Unreinforced Masonry Structures Research Articles

Effects of passive confinement on out-of-plane robustness of unreinforced masonry infill walls

Unreinforced masonry (URM) walls are vulnerable to out-of-plane (OOP) collapse under lateral loads; however, URM structures are still popular in developing countries. Even in many countries classified as developed by the United Nations, URM walls are still commonly applied as exterior and interior infills in engineered buildings, resulting in severe OOP failure during strong earthquakes. The objectives of this study were: 1) to experimentally examine the OOP behavior and performance of URM infills with construction deficiencies commonly found in Asian countries, 2) to investigate the impact of construction quality on resistance to OOP collapse and 3) to propose a promising approach using passive confinement to improve the lateral resistance of URM infills and a method to evaluate their seismic performance. To attain these objectives, the study used two full-scale, single wythe, brick masonry specimens with specific boundary conditions at the top, a control specimen representing URM infills with construction flaws frequently found in Asian countries and a well-constructed specimen subjected to passive confinement under lateral excitation. A series of incremental dynamic shaking table tests was performed on both specimens under OOP unidirectional one-way bending. The test results revealed the positive effects of construction with passive confinement on OOP performance. The results indicated that the OOP robustness of infills can be improved without specific retrofit materials, leading to advantages such as simplicity, cost-effectiveness, and swift execution. Moreover, a theoretical method to evaluate the lateral resistance of infills was presented and verified for practical applications. This method can contribute to reducing the potential for earthquake damage in engineered buildings.

Assessing fragility functions of unreinforced masonry buildings

The seismic vulnerability entrenched within the historical urban landscapes of Algeria serves as a poignant reminder of the pressing need for comprehensive risk management strategies, particularly concerning low and medium height unreinforced masonry (URM) structures. These architectural gems, rich in cultural significance, stand as tangible embodiments of Algeria's storied past. However, their susceptibility to seismic disturbances presents an immediate threat, demanding meticulous attention and innovative methodologies for effective risk assessment and mitigation. In a pioneering scholarly pursuit, this research embarks on an ambitious exploration, melding fragility functions and a sophisticated vulnerability index method to unravel the intricate tapestry of seismic risk assessment. Through the lens of fragility functions, the study delves into a nuanced analysis, probabilistically delineating the vulnerabilities entrenched within the complex framework of URM buildings. Furthermore, the meticulous derivation of vulnerability functions via the vulnerability index method enhances the granularity of risk assessment, providing a multifaceted perspective to discern vulnerabilities accurately. Central to this investigation is the Belouizdad district, a microcosm of historical significance nestled within the vibrant metropolis of Algiers. By scrutinizing seismic scenarios spanning a spectrum of intensities (ranging from VII to X), the study orchestrates a symphony of empirical data and analytical prowess, unraveling the potential seismic reverberations poised to impact the district's architectural heritage. In essence, this scholarly endeavor transcends the confines of mere academic pursuit, serving as a guiding light illuminating the path towards tailored seismic reduction policies. It stands as a clarion call to safeguard Algeria's architectural patrimony, fostering resilience amidst the tremors of uncertainty and ensuring the preservation of its cultural legacy for generations to come.

Development of engineered geopolymer composites containing low-activity fly ashes and ground granulated blast furnace slags with hybrid fibers

Engineered geopolymer composites, a green version of engineered cementitious composites, replace the cement by industry byproducts such as ground granulated blast furnace slag and/or fly ash with alkaline activators. However, fly ashes largely available in Kazakhstan termed here as low-activity fly ash, do not meet ASTM standard in terms of the particle size distribution and strength activity index due to lack of quality control during collection. Therefore, this paper aims to develop an engineered geopolymer composite containing low-activity fly ash and ground granulated blast furnace slag with hybrid fibers. Different combinations of fibers including polyvinyl alcohol, polypropylene, and steel fibers were investigated. The developed composite is targeted for multi-functional retrofitting on unreinforced masonry structures which requires high strain hardening capacity and low thermal conductivity. To achieve this target, the workability, dry shrinkage, compressive strength, tensile strength, strain hardening capacity, and thermal conductivity were examined through extensive experimental studies. From test results, it has been found that the geopolymer composite containing low-activity fly ash with a proper fiber combination can exhibit desirable mechanical and thermal properties with more than 20 MPa compressive strength, more than 2% of the tensile strain hardening capacity, and less than 0.25 W/mK thermal conductivity. Based on test results, micromechanical models were also developed for the composite to identify suitable fiber combinations for required strain hardening capacities.

Strategic Placement of Accelerometers for Structural Health Monitoring of a Complex Unreinforced Stone Masonry Hindu Mandir

ABSTRACT The BAPS Hindu Mandir, recently constructed in Abu Dhabi, UAE, is a complex unreinforced stone masonry structure built from thousands of sculpted sandstone and marble pieces employing ancient Indian techniques called Shilp Shastra. The entire structure is substantially large with a footprint size of 5,100 m 2 and unique so that it is not covered by modern seismic design standards. Its performance was verified by conducting dynamic field tests presented herein. The most vulnerable substructure was identified based on both engineering judgement and modal analysis of the entire structure employing a detailed 3D finite element model, which was validated via the field experiments. A “local” model was developed for the identified vulnerable substructure which significantly reduced model complexity and allowed to overcome computational limitations. Based on the response of the local model, the relative importance of the sensor locations was determined via a Displacement Index method in addition to a reduction of the total variance of spectral accelerations using conditional probability theory. Through this approach, a methodology for selecting the optimal sensor placement with application on the complex unreinforced stone masonry Hindu Mandir is proposed.

Out-of-plane behavior of U-shaped unreinforced masonry structures

The out-of-plane behavior of masonry walls is an important aspect of their response, particularly in seismic regions. While previous studies have examined various geometries and materials, there exists a dearth of research on U-shaped walls, specifically with diverse geometric configurations. Moreover, the influence of axial load on out-of-plane behavior remains largely unexplored, limiting practical applications. This study aims to bridge these gaps by comprehensively investigating the out-of-plane behavior of U-shaped walls, focusing on the quantification of variable parameters encompassing geometry and axial load. A three-dimensional finite element model using the macro-modeling approach is used to analyze the structural response of U-shaped masonry walls under out-of-plane loading. The accuracy of the numerical modeling approach has been validated by available experimental test results to demonstrate the effectiveness of the modeling approach in predicting the out-of-plane behavior of U-shaped masonry structures. Subsequently, a series of parametric studies delves into the impacts of distinct geometrical attributes, including thickness, height, and span size, shedding light on the behavior of these walls across diverse geometric configurations. The results indicate that the capacity of the wall at the connection between out-of-plane and cross walls significantly influences the overall behavior and the failure mode. It was found that axial load substantially increases the strength of walls, in order of 1.5–3 times, and the positive effect of axial load was more significant than the secondary effects in most cases, though the significance of secondary effect drastically increases by increasing the height. It is shown that walls with longer span have much better behavior in the presence of axial load. Based on the results, for each specific geometrical parameter, a linear function can be considered for relation between strength and axial load. These findings can provide useful insights for the design and analysis of U-shaped masonry structures, particularly in seismic regions where out-of-plane behavior is a critical factor in their performance.

Numerical Modeling of Two Adjacent Interacting URM Structures

Masonry structures in addition to their long heritage are still widely used in civil engineering practice. It should be emphasized that a lot of research has already been done on the seismic behavior of masonry structures. However, due to the nature of such a problem, its complexity and seriousness, the development of numerical models and their connection with experimental tests are always important. This is particularly significant considering their vulnerability to the action of horizontal forces generated during seismic excitations. In recent decades, many researchers have tried to capture the behavior of unreinforced masonry (URM) structures or reinforced concrete (RC) frames with masonry infills exposed to earthquakes, using different approaches. This paper tackles numerical modeling based on the finite element method (FEM) for the estimation of the dynamic response of two adjacent interacting URM units, subjected to shaking table motions. Geometrical and material properties of the specimen are provided by the Horizon 2020 project SERA-AIMS (The Seismology and Earthquake Engineering Research Infrastructure Alliance for Europe – Seismic Testing of Adjacent Interacting Masonry Structures). The analyses of dynamic performance were executed in SAP2000 software. Obtained results on the numerical model provide useful guidelines for modeling the nonlinear seismic behavior of masonry buildings.

Observed failure modes in existing URM buildings after November 26, 2019 earthquake in Albania

This study explores the vulnerability of Unreinforced Masonry (URM) structures to seismic events, aiming to uncover the causes of failure and propose reconstruction strategies. Despite often being devastated in powerful earthquakes, URM buildings show damage even in mild to moderate seismic events. Field assessments after a magnitude 6.4 earthquake near Durres, Albania, in 2019, revealed severe damage in masonry buildings and adobe dwellings. Weak structural connections, insufficient roof support, and the absence of bond beams in load-bearing walls were identified as key contributors to the observed damage. The survival of masonry buildings post-earthquake does not guarantee seismic safety. The study recommends reinforcement techniques like shotcrete application, space reduction, and corner reinforcement, along with innovative methods such as Fiber Reinforced Polymer (FRP) use, for existing undamaged unreinforced buildings. These measures aim to prevent damage in the aftermath of destructive earthquakes, offering insights for the resilience of URM structures.

Soil-structure interaction effects on out-of-plane seismic response and damage of masonry buildings with shallow foundations

A significant amount of damage and casualties induced by several strong-motion earthquakes which recently stroke South-East Mediterranean area is due to the major seismic vulnerability of residential buildings. In small villages and mid-size towns, those buildings very often consist of two- to four-story, unreinforced masonry (URM) structures not designed for earthquake resistance, with direct foundations usually corresponding to an in-depth extension of load-bearing walls. For such structures, especially when founded on soft soils, site amplification and soil-foundation-structure interaction (SFSI) can significantly affect the seismic performance; conversely, such phenomena should be investigated through methods that allow a trade-off between accuracy and computational effort, hence encouraging their implementation in engineering practice. This paper provides a comprehensive updated description of the studies carried out in the last years by the authors, which are based on both linear and nonlinear, parametric, dynamic analyses of complete soil-foundation-structure (SFS) models representative of existing residential building configurations on different soils. Specifically, the parametric study investigated SFS models with different masonry types, aspect ratios, and code-conforming homogeneous and heterogeneous soil profiles. The methodology and analysis results allowed for reaching the following objectives: (i) predicting the elongation of the fundamental period and the variation of equivalent damping of the SFS system with respect to fixed-base conditions, through a simplified approach based on an equivalent simple oscillator; and (ii) estimating the probability of exceeding increasing damage levels associated with out-of-plane overturning of URM walls, through fragility functions that take into account SFS interaction. The effectiveness of these simplified tools was successfully validated against well-documented case studies, at the scales of both single instrumented buildings and urban area.

Quantitative Contribution of Timber Ring Beams in the Dynamic Response of Adobe Masonry Structures

Earthen structures made of adobe bricks are complex systems that making the identification of their behavior difficult, especially when they have to sustain lateral forces such as seismic forces. This paper presents a numerical investigation for the assessment of the structural response of unreinforced adobe masonry structures and how the installation of wooden ring beams contributes to their overall resistance. In the framework of the numerical investigation, finite element models were created to simulate the response of an adobe building with and without the presence of wooden ring beams. The test building is located in Cyprus, in the South Eastern Mediterranean region which is a seismic area. The material properties used in this study were found in the literature and were based on experimental data for local materials. The models were subjected to earthquake loads, performing time history analyses for the calculation of pertinent displacements and stresses. The findings indicate that integrating wooden ring beams reduces the fundamental period by 6% and modifies the building’s seismic behavior. This modification is evident not just in the magnitude of the stresses but also in their distribution, leading to a stratified stress profile. Peak stresses are primarily concentrated around the ring beams.

Experimental validation of analytical equations for retrofitting masonry buildings with timber frames and boards

This paper is the second of a series of two, discussing the analytical assessment of the seismic behavior of an innovative timber retrofit for unreinforced masonry structures. The retrofit system consists of timber frames, connected to the masonry piers and to the floor diaphragms, with oriented-strand board sheathing nailed to the frames and to flexible timber diaphragms. The companion paper [1] focused on the conceptual bases of the proposed solution, providing analytical equations for the design of its components, and a step-by-step design procedure for their practical application. In this paper, the proposed formulations are validated against the experimental data coming from in-plane quasi-static shear-compression tests on masonry piers and dynamic shake-table tests on a full-scale building prototype. These tests were performed on nominally-identical specimens in both bare and strengthened configurations, allowing a direct comparison between the experimental and calculated retrofit contribution. In particular, the analytical formulation was applied to reproduce the in-plane response of individual piers, to reproduce the base shear-interstory drift envelope of the building specimens, to assess the out-of-plane strength of walls, and to validate the flexible diaphragm enhancement. Overall, the proposed formulations revealed a good match with the observed retrofit performance and failure mechanisms.

Comparative numerical study to simulate masonry with bed joint reinforced repointing

Bed joint reinforced repointing is a retrofitting technique for unreinforced masonry structures that is commonly applied in the Netherlands to repair settlement-induced damage. Using this technique, the bed joints of masonry walls are reinforced with steel rebars that are embedded in a high strength repair mortar. Due to the increase of induced seismic events in the northern part of the Netherlands, an experimental study was carried out at Delft University of Technology to investigate the performance of this retrofitting technique for combined settlement and seismic loading. This paper aims to simulate the experimental results, with a focus on the comparison of different finite element modelling approaches for studying both un-strengthened and strengthened full-scale tested walls. To that end, three different models are investigated – comprising both macro (continuum) and simplified and detailed micro (brick-to-brick) modelling approaches. The bricks and mortar joints are modelled as one homogenous continuum in the macro model, whereas in the two brick-to-brick models these structural components are modelled separately, with the detailed model including interface elements to simulate the brick–mortar bonds. Nonlinear pushover analyses are subsequently carried out using all three modelling approaches, for both monotonic and cyclic loading cases. Based on these analyses, the detailed brick-to-brick model was found unsuitable to simulate the strengthened wall because cracks in the model mainly occur in the form of opening of the brick–mortar bond interfaces, while smeared cracking in the plane stress elements of the mortar joints is very limited. Similarly, the continuum damage model was found to be inaccurate when pre-existing damage in the experiment needed to be taken into account. The continuum damage model also showed lower axial stresses in the rebars, compared with the simplified brick-to-brick model, as the former does not allow for the direct assignment of material properties for the high strength repair mortar in the strengthened joints.

Sensitivity analysis on mechanical parameters of continuum models for simulating in-plane tests on existing masonry panels

Understanding and modeling the structural behavior of existing masonry buildings is a fundamental task in view of their management and conservation, especially in seismic-prone areas. Among the different modeling approaches for the computational analysis of unreinforced masonry structures, this work investigates the ability of two material continuum models, the modified masonry-like material and Mazars’ material, to simulate real in-plane failure modes of masonry panels, considering the influence of the variability of mechanical parameters on the results. As the first part of a broader benchmark, this work focuses on the study of two recurring test patterns for masonry panels subject to both horizontal and vertical loads, given the availability of experimental data. To account for the parameter variability that typically affects masonry materials, uncertainty propagation is performed in a probabilistic framework, and a gPCE-based proxy model makes it possible to turn time-consuming numerical runs into significantly faster analytical evaluations. Sensitivity analyses are carried out in terms of Sobol’ indices to evaluate the influence of input variability on the ultimate horizontal load. The most relevant masonry parameters for each model are thus highlighted and the issue of model calibration is finally addressed by relying on Bayesian inverse methods. This work also underlines the pros and cons of the different hypotheses, which underlie the two nonlinear constitutive laws.

In-plane shear behaviour of concrete sandwich panels reinforced with various geogrids

Seismic occurrences have caused severe damage and even the collapse of fragile unreinforced masonry structures. New technologies are replacing these conventional methods of construction; concrete sandwich panels (CSP) are one such technique gaining popularity. In-plane diagonal shear strength of concrete sandwich panels is studied experimentally, and the effect of incorporating geogrids on the deformation capability and load-bearing capacity is reported. Two forms of geogrid material, plastic uniaxial geogrid (PUG) and polyester biaxial geogrids (PBG2 and PBG3) are used to improve the strength and ductility of concrete sandwich panels. Diagonal compression tests have been carried out better to understand the behavior of CSP under in-plane strength. This paper also discusses the effect of the different mixes, the effect of geogrids, load-deformation behaviour, shear capacity, deflection ductility factor, mode of failure, shear strength, and energy dissipation. In contrast to the control specimen, specimens cast with plastic geogrid had a 13.7% and 26% improvement in shear capacity and load-bearing capability for the two types of micro-concrete mixes used in this study. The conclusion reached is that plastic uniaxial geogrid is effective in improving the concrete sandwich panels’ load-bearing capacity, shear capacity, and deformation ability. Polyester biaxial geogrids enhanced the ductility of the concrete sandwich panels.

Research on the bond performance of ribbed bars and brick masonry in embedded reinforcement method

The embedded reinforcement method (ERM) is widely used in the seismic strengthening of unreinforced masonry structures (URM). One of the key factors that determines the reliability of this reinforcement method is the bond performance between the ribbed bars and the masonry. In this study, pull-out tests were adopted, ribbed bars, ordinary cement mortar and high-strength-polymer mortar with different strengths were selected to conduct caulking reinforcement on the specimens. The form of damage, bond-slip curves and bond strengths of the specimens were obtained. The bond failure mechanisms of the specimens and the influences of the mortar strength and the bond length were discussed. The mathematical expressions of the bonding parameters between the ordinary mortar-ribbed bars and masonry were obtained, and the bond energy between the ribbed bars and masonry was analyzed. The research results provide the basis for the popularization and application of the embedded reinforcement method.

Correlation between sonic pulse velocity and flat-jack tests for the estimation of the elastic properties of unreinforced brick masonry: Case studies from Croatia

Historic urban cores in southern Europe are characterized by a significant number of existing unreinforced masonry structures that are known for their seismic vulnerability. The first step in the preservation of such damaged buildings is condition assessment and characterization of the mechanical properties of the masonry, which can be achieved using non-destructive, semi-destructive and destructive methods. In this paper, the emphasis is on determining the modulus of elasticity of unreinforced masonry through flat-jack and sonic pulse velocity tests. Both methods are explained in detail and results from eight different case studies are presented. The results are compared and correlations among the techniques are observed, showing that both methods can be suitable for estimating the elastic properties of existing masonry. The paper provides a wide set of experimental data that has been made available. Finally, a sensitivity analysis was performed. Differences between static and dynamic modulus of elasticity and operational and practicability issues regarding on-site testing are discussed.

Haiti: architecture to save the soil

The use of bio-based construction materials in Haiti could help the construction sector to transition, from non-seismic unreinforced masonry structures towards materials that can reverse catastrophic deforestation, promote ecological regeneration, and help save the soil. Architecture has a role to play in this vision, by creating new designs and incentivising clients to invest in new materials.

The Sensitivity of Global Structural Parameters for Unreinforced Masonry Buildings Subjected to Simulated Ground Motions

This research performs a parametric study based on Equivalent Single Degree of Freedom (ESDOF) models for simplified seismic analysis of unreinforced masonry (URM) structures. This is a necessary action due to the fact that it is not affordable to model and analyze populations of masonry buildings by using detailed continuum-based models during regional seismic damage and loss estimation studies. Hence, this study focuses on the sensitivity of major structural parameters of a selected idealized hysteretic model for URM buildings. The numerical models are subjected to region-specific simulated ground motion time histories generated using validated seismological parameters. The variations in dynamic analysis results are evaluated using statistical tools for major structural and seismological parameters. The results reveal that the strength factor is the most influential structural parameter, whereas magnitude and distance have a significant impact on the response of idealized URM models as seismological parameters. Furthermore, the specific seismic performance exhibiting limited ductility capacity and the narrow margin of safety between the initial state of inelastic behavior and the ultimate (collapse) state for URM buildings is verified by the statistical approaches employed in this study.

Performance criteria expressed by means of relative displacements for a retrofitted masonry school building

In order to study how retrofitting techniques applied to URM structures influence the failure modes and the displacement capacity of the buildings, a school building from Bucharest was modelled using 3DMacro software. The mixed structural system required a particular approach for the calculation model, in a software able to capture the interaction between the reinforced concrete elements (columns and tie beams) and the URM masonry panels.Damage pattern evolution was analyzed for the three models capturing the gradual seismic improvements done in the past century, transforming the unreinforced masonry structure into a confined masonry one, with few RC elements, and eventually into a fully jacketed structure with densely spaced RC columns. The results obtained from the nonlinear static analyses were validated by means of global and local displacement limits associated with specific damage criteria.The damage suffered after 1977 Vrancea earthquake and the current performance requirements in terms of seismic safety led to structural changes aimed at improving the behavior of masonry structures by means of interventions that marked the transition from an unreinforced masonry structural system to a retrofitted masonry structural system. The comparative analyses carried out between the behavior of the initial and the strengthened structure of the school intend to highlight the benefits of the retrofitting interventions performed, but also the situations in which the applied strengthening measures might prove to be excessive. Considering the case study is representative for the existing building stock in the education sector in Romania which needed subsequent seismic retrofitting in order to reach the required seismic safety, the results highlight the improved behavior after these interventions. The present works represent a topic of interest at national level, since it targets a large part of the existing building stock subjected to losses in case of an earthquake. Moreover, the past retrofitting interventions assessed in this paper represent a typical intervention done for similar structures, and therefore, the results obtained are representative for the Romanian building stock.

Enhancing unreinforced masonry wall resilience through nano-silica modified steel fiber reinforced mortar: A study on in-plane cyclic loading

Strengthening existing unreinforced masonry (URM) structures with plaster is a common technique used to improve the structural integrity and durability of buildings. However, using a thin layer of plaster alone may not provide sufficient reinforcement to adequately enhance the URM wall’s resistance to lateral or other types of loading. Therefore, incorporating additional materials, such as steel fibers and nano-silica, into the plaster mix has been explored in this study to further improve the URM wall’s resilience performance. The effect of nano-silica modified steel fibers reinforced (NS-SFR) mortar plastering on the axial and lateral load capacity of concrete masonry walls was experimentally investigated. The investigated URM walls were constructed with a dimension of 830 × 810 × 100 mm (length × width × thickness) using hollow concrete masonry blocks. These walls were categorized as a reference wall, plastered on one side only wall, and plastered on both sides wall. During the experimental testing, each sample was exposed to a combination of precompression and cyclic in-plane loading to assess its shear capacity. The tests also involved closely observing and identifying any cracks or types of failures that occurred. The results of the tests indicated that the walls that were strengthened with NS-SFR mortar exhibited a significant improvement in their compressive strength, shear strength capacity, and stiffness, in comparison to the walls that were not strengthened. Applying NS-SFR mortar as a plaster on the URM wall resulted in significant improvements in both compressive strength and shear capacity. Specifically, for the one-side plastered wall, the use of NS-SFR mortar increased compressive strength and shear capacity by 18.5 % and 40 %, respectively, while for the two sides plastered wall, the increase was even more significant, with improvements of 86.5 % and 132 % in compressive strength and shear capacity, respectively.

Out-of-plane flexural behavior of brick masonry strengthened with engineered cementitious composites embedded in the horizontal mortar joint

Unreinforced masonry (URM) structures exhibit high vulnerability under out-of-plane loads. Engineered cementitious composite (ECC) is suitable for the reinforcement of URM structures due to its excellent mechanical properties. This study presents a promising method to strengthen brick masonry structures using ECC embedded in the horizontal mortar joints. In order to investigate the flexural behavior of brick masonry strengthened by the proposed method under one-way horizontal bending loads, a total of nine groups of masonry specimens were tested under four-point bending loads, including one group of un-strengthened specimens and eight groups of ECC-strengthened specimens. The mix proportion and reinforcement ratio of the ECC were considered as variables in the experimental schemes. The failure mode, load–displacement curves, stiffness, strength, energy dissipation capacity, and ductility of the specimens were discussed. The test results demonstrate the effectiveness of this strengthening system in improving the out-of-plane performance of URM structures. The tensile properties of the ECC were closely related to the out-of-plane flexural behavior of the strengthened specimens. The flexural performance of the strengthened specimens generally improved as the tensile properties of the ECC increased. The maximum mid-span loads and corresponding displacements increased by 34%-171% and 33%-163%, respectively, compared to the control specimens. Furthermore, the flexural stiffness of the strengthened specimens was enhanced by a maximum of 63%. The flexural strength and energy dissipation capacity of the strengthened specimens increased with the reinforcement ratio of the ECC. The maximum flexural strength and energy dissipation capacity of ECC-strengthened specimens were 2.39 and 7.01 times higher than those of un-strengthened specimens, respectively. In addition, the results showed that the maximum bending moments of un-strengthened and strengthened specimens could be better predicted by the existing theoretical model.