams and Bio-Based Infill Panels: Application to an Existing Masonry Building in Switzerland

This paper presents an optimization process for the design of a novel synergetic seismic and energy retrofitting strategy that combines the favorable mechanical properties of timber and the attractive thermal insulation properties of bio-based materials. The novel method, defined as Strong Thermal and Seismic Backs (STSB), comprises the attachment of timber frames and bio-based thermal insulation panels on the vertical envelope and the facade walls of existing masonry buildings, thus improving both the seismic behavior and the energy performance of these buildings. This strategy is integrated and visualized in a novel synergetic framework for the holistic evaluation of the seismic behavior, the energy performance and the carbon footprint of existing buildings, defined as the Seismic and Energy Retrofitting Scoreboard (SERS). The benefit of the novel retrofitting strategy is quantified based on the numerical simulation of the seismic behavior of an unreinforced masonry building located in Switzerland, an assessment of the energy performance of the building and an evaluation of the carbon footprint of the proposed retrofit solution. Three retrofitting alternatives are investigated for the synergetic seismic and energy retrofitting of the building, comprising timber beams and two different bio-based materials for the thermal insulation of the vertical envelope of the building: cork and recycled natural grass. The optimal seismic and energy retrofitting strategy for the building among the alternatives assessed in this study is chosen based on a Multi-Criteria Decision Making (MCDM) procedure.

Seismic Vulnerability Assessment of the Yungay’s Historic Urban Center in Santiago, Chile

ABSTRACT In this paper, a modified empirical large-territorial-scale procedure to assess the seismic vulnerability of aggregate buildings in urban areas located within a highly seismic context is proposed. The methodology is applied to a group of unreinforced masonry (URM) aggregates located in the Yungay’s historic center in downtown Santiago, Chile. This assessment was carried out by modifying and integrating a vulnerability-index-based procedure, a specific existing approach called first level macro-seismic method used to predict seismic damage and loss scenarios through fragility curves of structural components. The procedure was calibrated using the observed damage after the 2010 Chile earthquake in 292 URM structural units. The results of this new damage index for URM aggregate buildings are used to derive Probability Mass Functions (PMFs) of the different damage states and preliminary Empirical Fragility Functions (EFFs). To the best of the authors’ knowledge, these EFFs are the first publicly available for aggregate Chilean URM buildings. This research and its results are the starting point for a more comprehensive study for assessing damage scenarios and vulnerabilities of aggregate URM buildings in historic centers of older Chilean cities. These functions may be used in risk assessment, and are useful to prioritize possible future damage mitigation strategies and other processes involving this historical heritage. We foresee that these results could be at least partially applied to other historical urban areas in Chile, and perhaps other old Latin American cities located along the subduction convergence region between the Pacific and Continental plates, which were founded similarly.

Modelling and Seismic Response Analysis of Existing URM Structures. Part 1: Archetypes of Italian Modern Buildings

ABSTRACT Five existing unreinforced masonry buildings, built in different Italian cities in a relatively recent time frame, were selected to study the seismic behaviour of modern existing structures, not designed to withstand seismic actions. The buildings were modelled using an equivalent-frame approach, adopting, and comparing two nonlinear constitutive laws and introducing specific modelling strategies to represent the distinctive features of modern existing URM structures. Nonlinear static analyses were performed to determine capacity and multi-stripe time history analyses to evaluate displacement demand at increasing return periods. Although non-exhaustive, the results show a trend of generally decreasing seismic vulnerability for more recent buildings.

Seismic vulnerability assessment and fragility analysis of pre-code masonry buildings in Portugal

Despite the fact that in recent years Portugal has not seen the occurrence of high-magnitude earthquakes, it remains threatened by these events due to its geographic location. Since the 1960s, reinforced concrete has been the most used material for new constructions; however, the historic urban centers are dominated by old unreinforced masonry (URM) buildings, which techniques and construction materials have evolved since the Great Lisbon earthquake that occurred in 1755 (Mw = 8.5). Given the presence of these buildings in areas of significant seismicity, extensive research is needed to assess the seismic risk and define mitigation policies. This kind of studies is often supported by empirical methods and based on expert judgment due to the high variability of the building stock and lack of information. The main purpose of this work is: (i) to provide analytical fragility curves, supported by nonlinear static analysis, for the entire population of old masonry buildings, built before the introduction of the first design code for building safety against earthquakes (RSSCS) in 1958; (ii) define vulnerability curves to be used by the technical community for seismic assessment of pre code URM buildings. The characterization of the building stock geometry and material properties is based on information previously collected, which was essential to define representative archetypes and typologies.

Latest findings on the behaviour factor q for the seismic design of URM buildings

Recent earthquakes as the 2012 Emilia earthquake sequence showed that recently built unreinforced masonry (URM) buildings behaved much better than expected and sustained, despite the maximum PGA values ranged between 0.20–0.30 g, either minor damage or structural damage that is deemed repairable. Especially low-rise residential and commercial masonry buildings with a code-conforming seismic design and detailing behaved in general very well without substantial damages. The low damage grades of modern masonry buildings that was observed during this earthquake series highlighted again that codified design procedures based on linear analysis can be rather conservative. Although advances in simulation tools make nonlinear calculation methods more readily accessible to designers, linear analyses will still be the standard design method for years to come. The present paper aims to improve the linear seismic design method by providing a proper definition of the q-factor of URM buildings. These q-factors are derived for low-rise URM buildings with rigid diaphragms which represent recent construction practise in low to moderate seismic areas of Italy and Germany. The behaviour factor components for deformation and energy dissipation capacity and for overstrength due to the redistribution of forces are derived by means of pushover analyses. Furthermore, considerations on the behaviour factor component due to other sources of overstrength in masonry buildings are presented. As a result of the investigations, rationally based values of the behaviour factor q to be used in linear analyses in the range of 2.0–3.0 are proposed.

Simplified Approach for In-Plane Strength Capacity of URM Walls by Using Lower-Bound Limit Analysis and Predefined Damage Patterns

In this study, a two-phase simplified approach is proposed to predict the in-plane strength capacity of unreinforced masonry (URM) walls. In the first phase, in-plane damage and failure patterns of URM walls are determined from available observational (field) data, experimental data and also from numerical analysis data. Then, a set of rules are proposed to estimate damage and failure patterns of URM wall panels. In the second phase, this valuable information is employed to develop a simplified numerical model with a coarse mesh for the masonry wall, which is consistent with the crack formation at the ultimate state. Then, lower-bound limit analysis approach is used to predict the failure load of the wall without any detailed micro-element analysis. At the final stage, the proposed approach is verified by comparing the numerical results with experimental data from URM wall tests. By the assistance of this approach, it becomes possible to estimate the lateral capacities of ordinary, non-engineered URM walls and buildings from damage patterns at failure state. As an ultimate goal, this structural information can be used for seismic risk assessment of regions where the building typology considered in this study governs the building stock.