Seismic behaviour of masonry buildings built of low compressive strength units

Seismic behaviour of masonry buildings, built of low compressive strength units, is discussed. Although such materials have already been tested and approved for use from mechanical and thermal insulation point of view, the knowledge regarding their structural behaviour is still lacking. In order to investigate the resistance and deformation capacity of this particular type of masonry construction in seismic conditions, a series of eight walls and model of a two-storey full scale confined masonry building have been tested by subjecting the specimens to cyclic shear loads. All tests were conducted under a combination of constant vertical load and quasi static, cyclically imposed horizontal load. The behaviour of tested specimens was of typical shear type. Compared with the behaviour of plain masonry walls, the presence of tie-columns resulted into higher resistance and displacement capacity, as well as smaller lateral resistance degradation. The response of the model was determined by storey mechanism with predominant shear behaviour of the walls and failure mechanism of the same type as in the case of individual confined masonry walls. Adequate seismic behaviour of this particular masonry structural type can be expected under the condition that the buildings are built as confined masonry system with limited number of stories.     

Estimation of load reduction factors for clay masonry walls

The in‐plane cyclic behaviour of three types of unreinforced clay masonry was characterized by means of laboratory tests on full‐scale specimens. The masonry walls were assembled with various bonding arrangements (head joints made with mortar pockets, dry head joints with mechanical interlocking, thin‐layer mortar bed joints), which are not yet inserted in seismic codes. Experimental behaviour was modelled with an analytical hysteretic model able to predict lateral load–displacement curves in case of shear failure of the unreinforced walls. According to the experimental results and those of the selected analytical model, parametric study to evaluate the reduction in lateral strength demand produced by non‐linear behaviour in masonry walls, i.e. the load reduction factor was carried out by non‐linear dynamic analyses. The calculated values of the load reduction factor were modest. The differences in values found for the three masonry types, although consistent with them, were not great. This may indicate that, in the ultimate limit state, the type of masonry cannot significantly affect the behaviour of an entire building. Copyright © 2009 John Wiley & Sons, Ltd.     

Experimental and analytical studies on earthquake resisting behaviour of confined concrete block masonry structures

To improve the seismic performance of masonry structures, confined masonry that improves the seismic resistance of masonry structures by the confining effect of surrounding bond beams and tie columns is constructed. This study investigated the earthquake resisting behaviour of confined masonry structures that are being studied and constructed in China. The structural system consists of unreinforced block masonry walls with surrounding reinforced concrete bond beams and tie columns. The characteristics of the structure include: (1) damage to blocks is reduced and brittle failure is avoided by the comparatively lower strength of the joint mortar than that of the blocks, (2) the masonry walls and surrounding reinforced concrete bond beams and tie columns are securely jointed by the shear keys of the tie columns. In this study, wall specimens made of concrete blocks were tested under a cyclic lateral load and simulated by a rigid body spring model that models non‐linear behaviour by rigid bodies and boundary springs. The results of studies outline the resisting mechanism, indicating that a rigid body spring model is considered appropriate for analysing this type of structure. Copyright © 2006 John Wiley & Sons, Ltd.     

ECC面层加固空斗墙抗震性能试验研究

高延性纤维增强水泥基复合材料(ECC)具有高强度、高延性和水硬过程中粘结性能良好,在砌体加固工程中具有广泛的应用前景。通过对2片未加固、2片单面和2片双面ECC面层加固后空斗墙进行水平拟静力试验,对比加固不同砌法下单面和双面空斗墙破坏模式、滞回曲线、刚度退化、承载能力和延性性能,进而研究ECC面层加固对墙体抗震性能的影响。利用ABAQUS有限元软件,采用预留孔洞的整体建模方式进行有限元分析,对比了试验值与计算值。结果表明:ECC面层与砖墙之间粘结较可靠,ECC面层优越的材料抗力可以抵抗墙体承担的水平剪力;单、双面ECC面层加固后空斗墙体极限变形能力及极限承载力均得到大幅提升,试件延性和抗震性能同步改善;有限元计算的水平承载力值与试验值相差不大。     

A macro‐model with nonlinear springs for seismic analysis of URM buildings

Seismic assessment of existing unreinforced masonry buildings represents a current challenge in structural engineering. Many historical masonry buildings in earthquake regions were not designed to withstand seismic loading; thus, these structures often do not meet the basic safety requirements recommended by current seismic codes and need to be strengthened considering the results from realistic structural analysis. This paper presents an efficient modelling strategy for representing the nonlinear response of unreinforced masonry components under in‐plane cyclic loading, which can be used for practical and accurate seismic assessment of masonry buildings. According to the proposed strategy, generic masonry perforated walls are modelled using an equivalent frame approach, where each masonry component is described utilising multi‐spring nonlinear elements connected by rigid links. When modelling piers and spandrels, nonlinear springs are placed at the two ends of the masonry element for describing the flexural behaviour and in the middle for representing the response in shear. Specific hysteretic rules allowing for degradation of stiffness and strength are then used for modelling the member response under cyclic loading. The accuracy and the significant potential of the proposed modelling approach are shown in several numerical examples, including comparisons against experimental results and the nonlinear dynamic analysis of a building structure. Copyright © 2016 John Wiley & Sons, Ltd.     

单调及低周往复荷载作用下砌体非线性分析

带构造柱和圈梁的约束砌体结构在四川灾区乡镇房屋重建中被广泛采用,其抗震性能是人们所关心的.基于绵竹市土门镇当地重建房屋常用建筑材料的实验数据以及通用有限元软件ANSYS中Solid 65单元的性质和特点,用有限元模型模拟了粘土砖砌体在不同压应力状态(σ-/fm)下沿通缝截面抗剪强度试验,给出了相关单元在模拟砖砌体开裂中闭合及开口剪力传递系数的建议值;利用这些结果,分别建立了带约束(构造柱、圈梁等)和不带约束砌体墙的有限元模型,进而分析了他们在单调荷载以及低周往复荷载作用下的抗震性能.结果表明,与不带约束的墙体相比,带约束墙体在单调水平荷载作用下的初裂性能、极值荷载和延性都有很大的提高,在低周往复荷载作用下其耗能能力得到了改善.所得结果可供相应结构抗震设计的参考.     

水泥砂浆面层加固砌体墙抗震可靠指标研究

采用水泥砂浆面层加固方法加固砌体房屋是一种简单有效的方法。对比分析了《砌体结构加固设计规范》(GB 50702-2011)^[3]和《建筑抗震加固技术规程》(JGJ 116-2009)^[4]中钢筋网水泥砂浆面层加固砌体墙的可靠性;对10片未加固低强度砖墙和20片单面钢筋网水泥砂浆面层加固低强度砖墙进行拟静力试验,分析了未加固和加固墙体的破坏模式和机理,并提出了适用于钢筋网水泥砂浆面层加固砌体墙的抗震验算公式。研究表明：两标准的可靠水平差别较大,实际应用时,易产生矛盾;未加固墙主要发生沿阶梯形斜裂缝受剪破坏,加固墙主要发生沿通缝受剪破坏和沿阶梯形斜裂缝受剪破坏;建议公式的计算值与试验值吻合良好,未加固墙抗震可靠指标为2.2,加固墙抗震可靠指标为2.5～3.1;砂浆面层加固砌体结构可以显著提高结构的抗震性能。     

Bidirectional behavior of unreinforced masonry walls

Most of the studies related to the modeling of masonry structures have by far investigated either the in‐plane (IP) or the out‐of‐plane (OP) behavior of walls. However, seismic loads mostly impose simultaneous IP and OP demands on load‐bearing or shear masonry walls. Thus, there is a need to reconsider design equations of unreinforced masonry walls by taking into account bidirectional effects. The intent of this study is to investigate the bidirectional behavior of an unreinforced masonry wall with a typical aspect ratio under different displacement‐controlled loading directions making use of finite element analysis. For this purpose, the numerical procedure is first validated against the results of the tests on walls with different failure modes conducted by the authors. Afterward, the response of the wall systems is evaluated with increasing top displacement having different orientations. A set of 19 monotonic and three cyclic loading analyses are performed, and the results are discussed in terms of the variation of failure modes and load–displacement diagrams. Moreover, the results of wall capacity in each loading condition are compared with those of the ASCE41‐06 formulations. The results indicate that the direction of the resultant force, vectorial summation of IP and OP forces, of the wall is initially proportional to the ratio of stiffness in the IP and the OP directions. However, with the increase of damage, the resultant force direction inclines towards the wall's longitudinal direction regardless of the direction of the imposed displacement. Finally, recommendations are made for applicability of ASCE41‐06 formulations under different bidirectional loading conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

双层打包带加固窗间墙抗震性能试验研究

地震时砌体结构窗间墙易发生破坏,为了提高其抗震性能,对高宽比为1的2组共4片墙体,其中:2片为双层打包带加固墙体,2片为原墙,进行了拟静力试验,研究墙体的破坏形态、水平承载力、滞回曲线和耗能等抗震性能。试验发现原墙发生剪切破坏,加固后墙体发生摇摆破坏,加固改变了墙体破坏模式,加固后墙体滞回曲线饱满但有捏笼,破坏荷载、延性和耗能能力都有提高,破坏时未发生剥离,表明双层打包带加固法有效地提高了窗间墙体抗震性能,对承受较大竖向应力墙体效果更好,建议加固时要加强加固层与窗下和窗上墙体的连接。     

Using the applied element method for modelling calcium silicate brick masonry subjected to in‐plane cyclic loading

The response of calcium silicate unreinforced masonry construction to horizontal cyclic loading has recently become the focus of experimental and numerical research, given its extensive use in some areas of the world that are now exposed to induced earthquakes (eg, north of the Netherlands). To assess the seismic behaviour of such construction, a relatively wide range of modelling methodologies are available, amongst which the discrete elements approach, which takes into account the intrinsic heterogeneity of a brick‐mortar assembly, can probably be deemed as the most appropriate computational procedure. On the other hand, however, since discrete elements numerical methods are based on a discontinuum domain, often they are not able to model every stage of the structural response adequately, and because of the high computational burden required, the analysis scale should be chosen carefully. The applied element method is a relatively recent addition to the discrete elements family, with a high potential for overcoming the aforementioned limitations or difficulties. Initially conceived to model blast events and concrete structures, its use in the earthquake engineering field is, of late, increasing noticeably. In this paper, the use of the applied element method to model the in‐plane cyclic response of calcium silicate masonry walls is discussed and scrutinised, also through the comparison with experimental results of in‐plane cyclic shear‐compression tests on unreinforced masonry walls.     

Scaling unreinforced masonry for reduced-scale seismic testing

When testing multi-storey structures, most testing facilities require the testing of a reduced-scale model. A literature review on tests of scaled masonry structural components revealed that scaling of masonry was rather challenging and often significant differences in stiffness, strength and failure mechanisms between the different sized masonry were reported. This paper addresses the scaling of hollow clay brick masonry with fully mortared head and bed joints. We investigate different choices of scaling brick units and mortar joints. Based on the results of an extensive test programme including standard material tests and quasi-static cyclic tests on masonry walls subjected to horizontal and axial loads, we formulate recommendations for the production of a half-scale model of unreinforced masonry structures. The experimental results show a good match between full-scale and half-scale masonry. We discuss the differences in material properties that remained and compare the force-displacement hystereses obtained for the wall tests.     

Effect of out-of-plane behavior on seismic fragility of masonry buildings in Turkey

This study focuses on the evaluation of seismic safety of unreinforced masonry buildings in Turkey by using fragility curves generated for two behavior modes of load bearing walls: in-plane and out-of-plane. During generation of fragility curves, a force-based approach has been used. There exist two limit states in terms of base shear strength for in-plane behavior mode and flexural strength for out-of-plane behavior mode. To assess the seismic vulnerability of unreinforced masonry buildings in Turkey, fragility curves generated for in-plane behavior were verified by the observed damage during the 1995 Dinar (Turkey) earthquake and fragility curves generated for out-of-plane behavior were verified by the observed damage during the 2010 Elaz?? (Turkey) earthquake. The verification results reveal that the proposed fragility-based procedure can provide an alternative for the seismic safety evaluation of unreinforced masonry buildings in Turkey. Using this procedure, it becomes possible to investigate a large population of masonry buildings located in regions of high seismic risk in a short period of time. The obtained results are valuable in the sense that they can be used as a database during the development of strategies for pre-earthquake planning and risk mitigation for earthquake prone regions of Turkey.     

水平嵌筋加固砖砌体墙抗震性能试验研究

以嵌缝胶泥作为嵌缝材料,针对不同高宽比和不同配筋率的6片墙体进行了拟静力试验,探讨了嵌筋加固砖砌体墙的破坏特征、变形能力、承载能力、耗能能力、滞回特征及刚度退化等抗震性能,建立了以试验为基础的嵌筋加固砖砌体墙的抗剪承载力计算公式。研究结果表明:高宽比为1.8的试件,嵌筋墙体较无筋墙体水平抗剪极限承载力提高了17%~31%,延性提高了54%~83%;高宽比为0.5的试件,嵌筋墙体较无筋墙体水平抗剪极限承载力提高了13%~17%,延性提高了17%~20%,嵌筋加固墙体滞回环饱满,耗能能力有较大幅度提高,破坏形式由脆性破坏转变为延性破坏,嵌筋对墙体初始刚度的影响较小,给出的抗剪承载力公式计算值与试验值接近,为工程应用奠定了基础。     

Effects of load irregularity on the cyclic behaviour of sand

In an effort to investigate the effects of irregular nature of load application on the behaviour of soils during earthquakes, a series of uni-directional irregular loading tests were performed on saturated Fuji river snad under undrained condition using a simple shear test apparatus. The samples prepared to three different densities in this test device were subjected to twelve irregular time histories of shear stress alteration. For comparison purposes, the conventional type of constant amplitude cyclic simple shear tests were also conducted on samples prepared under identical conditions. On the basis of the test results, values of coefficients allowing for the effect of load irregularity were elucidated for the samples with three different densities. It was then suggested that the values of coefficient thus determined be used to facilitate the evaluation of the soil strength under actual seismic loading conditions, on the basis of the cyclic shear strength obtained from the conventional type of constant-amplitude triaxial tests.     

Cyclic shear-compression tests on masonry walls strengthened with alternative configurations of CFRP strips

This work reports the results of an experimental programme aimed at investigating the in-plane behaviour of clay-brick masonry walls externally strengthened by carbon fiber reinforced polymer (CFRP) strips. Particularly, four different geometrical layouts were considered for the CFRP strips, though keeping unchanged the quantity of composites employed in each wall. Firstly, a preliminary experimental work was carried out on samples of the constitutive materials for quantifying their key mechanical properties and evaluating the bond behaviour of FRP strips on the masonry substrates. Then, eleven cyclic shear-compression tests were performed to observe the response of strengthened walls and the influence of the strengthening layouts under investigation. The proposed experimental report is intended as a contribution to the current state of knowledge about the behaviour of FRP-strengthened masonry walls: it is available to assess the accuracy and possibly improve the predictive capacity of design-oriented capacity models. Finally, the comparison of the reported experimental results with the predictions obtained by applying the analytical relationships proposed by a recently issued guideline for FRP strengthening of masonry structures is proposed.     

The effective stiffness of modern unreinforced masonry walls

Code 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.     

Development of an innovative seismic strengthening technique for traditional load-bearing masonry walls

Traditional non-reinforced masonry walls are particularly prone to failure when subjected to out-of-plane loads and displacements caused by earthquakes. Moreover, singularities such as openings in fa?ades may trigger local collapse, for either in-plane or out-of plane motion. Bearing in mind all the former limitations, STAP, with the scientific support of ICIST and LNEC, has been developing a reduced intrusiveness seismic strengthening methodology for traditional masonry structures. The technique consists in externally applying Glass Fibre Reinforced Polymer (GFRP) composite strips to one or both faces of walls. Connection between GFRP composite strips and masonry substrate is enhanced through specifically detailed anchorages or confinement connectors. This technique has been developed and studied through an extensive series of experimental tests, which are briefly reviewed. This paper focuses more deeply on the latest experimental program, aimed at the characterization of the masonry-GFRP composite interface behaviour. This testing program comprised 29 masonry specimens, strengthened with externally bonded GFRP composite strips with anchorages. The testing variables were the number and spacing of anchorages as well as the loading history type: monotonic or repeated. Results clearly show that the use of anchorages dramatically enhances bond behaviour and that its number and spacing have a significant effect on deformation capacity and a less pronounced effect on strength. Based on experimental evidence, this paper also provides a calculation model and ULS safety assessment procedure for out-of-plane strength of reinforced masonry walls. This calculation model leads to interaction curves on strengthened masonry walls subjected to compression and out-of-plane flexure.     

Seismic vulnerability of roof systems combining URM gable walls and timber diaphragms

Typical low-rise masonry buildings consist of unreinforced masonry (URM) walls covered with various timber roof configurations generally supported or finished by masonry gables. Post-earthquake observations and experimental outcomes highlighted the large vulnerability of the URM gables to the development of overturning mechanisms, both because of the inertial out-of-plane excitation and the in-plane timber diaphragm deformability. This paper presents the static and dynamic experimental seismic performance of three full-scale roofs tested via quasi-static cyclic and shake table tests. Two of them were tested as part of a whole full scale one-storey and two-storey building. A single-degree-of-freedom (SDOF) numerical model is calibrated against experimental data and proposed for the analysis of this roof typology's dynamic behaviour. Several sets of analyses were conducted to assess the vulnerability of these structural components and to study the effect of the whole building's characteristics (eg, number of storeys and structural stiffness and strength) on the seismic performance of this roof typology.     

Shaking table testing for masonry infill walls: unreinforced versus reinforced solutions

Several factors influence the behaviour of infilled frames, which have been a subject of research in the past with moderate success. The new generation of European design standards imposes the need to prevent brittle collapse of the infills and makes the structural engineer accountable for this requirement, yet it fails to provide sufficient information for masonry infills design. Therefore, the present work aims at understanding the seismic behaviour of masonry infill walls within reinforced concrete frames, using both unreinforced and reinforced solutions (bed joint reinforcement and reinforced plaster). For this purpose, three reinforced concrete buildings with different infill solutions were constructed at a scale of 1:1.5, all with the same geometry, and were tested on the shaking table of the National Laboratory for Civil Engineering, Portugal. All solutions performed adequately for the design earthquake, with no visible damage. Still, the experimental tests show that the double‐leaf‐unreinforced infill walls underperformed during a large earthquake, collapsing out of plane by rotating as rigid bodies with multiple configurations. Also the reinforced concrete buildings collapsed, because of the adverse interaction with the infill walls. The infill walls with bed joint reinforcement and reinforced plaster did not collapse out of plane, because of their connection to the concrete frame, which is an essential requirement. Copyright © 2016 John Wiley & Sons, Ltd.     

A nonlinear macroelement model for the seismic analysis of masonry buildings

The macroelement technique for modelling the nonlinear response of masonry panels is particularly efficient and suitable for the analysis of the seismic behaviour of complex walls and buildings. The paper presents a macroelement model specifically developed for simulating the cyclic in‐plane response of masonry walls, with possible applications in nonlinear static and dynamic analysis of masonry structures. The model, starting from a previously developed macroelement model, has been refined in the representation of flexural–rocking and shear damage modes, and it is capable of fairly simulating the experimental response of cyclic tests performed on masonry piers. By means of two internal degrees of freedom, the two‐node macroelement permits to represent the coupling of axial and flexural response as well as the interaction of shear and flexural damage. Copyright © 2013 John Wiley & Sons, Ltd.