3 rd International Symposium on Advances in Civil and Environmental Engineering Practices for Sustainable Development (ACEPS – 2015) Comparison of Mesh Type Seismic Retrofitting for Masonry Structures N.A.A.C Nissanka, R.L.S Priyankara and N. Sathiparan Department of Civil and Environmental Engineering Faculty of Engineering University of Ruhuna SRILANKA E-mail: [email protected]Abstract: The tremendous loss of life that resulted in the aftermath of recent earthquakes in developing countries is mostly due to the collapse of non-engineered building structures. It has been observed that these buildings cannot withstand the lateral loads imposed by an earthquake and often fails, in a brittle manner. This underscores the urgency to find simple and economic solutions to reinforce these buildings. Different conventional retrofitting techniques are available to increase the strength and/or ductility of unreinforced masonry walls. Recent years, several researches work on mesh type retrofitting for masonry structures to delay or prevent the collapse of buildings and reduce the number of lives lost during devastating earthquake events. This paper reviews and discusses the state-of-the-art on seismic retrofitting of masonry walls with emphasis on the mesh type retrofitting techniques include retrofitting procedures, cost, improvement in structural performance and limitations. Keywords: Masonry, retrofitting, meshes, seismic loading, in-plane tests. 1. INTRODUCTION The result of earthquake damage investigations and studies conducted in earthquake-prone regions have revealed that the masonry constructed type buildings would collapse within a few seconds during earthquake movement, and does become a major cause of human fatalities. Lack of structural integrity is one of the principal sources of weakness responsible for severe damage leading to collapse. Lack of interlocking units between external and internal wythes of the wall sections and the lack of connection between crossing walls give rise to the possibility of out-of-plane behavior, as their formation increases the net length of the walls. In-plane lateral loads induce shearing deformations in masonry walls. This deformation elongates one diagonal, including tension, and shortens the other, including compression perpendicular to the tension. Since masonry materials have much lower strength in tension than compression, in-plane forces typically induce diagonal cracking perpendicular to the tension axis. Out-of-plane wall collapse is another major cause of destruction of masonry buildings, particularly in buildings with flexible floors and roofs. The inadequacy of connections between the cross walls and long walls is one of the key factors influencing out-of-plane wall collapse. When the walls are not connected to the roof, collapse is often caused. Roof collapse can also be caused by the collapse of walls subjected to shear forces and gravity loads. Heavy roofs also contribute to the seismic vulnerability of masonry buildings. This study focus on an experimental program of in-plane tests of masonry specimens retrofitted with different mesh types of steel, soft polymer, industrial geo-grid, PP-band and plastic carrier bags. These tests performed to investigate the effectiveness of different mesh types in preventing brittle failure of unreinforced masonry specimens, under in-plane loading. 2. MESH TYPE RETROFITTING In order to reduce damage on these masonry buildings during earthquakes, which could happen anywhere in the world, it is important to examine at the early stage how to improve and upgrade the 334
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3rd International Symposium on Advances in Civil and Environmental Engineering Practices for Sustainable Development (ACEPS – 2015)
Comparison of Mesh Type Seismic Retrofitting for Masonry Structures
N.A.A.C Nissanka, R.L.S Priyankara and N. Sathiparan
Abstract: The tremendous loss of life that resulted in the aftermath of recent earthquakes in developing countries is mostly due to the collapse of non-engineered building structures. It has been observed that these buildings cannot withstand the lateral loads imposed by an earthquake and often fails, in a brittle manner. This underscores the urgency to find simple and economic solutions to reinforce these buildings. Different conventional retrofitting techniques are available to increase the strength and/or ductility of unreinforced masonry walls. Recent years, several researches work on mesh type retrofitting for masonry structures to delay or prevent the collapse of buildings and reduce the number of lives lost during devastating earthquake events. This paper reviews and discusses the state-of-the-art on seismic retrofitting of masonry walls with emphasis on the mesh type retrofitting techniques include retrofitting procedures, cost, improvement in structural performance and limitations. Keywords: Masonry, retrofitting, meshes, seismic loading, in-plane tests.
1. INTRODUCTION
The result of earthquake damage investigations and studies conducted in earthquake-prone regions have revealed that the masonry constructed type buildings would collapse within a few seconds during earthquake movement, and does become a major cause of human fatalities. Lack of structural integrity is one of the principal sources of weakness responsible for severe damage leading to collapse. Lack of interlocking units between external and internal wythes of the wall sections and the lack of connection between crossing walls give rise to the possibility of out-of-plane behavior, as their formation increases the net length of the walls. In-plane lateral loads induce shearing deformations in masonry walls. This deformation elongates one diagonal, including tension, and shortens the other, including compression perpendicular to the tension. Since masonry materials have much lower strength in tension than compression, in-plane forces typically induce diagonal cracking perpendicular to the tension axis. Out-of-plane wall collapse is another major cause of destruction of masonry buildings, particularly in buildings with flexible floors and roofs. The inadequacy of connections between the cross walls and long walls is one of the key factors influencing out-of-plane wall collapse. When the walls are not connected to the roof, collapse is often caused. Roof collapse can also be caused by the collapse of walls subjected to shear forces and gravity loads. Heavy roofs also contribute to the seismic vulnerability of masonry buildings. This study focus on an experimental program of in-plane tests of masonry specimens retrofitted with different mesh types of steel, soft polymer, industrial geo-grid, PP-band and plastic carrier bags. These tests performed to investigate the effectiveness of different mesh types in preventing brittle failure of unreinforced masonry specimens, under in-plane loading.
2. MESH TYPE RETROFITTING
In order to reduce damage on these masonry buildings during earthquakes, which could happen anywhere in the world, it is important to examine at the early stage how to improve and upgrade the
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earthquake resistance of an existing masonry construction and to propose a concrete countermeasure method. Researchers assessed the feasibility of applying the various strengthening methods for existing masonry structures in developing countries; it is difficult to make direct comparisons regarding the structural performance of the techniques. The existing methods of retrofitting un-reinforced masonry buildings currently under research or early stages of application, including:
• External reinforcement: bamboo, seismic wallpaper • Post-tensioning using material such as rubber tyres • Mesh reinforcement: steel mesh cage, polymer mesh, polypropylene band (PP-band) mesh,
Plastic Carrier Bag Mesh
For the seismic safety of the structure, good connections between walls and floors or foundations, between adjacent walls and between walls and roof are essential. Integrity of masonry can prevent large pieces of debris to fall out and injure inhabitants. Considering these facts, recent years, several researches work on mesh type retrofitting for masonry structures to delay or prevent the collapse of buildings and reduce the number of lives lost during devastating earthquake events. The main objective of mesh retrofitting is to hold the masonry components into a single unit and to prevent the collapse of masonry structures. The mesh type retrofitting can be made of any ductile material, including: steel cage, industrial geo-grid, soft polymer, polypropylene band and plastic carrier bag as shown in Figure 1.
Figure 1. Various mesh types retrofitting techniques used for masonry structures.
Vertical bands help to tie the wall to the foundation and to the ring beam and restrains out-of-plane bending force and in-plane shear force. Horizontal bands help to transmit the bending and inertia forces in transverse walls (out-of-plane) to the supporting shear walls (in-plane), as well as restraining the shear force between adjoining walls and minimizing vertical crack propagation. The horizontal and vertical bands should be tied together and to the other structural elements (foundations, roof, etc.) by means of nylon string. This attachment provides a stable matrix, which is inherently stronger than the individual components. According to Sathiparan et al (2008), there is a trend toward a significant increase in cracking in masonry wall retrofitted by meshes, which is generated by the incompatibility of deformation between the masonry and meshes. Generally mesh distributed the stress through the cracks, transferring it to the undamaged portions of the structure. As a result, new cracks appeared. By allowing cracking without the loss of wall integrity, the meshes enhance structural ductility and energy dissipation capacity while holding disintegrated structural elements together, thus preventing collapse. The following are the major types of methods used for mesh type seismic retrofitting of masonry structures; • Steel Cage: The use of external welded wire mesh has been studied by several researchers as a
reinforcement system that could be applied both to new and existing earthen construction (San Bartolome et al, 2008; Tetley & Madabhushi, 2007). The mesh is placed in horizontal and vertical strips nailed with metal bottle caps to the adobe walls, and it is covered with a thick cement and sand mortar. Shaking table test results show that, the steel mesh retrofitted model suffered damage, but did not collapse. Clearly the use of reinforcement is successful in improving seismic performance of adobe buildings, but it has been recognized that the use of steel is problematic as
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it is expensive and susceptible to corrosion problems. • Polymer mesh (Industrial geo-grid and soft polymer): This technique uses polymer mesh
(geomesh) commonly used in geotechnical applications. The advantage of this material lies in the compatibility with the earthen wall deformation and its ability to provide an adequate transmission of tensile strength to the walls up to the final state. The mesh is attached to adobe walls by plastic or nylon forming a confinement and consequently preventing the total collapse. The researchers found that it is possible for the walls to disintegrate into large blocks during severe ground shaking, however the mesh prevents the walls from falling apart, and collapse can be avoided (Blondet et al, 2006). Research performed in recent years also indicated that various polymer mesh retrofitting system such as fiber-reinforced polymer, polymer, textile and polymer carbon mesh are effective strengthening solutions for masonry structures.
• PP-band mesh: PP-band retrofitting is a simple and low-cost method that consists of confining all
masonry walls with a mesh of PP-bands. PP-bands are an inexpensive, durable, strong, and widely available material, commonly used for packing. PP-band retrofitting technique is simple enough to be understood and applied by craftsmen and homeowners without any prior knowledge and special expertise, thus, it is expected to meet the very critical requirement of developing countries, the “easy-to-use” method by this retrofitting technique. The applicability of PP-band meshes to retrofit unreinforced masonry structures has been tested under static (Sathiparan et al 2005), cyclic (Mayorca & Meguro, 2003) and dynamic loading (Sathiparan et al 2012).
• Plastic Carrier Bag Mesh: Thus an innovative new method was tested, using ordinary plastic
carrier bags made into a mesh to reinforce the model (Tetley & Madabhushi, 2007). Plastic bags were cut into 20mm strips, these were then plaited together to make “ropes”. These plaited ropes were then knotted together to make 50mm×50mm mesh. The mesh was wrapped around the wall in one continuous piece and fixed to the base using tacks to mimic pegging to the ground. The surface of the wall was plastered with the adobe mortar mixture; this was to hold it in place and for aesthetics. The addition of the carrier bags clearly made the model more ductile. The ductile failure mechanism would mean there would be advance warning of the collapse, which would improve the fatality rates in earthquakes.
3. COMPARISON OF MESH TYPE RETROFITTING METHODS
3.1. Retrofitting Material Strength
In order to obtain the tensile strength and deformation properties of the retrofitting materials, preliminary tensile testing was carried out by several researchers (Bischof & Suter, 2014; Blondet et al, 2006). Generally retrofitting material exhibited large tensile strength and deformation capacity. Table 1 summarized the properties of the material used for mesh type retrofitting.
Table 1 Retrofitting material properties comparison Retrofitting
The cost of retrofitting material and strengthening cost are major constraints to implement this strengthening method. Strengthen by different type of meshes can be done with 5-15% cost compares with construction cost, in many parts of the world. Retrofitting cost comparison of different strengthening methods is summarized in Table 1.
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3.3. Structural Performance - In-plane Behaviour
In order to assess the retrofitting by different meshes, six masonry walls as shown in Figure 1 were constructed. One wall without reinforcement and one each with steel, soft polymer, industrial geo-grid PP-band and plastic beg mesh were tested. The wall dimensions were 900×750×105 mm3 and consisted of 10 brick rows of 4 bricks each. The panel was placed on a steel I-beam for testing, without any special connection provided between the panel and the steel beam. In order to prevent the wall from sliding, two welded L angle steel plate stoppers at the wall toes were provided. The specimens were first subjected to vertical pre-compression and then to horizontal racking. The concrete beam was cast in dimension of 920x180x160 mm3 and weighted around 160 kg. The testing setup is shown in Figure 2. The horizontal load was applied by means of a hydraulic actuator. The actuator had hinges on both ends and hung from the loading frame to prevent that its weight induced any flexural moments on the specimen. The horizontal load was applied through a steel plate on the brick surface.
Figure 2. Test setup for in-plane test
Each panel was subjected to a static cyclic loading sequence of 11 successive motion steps (test phases) with increasing amplitude in the same direction, where in each phase wall becomes loaded and then unloaded. Half-triangle shape was adopted for one side cyclic loading up to 75 mm amplitude and ramp wave adopted for 100 mm amplitude.
3.3.1. General Response of the Walls
As observed during the experimental program; initially in the non-retrofitted panel, the first crack was appeared at the base of the panel near the second layer. Afterwards the panel was splited into two pieces subjected to sliding without propagating further cracks. In case of geo-grid retrofitting panel, initial crack was appeared in the eighth row of the panel and then sequentially, it was propagated to diagonal cracks in the downward direction. At the same time uniform diagonal crack was appeared along with small cracks in the loading side of the wall. The upper portion of the panel above eighth row remains as a rigid block while subjecting to sliding. In the polymer retrofitted wall, the first diagonal crack was propagated at an upper layer and then subsequently the cracks become gradually longer and wider as the horizontal load is increased. In the steel mesh retrofitting panel after having the initial cracking, when the horizontal load is increased few additional cracks were appearing progressively. In case of PP band retrofitted panel, the initial crack was propagated at the fourth row of the panel and then a larger diagonal crack appeared starting from the eighth row at the bottom of the wall while dividing the wall into two pieces. In the plastic bag mesh retrofitted panel, the first crack was progressed across the seventh layer and then it was not propagated towards the bottom of the panel. Crack patterns of non-retrofitted panel and each retrofitted panels were shown in Figure 3. Figure 4 shows the shear resistance - drift relationship for non-retrofitted and mesh strengthened masonry wall panels under in-plane loading. It shows that after the initial breaking point, mesh strengthened wall panels have comparatively higher residual strength values than non-retrofitted masonry panel.
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Figure 3. Crack pattern for different
mesh type retrofitting Figure 4. Shear resistance vs. deformation for different
mesh type retrofitting
3.3.2. Basic Parameter Comparison
In order to compare the behavior of retrofitted masonry walls in a common base, the behavior of each panel was idealized as shown in Figure 5. Initial strength (Vo) and Initial stiffness (Ko) mainly depends on the masonry properties of brick, mortar and workmanship. Maximum residual strength after the initial crack (Vr) and residual stiffness (Kr) mainly depend on mesh strength and density.
Figure 5. Real and ideal behavior of mesh retrofitted wall.
The effective stiffness of a wall is defined by the gradation of the force - deformation graph. According to the force deformation curves presented in Figure 4, residual stiffness values and strength values were obtained for each mesh type retrofitted panels under in-plane loads. The comparison of basic parameters for each mesh type panel is shown in Figure 6 and Figure 7. Those values show that the mesh retrofitted panels have relatively higher residual stiffness and strength than the non-retrofitted panel. Specifically steel mesh retrofitted panel has the highest residual strength among those different mesh types retrofitting materials. Subsequently residual strength is lesser for polymer, geo-grid, PP band and plastic bag mesh retrofitted panels. While, non-retrofitted masonry panel shows the lowest
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residual strength value after the initial breaking point. In terms of residual stiffness also mesh type retrofitted panels show a similar behaviour where steel mesh retrofitted panel has the highest residual stiffness and subsequently residual stiffness is lesser for polymer, geo-grid, PP band and plastic bag mesh retrofitted panels.
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Figure 6. Residual strength/Initial strength
ratios for various retrofitting methods Figure 7. Residual stiffness/Initial stiffness
ratio for various retrofitting methods
3.3.3. Stiffness Degradation
Stiffness is the ability of a material to deform beyond the elastic range without fracturing or breaking and is an important parameter in seismic design structures. When structures are subjected to reverse cyclic loading they exhibit a gradual loss of lateral stiffness normally referred as stiffness degradation due to the inelastic deformation. Therefore the determination of stiffness degradation of different mesh type retrofitted wall panels under the in-plane loads were analyzed and summarized in Figure 8. It shows that after the initial cracking point, even at 100mm loading cycle also residual stiffness of steel mesh retrofitted panel is comparatively higher than the geo-grid and polymer mesh retrofitted panels. Also, it shows that non retrofitted panel has the lowest stiffness at 100 mm loading circle.
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Figure 8. Stiffness degradation for various
retrofitting methods Figure 9. Cumulative energy dissipation
capacities for various retrofitting methods
3.3.4. Energy Dissipation
Energy dissipation through hysteretic damping is an important aspect in seismic design since it reduces the amplitude of the seismic response and reduces the ductility and strength demands of the structure. The cumulative energy dissipated under the in-plane load is evaluated for each mesh type retrofitting panel on the basis of the measured shear resistance - drift hysteretic loops and summarized in Figure 9. It shows that steel mesh has the highest cumulative energy dissipation under in-plane loads and subsequently polymer, geo-grid, PP band and plastic bags have lower energy dissipation capacities. Non-retrofitted panel has the lowest energy dissipation capacity.
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3.4. Construction Complexity and Seismic Safety
Table 2 presents a construction complexity and seismic safety comparison of various retrofitting method. The first parameter is related to the complexity of construction, ranging from simple to complex based on house owners or masons experience. Complexity mainly depended on retrofitting material flexibility, preparation of the mesh, amount of surface finishing required and knowledge required to retrofit the structures. The second parameter ranks the reinforcement systems, according to the seismic safety they provide. The objective of providing seismic retrofitting of the building is to protect the lives of the building occupants. Based on that, suitability of polymer mesh and PP-band mesh retrofitting method are considered as strong option; steel mesh and plastic carrier bag retrofitting method are considered as moderate option for application (Smith & Redman, 2009).
Table 2 Construction complexity and Seismic safety of retrofitting methods. Retrofitting Method Construction Complexity Seismic safety
Steel mesh Simple Moderate Soft polymer mesh Moderate High Industrial geo-grid Moderate High PP-band mesh Moderate High Plastic carrier bag Moderate Moderate
3.5. Advantages and Drawbacks of Retrofitting Methods
Table 3 summarized the advantages and drawbacks of various mesh type retrofitting methods (Sathiparan et al, 2012; Smith & Redman, 2009; Tetley & Madabhushi, 2007). Lack of flexibility of steel and industrial geo-grid makes its application difficult, but soft polymer, PP-band and plastic carrier bag can be easily deformed so application is easy. The major sustainability problem with these methods is demolished and disposal of retrofitting material at the end of the structural life.
Table 3 Advantages and drawbacks of different mesh type retrofitting methods. Method Advantages Drawbacks
Steel mesh
• High increment in lateral resistance. • Improves ductility.
• Lack of flexibility of steel cage makes installation around opening and the corner is difficult.
• Steel is susceptible to corrosion problems and disposal at the end of the design life will be difficult.
Soft Polymer
• Soft polymer mesh can be easily deformed so transportation, application and removal are easy.
• Polymer mesh requires use and processing of petrochemicals, which is not sustainable unless sourced from recycled or re-used units.
Industrial geo-grid
• High increment in lateral resistance. • Improves ductility.
• Tough nature of material and lack of flexibility makes Industrial geo-grid application and removal is difficult.
PP-band • PP-band is water proof and chemically stable, it is possible to use with mud mortar.
• PP-band is light weight and can be easily carried in the mountainous region and so on.
• Retrofitting is simple enough for application without any prior knowledge.
• PP-band is not available as mesh, so preparation of mesh required special equipment and time.
Plastic carrier bag
• The materials used are light and flexible. • Carrier bags are incredibly cheap. • Carrier bags are normally sent to landfill or
thrown away. Reusing these bags is therefore considered a sustainable solution.
• It takes a very long time to construct the mesh, which is a barrier for this method unless the mesh manufacture can be industrialized/streamlined in some way.
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4. CONCLUSIONS
This paper discusses the results of a series of in-plane tests that were carried out for non-retrofitted and different mesh type retrofitted panels. The test results show that mesh type retrofitted panels obtained comparatively higher residual seismic characteristics rather than a non retrofitted panel as the post collapse strength. So considering the overall performance, it can be concluded these mesh types can effectively increase the residual seismic capacity. But, tests conducted only for in-plane behaviour, so it is difficult to make direct comparisons regarding the overall structural performance of the techniques. It is therefore recommended that further work is carried out on the some more tests on out-of-plane and dynamic tests, which can provide some sort of structural performance comparison of the methods discussed in this study. Generally, when selecting the particular mesh type, in addition to structural performance following characteristics should be considered;
• Available in local or in the market • Low-cost when compared with others • Masonry have a high water absorption capacity, therefore, meshes should be non-corrodible • Mesh roughness, it is important that the mesh surface is unpolished to provide a good grip • Thickness of the mesh, without making the plaster difficult to apply • Flexible material, which can provide easy installation.
Even though above mention mesh types retrofitting have some drawbacks, still the mesh type material was considered for retrofitting of masonry houses because they are affordable and notably improve the structure seismic behavior.
5. ACKNOWLEDGMENTS
The authors express their sincere gratitude to the support given by Building Material and Construction laboratory, Department of Civil and Environmental Engineering, Faculty of Engineering, University of Ruhuna.
6. REFERENCES
Bischof, P., and Suter, R. (2014), Retrofitting masonry walls with carbon mesh, Polymers, 6, pp. 280-299. Blondet, M., Torrealva, D., Vargas, J., Velasquez, J., and Tarque, N. (2006), Seismic reinforcement of adobe houses using external polymer mesh, Proceedings of the 1st European Conference on Earthquake Engineering and Seismology, Geneva, Switzerland, September 3-8, 2006. Mayorca, P., and Meguro, K. (2003), Efficiency of polypropylene bands for the strengthening of masonry structures in developing countries, Proceedings of the 5th International Summer Symposium, Tokyo, Japan, July 26, 2003. San Bartolome, A., Quiun, D., and Zegarra, L. (2008), Performance of reinforced adobe houses in Pisco, Peru Earthquake, Proceedings of the 14th World Conference on Earthquake Engineering, Beijing, China, October 12-17, 2008. Sathiparan, N., Mayorca, P., Nesheli, K., Ramesh, G., and Meguro, K. (2005), Experimental study on in-plane and out-of-plane behavior of masonry wallettes retrofitted by PP-band meshes, Seisan Kenkyu, 57(6), pp. 530-533. Sathiparan, N., Mayorca, P., and Meguro, K. (2008), Parametric study on diagonal shear and out-of-plane behavior of masonry wallettes retrofitted by PP-band mesh, In Proceedings of 14th World Conference on Earthquake Engineering. Beijing, China, October 12-17, 2008. Sathiparan, N., Mayorca, P., and Meguro, K. (2012), Shake table tests on one-quarter scale models of masonry houses retrofitted with PP-band mesh, Earthquake Spectra, 28(1), pp. 277-299. Smith, A., and Redman, T. (2009), A critical review of retrofitting methods for unreinforced masonry structures. Proceedings of EWB-UK research conference, London, UK, February 20, 2009. Tetley, R., and Madabhushi, G. (2007), Vulnerability of adobe buildings under earthquake loading, Proceedings of 4th Conference Earthquake Geotechnical Engineering, Thessaloniki, Greece, June 25-27, 2007.
