Response Of Unreinforced Masonry Walls Research Articles

Mesoscale numerical modeling of unreinforced masonry wall response to underground dynamic loads: A comparative study utilizing FEM and DEM

This study investigates the response of unreinforced masonry walls to dynamic loading induced by underground explosions. A mesoscale numerical modeling approach is employed to capture the nonlinear behavior of the masonry wall. The Menetrey-Willam yield criterion is utilized to represent the wall's nonlinear behavior at the mesoscale level, while the cohesive zone model (CZM) is employed to simulate the behavior of mortar, thereby overcoming the limitations of linear elasticity. The soil is characterized using the modified Mohr-Coulomb model. After calibration against material characterization tests, a numerical model is developed to obtain results that closely match experimental data. Given the disturbed nature of the crater soil, both finite element and discrete element methods are utilized for soil modeling. Comparison between these methods reveals that the discrete element method yields more realistic results in terms of displacement and strain. The study's findings enhance our understanding of the response of unreinforced masonry walls to dynamic loading and offer valuable insights for future structural designs and safety assessments.

In-plane cyclic response of unreinforced masonry walls retrofitted with ferrocement

Unreinforced masonries are weak against seismic loads which necessitates their retrofitting. Ferrocement is widely used as it is cheaper than alternative methods of retrofitting. Some crucial parameters can significantly affect the performance of such retrofitted structures under cyclic loading conditions, such as: aspect ratio, difference of wire mesh arrangements & pattern of retrofitting. To quantify the impact of these parameters, this study has implemented two variations of each parameter. This study conducts experimental investigations of the ferrocement retrofitted unreinforced masonry walls (URM) and evaluation of their properties such as energy dissipation, stiffness degradation, hysteretic damping, ductility, and load deformation responses. Results show notable increase in strength after ferrocement retrofitting of URM walls. Retrofitting of URM walls with inexpensive material such as ferrocement is gaining popularity, our research is guided towards finding the in-plane cyclic load characteristics of such method of retrofitting. Sample walls were prepared in two categories: (1) Long walls (aspect ratio 0.57) and (2) Short walls (aspect ratio 1). A total of ten clay masonry walls (five for each category) were constructed with reinforced concrete base slab. The first variable parameter for the tests was ferrocement coating difference. Some sample walls were retrofitted completely even the base slab & wall panel connection point while on some walls the ferrocement lamination was only applied on the wall panels. Steel wire mesh’s opening size is another crucial variable test parameter. Two different wire mesh opening sizes: (1) 3.2 × 3.6 mm and (2) 8.5 × 8.5 mm were used on the walls. One control specimen with no retrofitting for both short and long walls were prepared for test comparison purpose. Laterally reversed cyclic loads and vertical loads both were applied to the sample walls during tests. Improvements in hysteretic damping, stiffness, energy dissipation and ductility were scrutinized in particular. Finally, a comparison was drawn between the test findings and allowable load provisions prescribed in the Bangladesh National Building Code (BNBC) 1993.

Experimental and Numerical Investigation of Unreinforced Masonry Walls with and without Opening

ABSTRACT This paper comparatively addresses the numerical modeling techniques for simulating the nonlinear behavior of unreinforced masonry (URM) walls. An experimental test program including two specimens with and without opening was conducted to document the behavior of the specimens. Both specimens were subjected to constant axial load and monotonically increasing in-plane lateral load during the tests. Two different modeling approaches, macro and detailed micro modeling, were considered to develop the finite element models (FEMs) of URM walls. The numerical simulation was performed using ABAQUS finite element software. The results of both modeling strategies were compared with experimental results in terms of the lateral load–displacement response and crack pattern of URM walls. Based on the comparison, the approaches implemented for numerical analyses are found reasonably applicable for simulating the in-plane response of URM walls with and without opening under lateral load.

Effects of mortar compressive strength on out of plane response of unreinforced masonry walls

In this study, the out-of-plane response of infill walls that are widely used in Turkey and the surrounding regions were experimentally investigated. Several out-of-plane wall tests were performed in the laboratory, with the walls specimens produced with lateral hollow clay bricks (LHCB) and different mortar qualities. The walls were tested in their out-of-plane (OOP) direction under static load conditions and evaluated based on the load-bearing and energy dissipation capacities, crack propagations, mortar strengths, and initial stiffnesses. These walls are experimentally investigated to understand the effects of the mortar strength on the infill wall structural behaviors and to assess the effectiveness of the out-of-plane strength formulations. It was found that when the mortar strength is low, the first major crack occurs at the mortar, however, because of the arch mechanism efficiency in this situation the OOP load-carrying and energy dissipation capacities of unreinforced walls can be significantly increased. When the first major crack in the wall occurs in the brick itself, the arc mechanism is provided with delicate sections in the brick, which leads to strength decreasing in the walls. In this case, excessive deviations occur in the out-of-plane strength formulations estimates. This study shows that the arc mechanism, the damage start region and progress can change significantly unreinforced masonry (URM) infill walls behaviors.

Application of Artificial Neural Network to Predict Load Bearing Capacity and Stiffness of Perforated Masonry Walls

Perforations adversely affect the structural response of unreinforced masonry walls (UMW) by reducing the wall’s load bearing capacity, which can cause serious structural damage. In the absence of a reliable procedure to accurately predict the load bearing capacity and stiffness of perforated masonry walls subjected to in-plane loadings, this study presents a novel approach to measure these parameters by developing simple but practical equations. In this regard, the Multi-Pier (MP) method as a numerical approach was employed along with the application of an Artificial Neural Network (ANN). The simulated responses of centrally perforated UMW by the MP method were validated utilizing full-scale experimental walls. The validated MP model was used to generate a simulated database. The simulated database includes results of analyses for 49 different configurations of perforated masonry walls and their corresponding solid masonry walls. The effect of the area and shape of the perforations on the UMW’s behavior was evaluated by the MP method. Following the outcomes of the verified MP method, the ANN is trained to develop empirical equations to accurately predict the reduction in the load bearing capacity and initial stiffness due to the perforation of UMW. The results of this study indicate that the perforations have a significant effect on the structural capacity of the UMW subjected to in-plane loadings.

Masonry subjected to semi-cyclic compression: Inelastic response modelling

A series of load tests on unreinforced masonry elements (triplets and wallettes with inclined bed joints) subjected to semi-cyclic compression have been performed in order to investigate the response of masonry under such loading conditions. The specimens exhibited inelastic and extremely nonlinear response and substantially different behaviour during the loading, unloading and reloading phases. Further investigations revealed that the observed behaviour was governed by the response of the block-mortar interfaces. A new mechanical model has been developed, which combines the conventional stress–strain model for the masonry with a non-linear model describing the response of the block-mortar interfaces. The model was calibrated using the Monte-Carlo simulation method and validated against the semi-cyclic compression tests on masonry specimens. The model developed is expected to improve significantly the simulation of the force–displacement response of unreinforced masonry walls subjected to seismic loading.

Spandrel panels in masonry buildings: Effectiveness of the diagonal strut model within the equivalent frame model

Modeling of masonry buildings subjected to seismic actions still represents an open problem in structural engineering. The equivalent frame model is the most used approach to analyze the seismic behavior of both existing and new masonry constructions but, despite this, several aspects are being researched to improve its effectiveness. Among these, modeling of spandrel panels plays a key role to correctly analyze the in-plane seismic response of unreinforced masonry walls. Actually, horizontal mono-dimensional beam-like elements provided of both strength and displacement capacity are used to schematize the spandrel panels, also according with the most of current national and international codes.In this paper an alternative strut and tie scheme to model the spandrel panels, suitable to be used within the equivalent frame approach, is presented: the compressed masonry material is schematized with an equivalent diagonal no-tension truss, whose length and effective compressed area are obtained with limit equilibrium conditions, while tensile-resistant element is represented by no-compression truss. Such model allows a more realistic representation of the mechanical behavior of spandrels because both amount of axial force within the spandrel and the reduced elastic flexural and shear stiffness due to progressive cracking of material are taken into account. Moreover, the local failures of both compressed masonry and/or tensile-resistant elements are better identified and, consequently, also the failure mechanisms of the whole equivalent frame are better detailed.The proposed schematization has been used to simulate the seismic behavior of some reference case studies of existing masonry walls, strengthened with different steel ties arrangement. These walls have been modelled with a hybrid equivalent frame model which consists of mono-dimensional beam-like element for piers, fully rigid offsets for the nodal panels and a strut and tie scheme for spandrels.

Improving in-plane seismic response of unreinforced masonry walls using steel strips

This article discusses the results of an experimental program to retrofit unreinforced masonry walls by means of diagonal and vertical steel strips. The technique has several advantages such as simplicity to apply, relatively low costs, and insignificant disruption of service functions during the repair. The tests were conducted on three specimens: two with both diagonal and vertical steel strips and one with only diagonal steel strips, and all attached to one face of the walls. The tests showed that the steel strips improved lateral strength, increased non-linear displacement capacity, and changed the nature of the failure modes.

Quantifying the out-of-plane response of unreinforced masonry walls subjected to relative support motion

TThe supports of out-of-plane loaded unreinforced masonry walls in buildings are subjected to a motion that is filtered and amplified by the building structure and, in some cases, can be significantly different from the ground motion. Moreover, because these walls span one or several storeys, their top and bottom supports are subjected to motions that differ in phase and amplitude. In state-of-the-art assessment procedures for the out-of-plane stability of masonry walls any effect of a relative support motion is neglected. The objective of this paper is to study the effect of the relative support motion on the response of out-of-plane loaded vertically-spanning unreinforced masonry walls. The acceleration capacity of the walls is investigated by means of a discrete element model representative of different wall configurations. A set of ground motions covering a wide range of peak ground acceleration and peak ground displacement is used as input to the simulations. The relative motion between the wall supports is included in the model in a systematic way: firstly, through a motion that is non-synchronous but of equal amplitude; secondly, through a motion that is synchronous but of different amplitude. The effect of the relative support motion is studied on different wall configurations where the elastic modulus of masonry, the wall height-to-thickness ratio, the wall effective thickness and the overburden at the top wall are varied. The study shows that, because of the relative support motion, the acceleration capacity of the walls can drop by 20% and, in the cases where the overburden is high, by more than 50%.

The effective stiffness of modern unreinforced masonry walls

SummaryCode design of unreinforced masonry (URM) buildings is based on elastic analysis, which requires as input parameter the effective stiffness of URM walls. Eurocode estimates the effective stiffness as 50% of the gross sectional elastic stiffness, but comparisons with experimental results have shown that this may not yield accurate predictions. In this paper, 79 shear‐compression tests of modern URM walls of different masonry typologies from the literature are investigated. It shows that both the initial and the effective stiffness increase with increasing axial load ratio and that the effective‐to‐initial stiffness ratios are approximately 75% rather than the stipulated 50%. An empirical relationship that estimates the E‐modulus as a function of the axial load and the masonry compressive strength is proposed, yielding better estimates of the elastic modulus than the provision in Eurocode 6, which calculates the E‐modulus as a multiple of the compressive strength. For computing the ratio of the effective to initial stiffness, a mechanics‐based formulation is built on a recently developed analytical model for the force‐displacement response of URM walls. The model attributes the loss in stiffness to diagonal cracking and brick crushing, both of which are taken into account using mechanical considerations. The obtained results of the effective‐to‐initial stiffness ratio agree well with the test data. A sensitivity analysis using the validated model shows that the ratio of effective‐to‐initial stiffness is for most axial load ratios and wall geometries around 75%. Therefore, a modification of the fixed ratio of effective‐to‐initial stiffness from 50% to 75% is suggested.

Plastic-Damage Modeling of Unreinforced Masonry Walls (URM) Subject to Lateral Loading

In this paper, the lateral response of unreinforced masonry walls (URM) subjected to different levels of axial load is studied numerically and experimentally. The walls in general are noted to exhibit increasing levels of lateral strength when the magnitude of applied compressive axial force is increased. Beyond a certain threshold magnitude of axial compression, this trend in the URM interaction diagram is then noted to be reversed in which the wall exhibits a weaker lateral response as the axial force exerted on the wall increases. An experimental program was conducted to study the lateral load response of sandstone and limestone walls. The finite element simulation using a plastic-damage model in terms of parameters that account for hardening and softening of the wall components in an ABAQUS environment was conducted, and the relationship between lateral and axial force that encompasses the entire spectrum of failure modes was established. The model was tested against available experimental data, and the interaction between lateral and axial force has been expressed in a ready to use form. The results obtained from the plastic-damage-based FEM simulation have also been compared to existing mechanistic models.

In-plane response of unreinforced masonry walls confined by reinforced concrete tie-columns and tie-beams

This article focuses on the experimental and analytical investigations of masonry walls surrounded by tie-elements under in-plane loads. The experimental results of an unconfined and a confined masonry wall, tested under reversed cyclic lateral loads, are presented. For numerical study, a micro-modeling strategy, using smeared-crack-based approach, is adopted. In order to validate the numerical approach, experimental test results and data obtained from the literature are used, and through a systematic parametric study, the influence of adjoining walls and number of tie-columns on the seismic behavior of confined masonry panels is numerically assessed and a simple but rational method for predicting the nonlinear behavior of these structures is proposed.

Out-of-Plane Seismic Response of Unreinforced Masonry Walls: Conceptual Discussion, Research Needs, and Modeling Issues

ABSTRACTModeling unreinforced masonry walls, subjected to seismic loads applied normal to their plane, has received much attention in the past. Yet, there is a general lack of conformance with regard to what aspects of seismic response a computational model should reflect. Boundary conditions are certainly an important aspect, as the response can involve two-way bending or just one-way bending and, in the second case, along vertical or horizontal directions. In this respect, flexural restraint of wall intersections can be significant in addition to size and placement of openings. Moreover, in-plane damage can modify the boundary conditions and the overall out-of-plane performance. Proper modeling of actions is also relevant, as they can be a result of distortions imposed upon wall elements and/or inertial forces along the span of a wall. Axial forces can markedly affect the out-of-plane response of the wall, particularly vertical compressive forces, which can enhance out-of-plane strength. The outcome of static verifications can be more conservative than that of dynamic analyses, but the latter are much more complex to carry out. These topics are discussed with reference to previous research, observations in the field and in the laboratory, as well as numerical analyses on three-dimensional models.

Structural Response of Unreinforced Masonry Walls

Historical buildings located in seismic regions have URM (unreinforced masonry) walls of considerable width as their main structure. In order to study the structural response of such type of masonry, it is necessary to know its mechanical and elastic characteristics. For such purpose, it is generally necessary to perform destructive and non-destructive tests. Other procedures, such as the study of masonry through numerical models, would make it possible to predict, with close approximation, the response of masonry to different actions. The purpose of this paper is to evaluate the behavior of URM walls subjected to axial and horizontal load by using a finite element model with the Abaqus code. Constitutive laws of materials, bricks and mortar are defined, using two constitutive models of plasticity and damage included in the materials library. The constitutive models used employ information of experimental tests performed on bricks and mortar. The results of an analysis for an URM wall subjected to axial and horizontal load are presented. Responses of walls were obtained in terms of load versus displacement for different constitutive models and for different qualities of bricks and mortar. The two constitutive models used show similar results, particularly in the response waveforms. However, the ultimate capacity values show mean variations of around 30%. The two constitutive models display high dependence upon the mechanical characteristics of bricks and mortar. Language: en

Multi‐directional response of unreinforced masonry walls: experimental and computational investigations

SummaryThis paper describes the results of an experimental and numerical study that focused on multi‐directional behavior of unreinforced masonry walls and established the requisite of the related proposed design equations. The tests were conducted following several sets of multi‐directional loading combinations imposed on the top plane of the wall along with considering monotonic and cyclic quasi‐static loading protocols. Various boundary conditions, representing possible wall–roof connections, were also considered for different walls to investigate the influence of rotation of the top plane of the wall on the failure modes. The results of the tests were recorded with a host of high precision data acquisition systems, showing three‐dimensional displacements of a grid on the surface of the wall. Finite element models of the walls are developed using the commercial software package ABAQUS/Explicit compiled with a FORTRAN subroutine (VUMAT) written by the authors. The experimental results were then used to validate the finite element models and the developed user‐defined material models. With the utility of validated models, a parametric study was performed on a set of parameters with dominant influence on the behavior of the wall system under in‐plane and out‐of‐plane loading combinations. The experimental and numerical results are finally used to investigate the adequacy of ASCE 41 empirical equations, and some insights and recommendations are made. Copyright © 2016 John Wiley & Sons, Ltd.

Masonry elements with multi-layer bed joints: Behaviour under monotonic and static-cyclic shear

The goal of the research project presented here is to investigate the effect of multi-layer bed joints on the shear behaviour of unreinforced masonry elements subjected to cyclic actions, and to assess the shear mechanical characteristics of multi-layer bed joints. Such joints consist of a core soft layer protected with two thin extruded elastomer membranes, which in turn are placed in a bed mortar joint. The extruded elastomer membranes are employed to prevent and/or limit the deterioration of the core soft layer during the cycling action, which has been observed in previous investigations. Five different core soft layer materials, namely rubber granulate, cork-rubber granulate, cork, bitumen and polyvinylchloride were employed. A total of 57 clay block masonry triplets were subjected to both monotonic and static-cyclic loading under varying levels of pre-compression.The shear behaviour of the specimens was greatly influenced by the applied pre-compression level and by the loading speed. Increasing the level of pre-compression leads to the higher values of shear strength. The same is true for increasing loading speed. Further, for the prediction of the failure shear force a Mohr–Coulomb failure criterion with zero cohesion can be applied. Finally, experimental findings showed that extruded elastomer layers largely fulfilled the intended protective role.The results indicate that multi-layer bed joints with adequate material properties have the potential to change the typical brittle shear response of unreinforced masonry walls to a more desirable quasi-ductile behaviour. Moreover, based on the observed hysteretic behaviour, considerable energy dissipation could be expected in masonry structures with multi-layer bed joints. Such behaviour would be desirable for enhanced seismic performance.

Masonry wallettes with a soft layer bed joint: Behaviour under static-cyclic loading

The main goal of the research project presented herein is to investigate the effect of damp proof courses and soft layer wall bearings on the seismic shear behaviour of unreinforced masonry walls subjected to cyclic actions, in particular how the lateral load resisting mechanism of the wall is influenced thereby. Unreinforced clay block masonry wallettes with soft layers of three different thicknesses (3, 5 and 10mm), and two different materials (rubber granulate and extruded elastomer) were subjected to static-cyclic shear under varying levels of pre-compression. The soft layer was placed either in the first bed joint of the wallette or in the interface joint between the wallette and the underlying concrete slab.Soft layers were beneficial when the tested wallettes failed in sliding. Such sliding failure was observed for wallettes with rubber granulate soft layers placed in the first bed joint. This resulted in quasi-ductile behaviour of the wallettes, with a displacement and energy dissipation capacities significantly larger than those of the conventional unreinforced masonry control specimens. Furthermore, wallettes failing by sliding along the joint containing the soft-layer showed considerably less damage compared to wallettes that exhibited other failure modes at the same displacements levels. The progressive deterioration of the rubber granulate layers lead to a decreasing shear strength at larger horizontal displacements. Wallettes with extruded elastomer layers and wallettes with layers placed in the interface joint did not fail in sliding. Apart from slightly increased energy dissipation, no beneficial effect on the shear force–deformation behaviour of the wallettes could be observed in these tests.The results indicate that soft-layers with adequate material properties placed in the first bed joint have the potential to change the typical brittle shear response of unreinforced masonry walls to a more desirable quasi-ductile behaviour.

Lateral Force–Displacement Response of Unreinforced Masonry Walls with Flanges

AbstractThe experimental in-plane force-displacement response of unreinforced masonry (URM) walls with flanges (return walls) subjected to pseudo-static cyclic lateral loading is presented. Each wall failed in a diagonal tension mode followed by bed-joint sliding. The effect of wall flanges was an increase in the displacement capacity of the in-plane loaded wall, in comparison with an in-plane loaded wall without flanges. The measured shear strengths of the walls were compared with an analytical model for determining the limiting diagonal tension strength of the walls, with a high level of correlation. The initial stiffness of the shear walls before the effective yield was compared with the initial stiffness as determined using conventional principles of mechanics for homogeneous materials, and it was found that with some approximations the initial stiffness could be satisfactorily determined. Because the bed-joint sliding failure mechanism exhibited by the walls is a deformation-controlled action, there ...

Deformation capacity of unreinforced masonry walls subjected to in-plane loading: a state-of-the-art review

A research project on the deformation capacity of unreinforced masonry structures is underway at the Institute of Structural Engineering of ETH Zurich. The development of the basic building blocks for the displacement-based design of unreinforced masonry structures is the objective of the present research project, which should be seen as a first step in an initiative to investigate the limits of the deformation capacity of unreinforced masonry walls. This paper presents a summary review of previous experimental and analytical studies on the deformation capacity of unreinforced masonry walls subjected to in-plane loading. This review is the first phase of the aforementioned research program. A summary of 71 shear tests on unreinforced masonry walls is presented in the form of a database, along with the statistical analysis and discussion of the tests results. Furthermore, three different computational approaches for structural masonry, i.e. micro-modelling, macro-modelling and macro-element discretization, are discussed, and a review of macro-elements for the in-plane response of unreinforced masonry walls is presented. The reviewed models are discussed and a set of conclusions is given. Special attention is devoted to the deformation capacity parameter throughout the paper. Finally, the paper shows the limitations of our current state of knowledge of the deformation capacity of structural masonry.

Finite element modeling and shake-table testing of unidirectional infill masonry walls under out-of-plane dynamic loads

The dynamic out-of-plane response of unreinforced masonry walls is investigated. The study combines analytical, numerical, and experimental methodologies. The paper focuses on structural schemes that involve supporting at the base and the top and yield a unidirectional (one-way) flexural action. First, the modeling concepts for the nonlinear dynamic analysis are discussed and used as a basis for a finite element formulation. The element is based on a first-order shear deformation theory with large displacements, moderate rotations, small strains, material nonlinearity, and a Rayleigh type of viscoelastic damping. The nonlinearities due to cracking and the inelastic response under cyclic compression are introduced through the constitutive model for the mortar. The experimental part includes shake-table testing under different levels of out-of-plane excitation and compressive loading. The experimental results and the numerical model quantify a range of physical phenomena, including the dynamic arching and rocking effects, the coupling of the axial (in the height direction) and the out-of-plane responses, the role of axial loading, and the vulnerability of the masonry construction to dynamic loads. The comparison between the numerical results and the experimental results examines the capabilities of the model and gains insight into the nonlinear dynamics of the masonry wall.