Unreinforced Masonry Walls Research Articles

Quantifying Hybrid Failure Modes of Unreinforced Masonry Walls through Experimental Data Analysis

Quantifying Hybrid Failure Modes of Unreinforced Masonry Walls through Experimental Data Analysis

Case Study on Behavior of a Masonry Buildings Under Seismic Loading October 2023 Earthquake in Nepal

Abstract: This paper assesses the behavior of the corners of unreinforced masonry walls during a magnitude 5.3 earthquake that struck western Nepal on October 3 at 14:40 local time. A strong aftershock with a 6.3 Richter scale occurred immediately after the main shock at 15:06, followed by a second aftershock of 5.1 Richter scale. The epicenter of the quake was located in Bajhang district, with tremors felt in neighboring districts (Doti, Achham, Bajura, Dadeldhura, Baitadi, and Darchula), as well as in Kathmandu. According to the National Society of the Red Cross, 334 houses have been fully damaged, and 1,185 have been partially damaged. The assessment is still ongoing in the affected areas, with displaced people staying in schools or with neighbors. The response is slow due to access constraints, requiring one to two days of walking to reach the affected communities. The majority of the buildings in the affected region are constructed with masonry, most of which were formed with random or coursed stone and mud brick walls without any reinforcement. Many of these buildings were damaged or had collapsed. The cracking and failure patterns of the buildings are examined and interpreted according to current provisions for earthquake resistance of masonry structures. The damages are attributed to several factors, such as poor construction quality and workmanship. This study aims to contribute to the body of knowledge necessary for improving earthquake resilience in Nepal and similar regions around the world

Seismic in-plane behavior of composite masonry walls strengthened with cold rolled steel sheets

The seismic behavior of masonry walls strengthened by cold rolled steel sheets is numerically evaluated in this study using finite element modeling in ABAQUS. Cold-rolled steel sheets cover two sides of an unreinforced masonry wall and are bolted together to form a composite masonry wall. The behavior of the composite masonry walls is compared to that of the corresponding unreinforced masonry walls with various failure mechanisms. The finite element models of the unreinforced masonry walls are validated through experimental data in the literature. Then, cold-rolled steel sheet-strengthened walls are simulated, and variations in strength, ductility, and failure mode are investigated. A thorough sensitivity analysis is conducted on the effects of the aspect ratio, boundary condition, axial load ratio, steel sheet thickness, and connection bolt configuration on the in-plane strength and failure mode of the walls. It is observed that the steel sheets affect the local and global behavior of the masonry walls and change their failure modes. The use of through-thickness bolts to connect steel sheets to the masonry wall provides a composite action between steel sheets and the masonry wall against in-plane lateral loads. The cold-formed steel sheets not only resist a portion of the lateral load but also significantly increase the lateral strength of the masonry walls and the ductility of the whole wall system. Based on the analysis results it can be concluded that strengthening masonry walls with cold-rolled steel sheets is a promising technique for the seismic improvement of existing conventional masonry buildings and for the design of new masonry buildings.

Rapid shear capacity prediction of TRM-strengthened unreinforced masonry walls through interpretable machine learning deployed in a web app

The presented study provides an efficient and reliable tool for rapid shear capacity estimation of TRM-strengthened unreinforced masonry walls. For this purpose, a data-driven methodology based on a machine learning system is proposed using a dataset constituted of experimental results selected from the bibliography. The outlier points of the dataset were detected using the Cook’s distance methodology and removed from the raw dataset, which consisted of 113 examples and 11 input variables. In the processed dataset, 17 machine learning methods were trained, optimized through hyperparameter tuning, and compared on the test set. The most effective models were the optimized instances of XGBoost and CatBoost methods, which combined into a voting model to leverage the predictive capacity of more than a single regressor. The final blended model shows remarkable predicting capacity with the determination factor (R2) equal to 0.95 and the mean absolute percentage error equal to 8.03%. Also, the model’s predictions are compared with those of existing analytical relationships, and it is found to perform the best of all. In sequence, machine learning interpretation methods are applied to find how the predictors influence the target output. Am, ft, and n⋅tf were identified as the most significant predictors with a mainly positive influence on the shear capacity. Finally, the built machine learning system is employed in a user-friendly web app for easy access and usage by professionals and researchers.

Discrete macro - models of nonlinear interlocking mechanisms in the out-of-plane failure of masonry walls

AbstractHistorical unreinforced masonry (URM) constructions are generally vulnerable to out-of-plane (OOP) failures due to the absence of rigid floors and poor connections between orthogonal walls. That leads to the activation of rocking mechanisms of external walls, whose ultimate force and displacement are affected by complex nonlinear interactions with sidewalls. These interactions are often neglected in the engineering practice, potentially leading to significant approximations, as demonstrated by experimental and numerical studies available in the literature. As a novel contribution to the field, this paper presents an upgraded discrete macro-element model (DMEM) to predict the rocking capacity of OOP loaded URM walls interacting with sidewalls. Considering both the onset and the evolution of the rocking mechanism of the front wall, interlocking effects with the sidewalls are first simulated through frictional resistances using the macro-block model (MBM) and the nonlinear kinematic approach of limit analysis. Then, the upgraded DMEM is implemented on the basis of the equivalence between the continuous distribution of these forces, introduced as a further novelty of the paper, and the discrete distribution of lateral elastic-plastic links, accounting for mechanical and geometrical nonlinearities. The results of the two models are discussed in terms of both frictional resistance-displacement and pushover curves, referring to a case study of a front wall belonging to a two-storey URM building. The wall response is also compared with the results derived from the original source of the case study and analysed by changing the number of nonlinear links to define different levels of accuracy.

A unified modeling approach for concrete masonry shear walls: Conception, validation and insights

Distinct modeling strategies have been developed to simulate concrete masonry shear walls; however, each is validated with a limited number of specimens tested under identical laboratory conditions and only covers specific detailing scenarios. This research seeks a unified finite element-based modeling approach integrating macro-modeling concepts for grouted masonry (GM) regions and simplified micro-modeling for ungrouted masonry (UGM) regions. The objective is to facilitate accurate simulation of the in-plane behavior of diverse typologies of concrete masonry shear walls: unreinforced masonry (URM), fully grouted reinforced masonry (FGRM), and partially grouted reinforced masonry (PGRM). A smeared crack approach is used to capture the crushing and cracking of grouted masonry (GM) and ungrouted masonry (UGM), and interface elements are incorporated into UGM regions to model the mortar joints, resulting in a combined smeared-discrete model. The validation process comprises two levels. The first-level evaluation focuses on URM and FGRM walls, emphasizing the analysis and comparison of lateral force–displacement curves. Second-level validation is done through simulation of PGRM walls by evaluating lateral force–displacement curves, damage patterns, and reinforcement strains. The implemented modeling approach generally simulates the response of the different experimental masonry shear walls evaluated with good accuracy. A sensitivity analysis was also performed to reveal the influence of some parameters on the numerical results, providing additional perceptions into the reliability of the modeling approach. Relevant insights into the modeling approach were obtained, allowing complex phenomena in the tested walls to be simulated, thus contributing to advances in the simulation of in-plane loaded concrete masonry walls.

Analytical Model for the In-Plane Lateral Capacity of Unreinforced Masonry Walls Based on Effective Compression Zone Failure Mechanism

ABSTRACT This study proposes a simplified theoretical model to predict the in-plane lateral capacity of unreinforced masonry (URM) walls, addressing limitations in current design guidelines based on semi-empirical approaches. Masonry walls are conceptualized as macroscale units subjected to vertical and lateral loads, with failure mechanisms governed by the effective compression zone. The model applicability is evaluated against comprehensive URM datasets and compared with existing guidelines. The results indicated the outperformance of the proposed model compared to existing guidelines. Additionally, a parametric study explores the influence of primary design parameters on the in-plane lateral capacities of URM walls, offering valuable predictive insights.

A study of the seismic resistance performance of strengthened masonry walls using polypropylene mesh-composite on the surface

To investigate the seismic performance of masonry walls in village buildings strengthened with polypropylene mesh materials on the surface, five reduced-scale masonry walls were designed for low-cyclic reversed loading tests. The research observed the failure processes and characteristic forms of masonry walls, conducting a comparative analysis of the hysteresis curves and energy dissipation of the walls. Then, ABAQUS finite element software was used to simulate the performance of the strengthened walls to analyze the damage mode of these walls. The results indicated that under compression-shear loading, unreinforced masonry walls experienced shear failure along horizontal mortar joints, exhibiting lower shear load, lateral stiffness, and ductility. The reinforced walls with polypropylene mesh-composite effectively delayed and mitigated the failure. Weak zones in the wall gradually extended from the structural sides towards the central core area. Ultimately, after the reinforcement material had fully engaged, delamination failure occurred, leading to the formation of "X" or "Y" shaped through cracks. After reinforcement with single-side polypropylene materials, the load-bearing capacity of the wall increased by 165 % compared to the unreinforced wall. The ductility is improved compared to the unreinforced wall, whereas the initial stiffness increased by 133 %. Upon reinforcement with double-sided materials, the wall's load-bearing capacity surged by 180 %. The initial stiffness climbed to 171.3 %. The finite element analysis results closely aligned with the experimental data, suggesting that reinforcing the surface with polypropylene mesh-composite cement mortar significantly bolstered the wall's shear strength and deformation capabilities. This method effectively strengthens the seismic resistance of masonry structures.

Accuracy of stochastic finite element analyses for the safety assessment of unreinforced masonry shear walls

ABSTRACT To examine structural safety and reliability, an accurate prediction of the variability of structural resistance must first be determined. This may be achieved through extensive physical testing or, more commonly in modern research, synthetic data generation, such as stochastic finite element analyses (SFEAs). Due to the prevalence and versatility of such techniques, and the need for a high level of confidence when performing a safety assessment, an understanding of the accuracy of data derived from SFEAs is essential. In this paper, SFEA models have been developed to predict the responses of 16 unreinforced masonry walls tested in a laboratory under cyclic in-plane lateral loading. Each SFEA has been developed to reflect the as-built conditions of these experimental specimens and focus on predicting the shear capacity of the failure mechanisms that unreinforced masonry shear walls are susceptible to. From the results of these SFEAs, the accuracy (quantified as model error) of this modelling strategy has been estimated by comparing the peak in-plane shear resistances of the laboratory and numerical models.

Evaluating the Role of Mortar Composition on the Cyclic Behavior of Unreinforced Masonry Shear Walls.

The mechanical behavior of unreinforced masonry (URM) shear walls under in-plane cyclic loading is crucial for assessing their seismic performance. Although masonry structures have been extensively studied, the specific influence of varying lime content in cement-lime mortars on the cyclic behavior of URM walls has not been adequately explored. This study addresses this gap by experimentally evaluating the effects of three mortar mixes with increasing lime content, 1:0:5, 1:1:6, and 1:2:9 (cement:lime:sand, by volume), on the cyclic performance of brick URM walls. Nine single-leaf wall specimens 900 mm × 900 mm were constructed and subjected to combined vertical compression and horizontal cyclic loading. Key parameters such as drift capacity, stiffness degradation, and energy dissipation were measured. The results indicated that the inclusion of lime leads to a moderate improvement in drift capacity and ductility of the walls, with the 1:1:6 mix showing the highest lateral capacity (0.55 MPa), drift at cracking (0.08%), and drift at peak capacity (0.31%). Stiffness degradation and energy dissipation were found to be comparable across all mortar types. These findings suggest that partial substitution of cement with lime can enhance certain aspects of masonry performance. Further research is recommended to optimize mortar compositions for unreinforced masonry applications.

Out-of-plane behavior of unreinforced masonry walls with horizontal slip layers in RC frames

In some cases, the infilled walls may be subjected to in-plane（IP） earthquake and out-of-plane(OOP) earthquake action due to the uncertainty of earthquake action and infills’ position. As a new type of unreinforced masonry(URM) walls with horizontal slip layers, it can effectively reduce the in-plane damage of infilled walls of reinforced concrete frames under an earthquake. However，the infill wall out-of-plane damage has a greater security threat than in-plane damage. In this study, a numerical study about the difference of OOP behavior of URM walls with/without horizontal sliding layers are analyzed, mainly including failure mode, bearing capacity, displacement capacity and stiffness. The spacing of horizontal slip layers, length of annectent reinforcing bars and the friction efficient between slip layer and masonry unit are chosen to investigate the differences of OOP mechanical behaviors. The results show that the horizontal sliding layers reduce the integrity of URM walls in reinforced concrete frames, causing the constraint of surrounding frame on infilled wall and the arching effect inside infilled wall become weaker, and OOP bearing capacity to drop. It is found that i) the annectent reinforcing bars is conducive to reducing the out-of-plane failure of the wall; 2)the smaller the slip layer spacing of the wall is, the more serious the damage of the infill wall under the out-of-plane load is, and the smaller the out-of-plane bearing capacity and the out-of-plane stiffness of the wall are; iii) the influence of the friction coefficient between slip layers and masonry unit on OOP mechanical behaviors is small. Finally, Chinese building design code is also selected to estimate the OOP bearing capacity of URM walls with horizontal sliding layers. The value of λcr is recommended to be greater than 1.0, and the value of λP is greater than 3.3. The spacing of the slip layer should be greater than 1 / 5 of the wall height, and the length of the annectent reinforcing bars should not be less than 1 / 5 of the infilled wall length.

Characterisation of timber joists-masonry connections in double-leaf cavity walls – Part 2: Mechanical model

The seismic assessment of the out-of-plane (OOP) behaviour of unreinforced masonry (URM) buildings is essential since the OOP is one of the primary collapse mechanisms in URM buildings. It is influenced by several parameters, including the poor connections between structural elements, a weakness highlighted by post-earthquake observations. The paper presents a mechanical model designed to predict the contributions of various resisting mechanisms to the strength capacity of timber-joist connections in masonry cavity walls. The research presented in this paper considers two different failure modes: joist-wall interface failure, and OOP rocking behaviour of the URM walls. Consequently, two mechanical models are introduced to examine these failure modes in timber-joist connections within masonry cavity walls. One model focuses on the joist-wall interface failure, adopting a Coulomb friction model for joist-sliding further extended to incorporate the arching effect. The other model investigates the OOP rocking failure mode of walls. The combined mechanical model has been validated against the outcomes of an earlier experimental campaign conducted by the authors. The considered model can accurately predict the peak capacity of the joist connection and successfully defines the contribution of each mechanism in terms of resistance at failure.

Characterisation of timber joists-masonry connections in double-leaf cavity walls – Part 1: Experimental results

Post-earthquake structural damage shows that out-of-plane (OOP) wall collapse is one of the most common failure mechanisms in unreinforced masonry (URM) buildings. This issue is particularly critical in Groningen, a province located in the northern part of the Netherlands, where low-intensity induced earthquakes have become an uprising problem in recent years. The majority of buildings in this area are constructed using URM and were not designed to withstand earthquakes, as the area had never been affected by tectonic seismic activity before. OOP failure in URM structures often stems from poor connections between structural elements, resulting in insufficient restraint to the URM walls. Therefore, investigating the mechanical behaviour of these connections is of prime importance for mitigating damages and collapses in URM structures.This paper presents the results of an experimental campaign conducted on timber joist-masonry cavity wall connections. The specimens consisted of timber joists pocketed into masonry wallets. The campaign aimed at providing a better understanding and characterisation of the cyclic axial behaviour of these connections. Both as-built and strengthened conditions were considered, with different variations, including two tie distributions, two pre-compression levels, two different as-built connections, and one strengthening solution. The experimental findings underscored that incorporating retrofitting bars not only restores the system’s initial capacity but also guarantees deformation compatibility between the wall and the joist. This effectively enhances the overall deformation capacity and ductility of the timber joist-cavity wall system.

An improved method for statistics estimation of out-of-plane load resistance of masonry walls using design-code models and mechanics-based finite element models

The susceptibility of masonry walls to out-of-plane failure under seismic loading presents a significant engineering concern. In adherence to contemporary limit state design philosophy and as mandated by performance-based design guidelines, it is crucial to accurately estimate statistics parameters such as the mean and variance of out-of-plane resistance. To achieve this, two deterministic models are often considered within the Monte Carlo simulation framework: design-code models, known for their computational efficiency but potential for significant model error, and mechanics-based finite element models, noted for their accuracy but computational intensity. Relying solely on either model for statistics estimation presents challenges due to their respective inherent limitations. This study introduces an improved strategy that synergizes the strengths of both models, resulting in enhanced statistics estimators for the out-of-plane resistance of masonry walls. The proposed methodology involves the use of the control variate method by integrating numerous design-code model evaluations to boost computational efficiency while incorporating a limited number of finite element model evaluations to ensure accuracy. The constructions of the proposed estimators follow rigorous optimization procedures to minimize associated errors and variances. Theoretical derivations are provided for essential elements in the estimator constructions, such as model evaluation numbers for finite element and design-code models and control variate coefficients. Moreover, the accuracy of the proposed estimators is compared with that of the crude Monte Carlo estimator. A key benefit of the proposed estimators is their unbiased nature, and their accuracy is not compromised by the discrepancies between the finite element model and the design-code model. To demonstrate the practicality of these estimators, two case studies are illustrated: one focusing on unreinforced masonry walls and the other on reinforced masonry walls. The results indicate that design-code model-based estimators exhibit large bias, and compared to the estimators that solely rely on the finite element model, the proposed estimators achieve higher accuracy given the same computational budget.

Load-Displacement Characteristics for Out-of-Plane of Unreinforced Masonry Walls

Load-Displacement Characteristics for Out-of-Plane of Unreinforced Masonry Walls

Experimental study on the pullout behavior of steel rebars in masonry shotcreted layer

Unreinforced masonry (URM) buildings are quite popular worldwide due to their low construction costs, despite being susceptible to significant damage even from moderate earthquakes. In many cases, reconstruction may not be the most viable option due to financial and time constraints. Among the available retrofit solutions, steel-reinforced shotcrete (SRS) proves to be a cost-effective, straightforward, yet effective method for repair/retrofitting. This paper presents the results of an experimental study aimed at evaluating the effects of various construction conditions on the rebar pullout resistance of steel-reinforced shotcrete used as a retrofit method for URM walls. To achieve this, a total of 46 full-scale URM specimens were manufactured and tested, each consisting of a one-sided 50 mm-thick shotcrete layer with a single 8 mm steel rebar. The variables examined included the condition of the URM surface before the shotcrete application (wet/dry, with/without finishing including plaster/scratched plaster/oil paint), the condition of the rebar surface (rusty/clean), the type of rebar (plain/ribbed), rebar cover in the shotcrete layer (20/25/30 mm), and the URM substrate temperature before application of shotcrete (20 °C/-16 °C). The results indicate that there is an increase in pullout resistance of the specimens under the following conditions: when the wall's finishing is removed, the wall's surface is wetted before shotcrete application, ribbed steel rebar is utilized, and the temperature of URM before shotcrete application is 20 °C, there is an increase in pullout resistance of the specimens by 58 %, 56 %, 32 %, and 13 %, respectively. Additionally, when the steel rebar cover is sufficient, ribbed steel rebar is utilized, and this rebar is rusted, there is an increase in pullout resistance of the specimens by 55 %, 10 %, and 6 %, respectively.

Variation in the Modal Response of Retrofitted Unreinforced Masonry Walls at Different Levels of Damage

Variation in the Modal Response of Retrofitted Unreinforced Masonry Walls at Different Levels of Damage

Flexural strength of surrounding columns in URM-Infilled RC frames retrofitted by thick hybrid wall technique

This study investigated the flexural strengthening effect of the Thick Hybrid Wall (THW) technique as a seismic retrofit approach for Unreinforced Masonry Wall (URM)-infilled Reinforced Concrete (RC) frames. THW is a strength–ductility-type seismic retrofit technique, which was developed for pilotis-type RC buildings. In this study, the flexural strength and failure mechanisms of URM-infilled RC frames retrofitted using the THW technique were investigated. Consequently, a comprehensive theoretical model was developed to measure the flexural strength, and a previously proposed equation was modified. Additionally, an equation was developed to determine the minimum additional-wall length, and another equation was developed to determine the demand length of the additional wall to obtain the demand flexural strength. These equations enhance the cost-efficiency and reduce the irregularities risks. The interaction between the URM infill and surrounding RC columns was modeled, and a cyclic loading test was performed under the influence of a constant axial force on the four specimens. The results indicate that THW technique changes the failure mechanism of URM-infilled RC frames from shear to flexure and that the retrofitted URM acts as part of the lateral resisting system owing to the active lateral confinement pressure. The accuracy of the analytical models was confirmed via experimental results, and the developed models facilitate the practical implementation of the THW technique on intended frames.

The Effect of CFRP Application on the Strength and Dynamic Characteristic of Unreinforced Masonry Wall

The Effect of CFRP Application on the Strength and Dynamic Characteristic of Unreinforced Masonry Wall

Effect of in-plane damage on the out-of-plane strength of masonry walls with openings

Past earthquakes have shown that masonry walls are vulnerable to fail near the openings. Openings such as doors and windows are an essential part of the structure. The presence of an opening reduces the strength and stiffness of the masonry structures. It also affects the load-resistant mechanism of the masonry walls. The walls are subjected to simultaneous in-plane and out-of-plane forces during seismic events. Therefore, the analysis of the masonry walls either in in-plane or out-of-plane direction may not give a proper picture of the global behavior of the masonry structures. The masonry walls deform primarily in the in-plane direction if the walls are properly tied with each other, and the structure is constructed by following codal provisions. Under this situation, it is necessary to investigate the effect of in-plane damage on the out-of-plane strength of the masonry walls. Few researchers have studied the behavior of solid unreinforced masonry walls under combined loading. However, limited work can be found for masonry walls with openings. Therefore, the present study focuses on the effect of different types of opening layouts on the combined in-plane and out-of-plane behavior of masonry walls incorporating varying pre-compression. Further, in masonry buildings, walls are interconnected, making the masonry walls of various shapes. In this study, masonry walls of rectangular and I shapes have been considered to examine the effect of cross-walls under combined loading. Interaction curves have been developed for the walls subjected to the in-plane loading followed by an out-of-plane pressure. Based on the results, the maximum opening percentage in the masonry walls has been suggested, which is desirable in the case of masonry buildings constructed in high seismic zones.