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ACEPS - 2013 89 Seismic Retrofitting of Non-Engineering Masonry Houses using Polypropylene Band Meshes N.Sathiparan 1 and K. Meguro 2 1 Department of Civil and Environmental Engineering Faculty of Engineering University of Ruhuna SRILANKA 2 ICUS, Institute of Industrial Science University of Tokyo JAPAN E-mail: [email protected] Abstract: This paper introduces a retrofitting technique aimed at preventing or prolonging the collapse of adobe (mud brick) houses under strong earthquakes. This technique uses common polypropylene packaging straps to form a mesh, which is then used to encase structural walls. The aim of this paper is to give an overview of the retrofitting technique’s development and implementation. The key development stages of static, dynamic and numerical testing are presented, showing that the proposed technique effectively prevents brittle masonry collapse and the loss of debris. Keywords: Masonry, retrofitting, polypropylene band, seismic loading, shaking table test. 1 INTRODUCTION More than 60% of people in the world are living in masonry buildings that are made by piling up bricks, sun-dried mud bricks (non fire-burnt brick, generally called Adobe), stone and/or concrete blocks. Population statistics showed that such ratio is rather high, especially in the developing countries. The masonry building without reinforcement against earthquake has claimed scores of victims in the history of all parts of the world. The result of earthquake damage investigations and studies conducted in earthquake-prone regions of the world have revealed that the masonry constructed type buildings would collapse within a few seconds during earthquake movement, and it becomes a major cause of human fatalities. Generally adobe type structures share two serious structural deficiencies: (1) of having little if any tension strength, and (2) brittleness. Typical earthquake damage patterns of earthen housing include: Poor connections between the different elements of the building that lead to walls separating at corners and falling outwards Diagonal cracking in walls. This weakens them and leaves them very vulnerable to total collapse. Out-of-plane failure of walls due to lack of cross walls Roof collapse These types of damage are serious. They lead to severe injuries and loss of life. Furthermore, they can occur at low intensities of earthquake shaking depending on the quality of construction materials and their maintenance. Therefore, retrofitting low earthquake-resistant masonry structures are a key issue for earthquake disaster mitigation in developing countries. 1.1 Currently available retrofitting techniques for masonry Several types of retrofitting methods have been developed for unreinforced masonry structures. A comprehensive review of them can be found in Blondet, M. et al (Blondet, M. et al, 2011). There is no doubt that these methods are useful for strengthening masonry structures. The methods required to meet the needs of the large populations in danger of non-engineered masonry collapse must be simple and inexpensive to match the available resources and skills. Considering above factors and lack of unreinforced masonry structural integrity, technically feasible and economically affordable retrofitting technique utilizing different type of meshes has been developed and many different aspects have been studied. The main objectives of this technique are to hold the disintegrated elements together thus
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Page 1: Seismic Retrofitting of Non-Engineering Masonry Houses ... · 1.1 Currently available retrofitting techniques for masonry Several types of retrofitting methods have been developed

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Seismic Retrofitting of Non-Engineering Masonry Houses using Polypropylene Band Meshes

N.Sathiparan1 and K. Meguro2 1Department of Civil and Environmental Engineering

Faculty of Engineering University of Ruhuna

SRILANKA 2ICUS, Institute of Industrial Science

University of Tokyo JAPAN

E-mail: [email protected]

Abstract: This paper introduces a retrofitting technique aimed at preventing or prolonging the collapse of adobe (mud brick) houses under strong earthquakes. This technique uses common polypropylene packaging straps to form a mesh, which is then used to encase structural walls. The aim of this paper is to give an overview of the retrofitting technique’s development and implementation. The key development stages of static, dynamic and numerical testing are presented, showing that the proposed technique effectively prevents brittle masonry collapse and the loss of debris. Keywords: Masonry, retrofitting, polypropylene band, seismic loading, shaking table test.

1 INTRODUCTION

More than 60% of people in the world are living in masonry buildings that are made by piling up bricks, sun-dried mud bricks (non fire-burnt brick, generally called Adobe), stone and/or concrete blocks. Population statistics showed that such ratio is rather high, especially in the developing countries. The masonry building without reinforcement against earthquake has claimed scores of victims in the history of all parts of the world. The result of earthquake damage investigations and studies conducted in earthquake-prone regions of the world have revealed that the masonry constructed type buildings would collapse within a few seconds during earthquake movement, and it becomes a major cause of human fatalities. Generally adobe type structures share two serious structural deficiencies: (1) of having little if any tension strength, and (2) brittleness. Typical earthquake damage patterns of earthen housing include:

• Poor connections between the different elements of the building that lead to walls separating at corners and falling outwards

• Diagonal cracking in walls. This weakens them and leaves them very vulnerable to total collapse. • Out-of-plane failure of walls due to lack of cross walls • Roof collapse

These types of damage are serious. They lead to severe injuries and loss of life. Furthermore, they can occur at low intensities of earthquake shaking depending on the quality of construction materials and their maintenance. Therefore, retrofitting low earthquake-resistant masonry structures are a key issue for earthquake disaster mitigation in developing countries.

1.1 Currently available retrofitting techniques for masonry

Several types of retrofitting methods have been developed for unreinforced masonry structures. A comprehensive review of them can be found in Blondet, M. et al (Blondet, M. et al, 2011). There is no doubt that these methods are useful for strengthening masonry structures. The methods required to meet the needs of the large populations in danger of non-engineered masonry collapse must be simple and inexpensive to match the available resources and skills. Considering above factors and lack of unreinforced masonry structural integrity, technically feasible and economically affordable retrofitting technique utilizing different type of meshes has been developed and many different aspects have been studied. The main objectives of this technique are to hold the disintegrated elements together thus

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preventing the collapse. Some examples of low-cost mesh type retrofitting techniques suitable for masonry dwellings are given in Table 1 (JBIC 2007; Smith and Redman 2009; Blondet et al. 2011)

Table 1 Summary of mesh type seismic retrofitting techniques for masonry buildings Type Developing institute Cost (per m²) Seismic

safety Complexity

Wire mesh Pontificia Universidad Cato´ lica del Peru, Peru.

$4.45 Moderate Simple

Polymer mesh

Industrial geogrid: $21 Soft polymer mesh: $4.5

High Moderate

PP-band Institute of Industrial Science, University of Tokyo, Japan.

Retrofitted by owner : $0.85 Retrofitted by outsider : $2.25

High Moderate

bamboo Sydney University, Australia. $2.5 Moderate Simple This paper focuses on the technique of polypropylene (PP) meshing and presents example numerical and physical tests that isolate the in-plane behavior of masonry walls.

1.2 PP-band meshing retrofitting

PP meshing uses common PP packaging straps to form a mesh, which is then used to encase masonry walls (i.e. fixing to both faces of each wall). The mesh prevents the separation of structural elements and the escape of debris, maintaining sufficient structural integrity to prevent collapse. This method for retrofitting masonry structures is economic, the material is accessible in all parts of the world and the installation method is an easy-to-use and culturally acceptable. The retrofitting procedure explained by photo in Figure 1 taken during the retrofitting of the full scale house.

Figure 1 PP-band retrofitting procedure

In case of strengthening for the existing house, it is necessary to make holes with the drill on the bearing walls. Since the seismic performance after reinforced with PP-band mesh will be up graded to certainly compare with original one, a temporary capacity drop due to drilled holes is no problem. In case of the new construction of houses, the sleeves of binding points can be provided at designed positions.

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2 STATIC LOADING TESTS

To evaluate the beneficial effects of the proposed PP-band mesh retrofit method, diagonal compression tests were carried out using masonry wallettes with and without retrofitting for unburned bricks. The wallette dimensions were 275×275×50mm

3 and consisted of 7 brick rows of 3.5 bricks each. The mortar

joint thickness was 5mm for both cases. A Cement/Water ratio equal to 0.33 was used. The pitch of meshes was 40mm. Four wire connectors were used to link the meshes attached from both surfaces of the wallette. The specimens were tested 28 days after construction under displacement control.

Figure 3 Failure patterns of brick masonry wallettes with and without retrofitting by PP-band mesh

Figure 4 Force vs vertical deformation for masonry wall specimens

Figure 3 shows the non-retrofitted and retrofitted specimens at the end of the test, which corresponded to vertical deformations equal to 1mm and 50mm, respectively. In the non-retrofitted case, the specimens split in two pieces after the first diagonal crack occurred and no residual strength was left. While in the retrofitted case, diagonal cracks appear progressively, each new crack followed by a strength drop. Although the PP-band mesh influence was not obvious before the first cracking, after it, each strength drop was quickly regained due to the PP-band mesh effect. Although at the end of the test, almost all the mortar joints were cracked, the retrofitted wallettes did not lose stability. Figure 4 shows the diagonal compression strength variation with vertical deformation for the non-retrofitted and retrofitted unburned brick specimens. In the non-retrofitted case, the average initial strength was 0.88kN and there was no residual strength after the first crack. While in the retrofitted case, although the initial cracking was followed by a sharp drop, at least 70% of the peak strength remained. Subsequent drops were associated with new cracks like the one observed at the deformation of 1.8mm. After this, the strength was regained by readjusting and packing by PP-band mesh. The final strength of the specimen was equal to 2.16kN much higher than initial strength of 0.88kN. Out-of-plane tests were carried out in order to investigate the PP-band mesh effectiveness in walls

exhibiting arching action. The nominal dimensions of these walls were 475mm×235mm; their thickness was 50mm. A total of 6 wire connectors were used to link the meshes installed on both sides of wallettes. The specimens were tested 28 days after construction under displacement control. The wallettes were simply supported with a 440mm span. Figure 5 shows the non-retrofitted and retrofitted masonry wallettes at the end of the test, which corresponded to a mid-span net deformation equal to 1.2mm and 70.0mm, respectively. In the non-retrofitted case, the specimens split into two pieces just after the first crack occurred at mid-span, and no residual strength was left. In the retrofitted case, on the other hand, although PP-band mesh influence was not observed before the first cracking, after it, strength was regained progressively due to the PP-band mesh effect. Figure 6 shows the out-of-plane load variation in terms of mid-span net vertical deformation for the non-retrofitted and retrofitted wallettes. In the non-retrofitted case, the initial strength was 0.08 kN and there was some residual strength remaining for further small amount of deformation after the first crack. This behavior was observed due to interlocking between bricks and also the application of load under displacement control method.

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Figure 5 Failure patterns of brick masonry wallettes

Figure 6 Out-of-plane load variation in terms of net vertical deformation

In the retrofitted case, although the initial cracking was followed by a small drop, but strength remained closer to peak strength. After this, the strength was regained by readjusting and packing by PP-band mesh. The final strength of the specimen was equal to 0.54kN much higher than the initial strength of 0.08 kN.

3 DYNAMIC TESTING

Limitation in designing of models was the shaking table dimensions and capacity. The steel platform measures 1.5×1.5 m

2 in plan, and can carry a maximum payload of 2000kg. It is capable of operating in

frequencies ranging from 0.1 to 50 Hz. The maximum acceleration input, under zero payload, is ±2.0g in longitudinal direction and maximum displacement is ±100mm. Considering the shaking table size and allowable loading condition, the model scaling factor adopted was 1:4 as shown in Figure 7.

Figure 7 House model for shaking table test (dimensions in mm) Specimens consist of 18 rows of 44 bricks in each layer except openings. It took two days for construction of one specimen. The first 11 rows were constructed in the first day and remaining rows were done in following day. The geometry, construction materials and mixture proportion, construction process and technique and other conditions that may affect the strength of the building models were kept identical to better comparison

3.1 Input motion

Tests were carried out as a series of test runs on the shaking table at Institute of Industrial Science, The University of Tokyo. Simple and easy-to-use sinusoidal motions of frequencies ranging from 2Hz to 35 Hz

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and amplitudes ranging from 0.05g to 1.4g were applied to the specimen to obtain the dynamic response of both retrofitted and non-retrofitted structures. This simple input motion was applied because of its adequacy for later use in the numerical modeling. Figure 8 shows the typical shape of the applied sinusoidal wave input motion.

Figure 8 Typical shape of the applied sinusoidal wave Loading was started with a sweep motion of amplitude 0.05g with all frequencies from 2Hz to 35Hz for identifying the dynamic properties of the models. The numbers in Table 2 indicate the run numbers. General trend of loading was from high frequency to low frequency and from lower amplitude to higher amplitude. Higher frequency motions were skipped towards the end of the runs.

Table 2 Loading sequence

Amplitude Frequency

2Hz 5Hz 10Hz 15Hz 20Hz 25Hz 30Hz 35Hz

1.4g 50

1.2g 54 49

1.0g 48

0.8g 53 47 43 40 37 34 31 28

0.6g 52 45 42 39 36 33 30 27

0.4g 51 44 41 38 35 32 29 26

0.2g 46 25 24 23 22 21 20 19

0.1g 18 17 16 15 14 13 12 11

0.05g 10 09 08 07 06 05 04 03

sweep 01,02

3.2 Shaking table test

In both specimens, due to shrinkage, some minor cracks were observed before the test. These cracks mainly appear closer to opening in the horizontal direction. For both specimens, up to Run 25, no major crack was observed. For non-retrofitted model, at run 26, major cracks were observed close to the connection between the roof and south wall and cracks widened with each successive run. At run 43, and lateral drift equal to 2.62mm, a lot of damage observed in the model. Separation between East wall and its adjacent walls was observed. Furthermore, a lot of surface finishing separated from the walls. At run 44, Top corner of the East wall and its adjacent walls was totally separated from a specimen. At run 45, all the top part of the North and South walls was totally separated from a specimen. Now the roof only supported by two walls, which are in the perpendicular direction of shaking. This finally led to the structure collapse at run 47. This input motion corresponds to a velocity and displacement equal to 250mm/s and 8mm, respectively. Figure 9 shows the non-retrofitted and retrofitted house model at the end of the Run 47. In case of the retrofitted model, up to Run 39, there are only few cracks were observed (Figure 10). However, after that, the process of widening of the cracks occurred and propagation of new cracks continues until the run 50. Although at the end of 50th run almost cracks observed in entire walls, the specimen did not lose stability. At this level lateral drift was equal to 24.3mm. Some bricks from the bottom part of the east wall were spilled out from the PP - band mesh. Therefore, some looseness was observed in the bottom part of the wall. Even this very high input motion, most of the surface finishing still

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attached to the walls. At the final stage of the test, run 54, with 74.6 mm base displacement, 9 times more than the input displacement applied in run 47 and 3.7 times more velocity, virtually all the brick joints were cracked, and the building had substantial permanent deformations. However, the building did not lose the overall integrity as well as stability and collapse was prevented in such a high intensity of shaking. Thus, PP-band retrofitting technique maintained the integrity of the structural elements. Further, the retrofitted model showed the better energy dissipation mechanism as many new cracks were propagated without losing the overall integrity and stability of the structure.

Figure 9 Non-retrofitted model (left) and retrofitted model (right) after Run 47

Figure 10 Crack patterns of Non-retrofitted (left) and retrofitted (right) models after Run 39

Figure 11 Non-retrofitted model (left) and retrofitted model (right) after Run 43 When we applied the surface finishing to house model, due to improve bond connections between PP-band and brick wall, surface plaster kept well with the wall. This behavior was not observed in the non-retrofitted model (Figure 11). In case of PP-band retrofitting technology, PP-band mesh effectively distributed cracks in the retrofitted models. In case of the non-retrofitted ones, after a limited number of cracks the structure was sectioned in

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several pieces, which shook and interacted with each other. The only stabilizing mechanism against toppling was each piece self weight. There was no possibility of transferring and distributing stresses to the undamaged structure pieces. In the case of retrofitted structures, the structure sectioning was similar to initial loading stages. However, PP-band mesh distributed stresses through the cracks, transferring them to undamaged portions of the structure. As a result, new cracks appeared. PP-band mesh was also an effective mechanism to stabilize the structure and prevent wall toppling. Furthermore, it controlled the structure corner damage, one of the common failure mechanisms of non-reinforced masonry structures. In order to analyze the test results, the Japan Meteorological Agency (JMA) intensity of the input motion as a function of time was calculated. The Japan Meteorological Agency seismic intensity scale (JMA) is a measure used in Japan to indicate the strength of earthquakes. Unlike the Richter magnitude scale (which measures the total magnitude of the earthquake, and represents the size of the earthquake with a single number) the JMA scale describes the degree of shaking at a point on the Earth's surface.

Figure 12 Performance evaluation based on JMA intensities Figure 12 shows the damage evaluation for all models. Total collapse of the non-retrofitted building was occurred at the 47th run at intensity JMA 5+. The retrofitted building performed moderate structural damage level at 47th run at which the non-retrofitted building was totally collapsed. Moreover, moderate structural damage level of performance was maintained until 48th run, leading to intensity JMA 5+. It should be noted again that this model survived 7 more shakings in which many runs were with higher intensities than JMA 5+ at which the non-retrofitted building was collapsed before reaching to the final stage at the 54th run. From this it can be concluded that a structure retrofitted with PP-band meshes would be able to resist strong aftershocks. Moreover, it proves that even though a house retrofitted with PP-band is cracked after a strong earthquake, it can be repaired and be expected to withstand subsequent strong shakes.

4 PROMOTION SYSTEM FOR SEISMIC RETROFIT

Even if technically attractive retrofit method is developed, if people’s disaster imagination capability is poor, retrofit of weaker houses cannot be popular. It is impossible for human to prepare well for unimaginable situation. We should pay much attention to increase disaster imagination of the people to understand the importance of seismic retrofit of weaker houses that is the main cause of casualty and to create some social systems by which house owners are encouraged to retrofit their own weaker houses by themselves. For this purpose, to increase people’s disaster awareness, demonstrations at earthquake affected areas have been carried out. The demonstration was designed to allow the masons to apply what they had learned, for the public to graphically witness the necessity to safeguard their homes. As part of the demonstration, two 1/6 scale models, with and without retrofitting, were shaken with an improvised shaking table in order to increase earthquake risk awareness. This event was attended by politicians,

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local practitioners, mass media people, NGO/NPO representatives, and the general public (Figure 13) (JBIC, 2007).

Figure 13 Shaking table test carried out at Muzaffarabad, Pakistan The main feedback from the community after the demonstration was that community members were also motivated on the need for earthquake safety, keen to retrofit their homes but concerned over the cost of retrofitting.

5 CONCLUSIONS

Past earthquake has shown that the collapse of seismically weak masonry structures is responsible for most of the fatalities in developing countries. It is, thus urgent to improve their seismic performance in order to deduce future fatalities and to protect the existing housing stock. To encourage seismic retrofitting, inexpensive and easy to implement technical solutions are desirable. Retrofitting by PP-band meshes satisfies these requirements. From all the experimental results, it was found that this retrofitting technique can enhance safety of both existing and new masonry buildings even in worst case scenario of earthquake ground motion like JMA 7 intensity. Therefore, PP-band retrofitting method can be one of the optimum solutions for promoting safer building construction in developing countries and can contribute earthquake disaster in the future.

6 REFERENCES

Blondet, M., Villa Garcia, G., Brzev, S., Rubiños, A., (2011), Earthquake-Resistant Construction of Adobe Buildings: A Tutorial, EERI/IAEE World Housing Encyclopedia, Earthquake Engineering Research Institute, Oakland, California.

Andrew Smith and Thomas Redman. (2009), “A Critical Review of Retrofitting Methods for Unreinforced Masonry Structures”, EWB-UK Research Conference, 02-2009, London, UK

Japan Bank for International Cooperation., International Center for Urban Safety Engineering., and OYO International Corporation., 2007. Pilot Studies for Knowledge Assistance for Verification and Promotion on a New Seismic Retrofitting Method for Existing Masonry Houses by Polypropylene Band Mesh (The Islamic Republic of IRAN), Tokyo, Japan 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, Earthquake Engineering

Research Institute, Vol. 28 (1), Page 277-299.


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