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AN INNOVATlVE SEISMIC PROTECTION SCHEME FOR MASONRY BUILDINGS Svetlana Nikolié-Brzev· 1. ABSTRACT To diminish the extent of damage to masomy buildings during severe earthquak:es, applicati0n of the seismic isolation technique is introduced in the papeL A cost-effective and technically feasible isolation system for seismic protection of multi-storey masonry buildings is developed. The proposed seismic protection scheme is based on the pure- friction sliding isolation concept, and it is evaluated both analytically and experimentally. Two one-third scale models of a three-storey high brick building are consuucted and tested under simulared earthquak:e excitations on a shak:e-table. Of those, one is a conventional system, whereas the other one is isolated at two leveis, according to the proposed scheme. The most important issues regarding the experimental performance of isolated versus conventional sUUcture are explored, such as acceleration amplification ratio, relationship between peak: ground acceleration and energy input to the structure, as well as the effect of vertical component of earthquak:e excitation. KEYWORDS : Brick masonry; buildings; earthquak:e-resistant; seismic isolation; cost- effective. * M. Sc .C. E. , Visiting Researcher, Department of Civil Engineering, University of Toronto, Toronto, Ontario M5S lA4, CANADA 273

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AN INNOVATlVE SEISMIC PROTECTION SCHEME FOR MASONRY BUILDINGS

Svetlana Nikolié-Brzev·

1. ABSTRACT

To diminish the extent of damage to masomy buildings during severe earthquak:es, applicati0n of the seismic isolation technique is introduced in the papeL A cost-effective and technically feasible isolation system for seismic protection of multi-storey masonry buildings is developed. The proposed seismic protection scheme is based on the pure­friction sliding isolation concept, and it is evaluated both analytically and experimentally. Two one-third scale models of a three-storey high brick building are consuucted and tested under simulared earthquak:e excitations on a shak:e-table. Of those, one is a conventional system, whereas the other one is isolated at two leveis, according to the proposed scheme. The most important issues regarding the experimental performance of isolated versus conventional sUUcture are explored, such as acceleration amplification ratio, relationship between peak: ground acceleration and energy input to the structure, as well as the effect of vertical component of earthquak:e excitation.

KEYWORDS : Brick masonry; buildings; earthquak:e-resistant; seismic isolation; cost­effective.

* M . Sc . C . E. , Visiting Researcher, Department of Civil Engineering, University of Toronto, Toronto, Ontario M5S lA4, CANADA

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2. INTRODUCTION

Millions of people in earthquake prone areas of the world have lost their lives under the ruins of collapsed masonry buildings during the past earthquakes. In many instances, such tragic events have occurred in deve10ping countries located in the areas of high seismic activity, such as China, India, Iran, Mexico, Russia, etc. However, recent severe earthquake attacks in some of the high1y deve10ped countries like the U.S.A. (Lo ma Prieta earthquake, California, 1989, and Northridge earthquake, California, 1994) point out to the pronounced vulnerability of masonry structures in the developed countries as welI.

Failure of umeinforced masonry during earthquakes is so common that it is almost taken for granted. To enhance seismic performance of masomy structures, rational strengthening arrangements with emphasis on minÍmum use of reinforcing steel, have been proposed, verified and incorporated in building Codes worldwide, such as [1]. Reconnaissance studies in the regions stricken by the severe earthquakes reveal the superior performance of recently constructed, reinforced masonry buildings as compared to the older, umeinforced ones [2]. Properly strengthened masomy buildings usually survive attacks of severe earthquakes, but with a more or less pronounced degree of damage to their structural and non-structural components. Such behaviour of masonry and other structures during severe earthquake attacks is in accordance with the presently adopted "fail-safe" design approach. It is not economically feasible to design structures which would resist attack of a major earthquake in elastic manner. Therefore, present Codes are based on the assumptions of inelastic, "ductile" structural behaviour in the event of a major earthquake. For reinforced concrete and steel structures, it is generally possible to achieve ductile behaviour without significant structural damage. However, for masonry, which is a brittle material, this is not possible.

It appears that for masonry structures alternative design philosophy should be sought. One possibility to achieve a "damage-fiee" behaviour of these structures even during major earthquake occurrences is by means of the seismic isolation technique.

3. SEISMIC ISOLATION SYSTEMS FOR MASONRY BUILDINGS

3.1 Outline of the Concept

Seismic (base) isolation consists of decoupling a structure fiom its base so as to reduce effects of the larger horizontal acceleration peaks of the earthquake ground motion. This concept has the merit of diminishing extent of damage to building structures or avoiding ir altogether in the event of a strong earthquake. To achieve this, flexibility is introduced at the structure base leveI by means of specially designed bearings or other devices.

By means of majority of existing base isolation schemes, effective fundamental period of a structure gets shifted to range well above the predominant period of earthquake ground motion. Moreover, a considerable part of earthquake input energy which havs been transferred to the structure, gets absorbed at its base leveI. In this way, both the lateral

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forces and ductility demand to a structure are greatly reduced. Acceleration amplification in the rigid structures, and interstorey drift in the flexible ones, are minimized. Thus, both the structural and nonstructural elements, as well as any valuable equipment items, are highly protected against the earthquak:e induced darnage in base isolated structures.

3.2 Basic Requirements for Seismic Isolation of Masonry Buildings

When discussing seismic isolation schemes for masonry buildings, special care must be tak:en of the basic requirements such a scheme is supposed to meet. These are the following ones [3]:

i) Earthguak:e input energy transferred from the ground to a superstructure should be controlled and lirnited to the desired leve!.

ii) The scheme should be as simple as possible, feasible for construction at any site location, and by trained but no more skilled labour than for the conventional building construction.

iii) The proposed solution should be cost-effective, as compared to the cost expenses of traditional seismic strengthening arrangements.

Various isolation systems have been proposed in the last few decades, most of them being effective in lirniting earthquak:e input energy transferred to the structure (fust requirement). However, the majority of presently existing base isolation systems are toa advanced and toa expensive for wide application to masonry buildings in developing countries, which means that these schemes are not meeting the second and the third requirements. Therefore, an innovative base isolation scheme for seismic protection of the masonry buildings is needed.

3.3 Pure-Friction Sliding Isolation Systems

It appears that Pure-Friction (P-F) sliding isolation systems satisfy the above mentioned requirements. P-F isolation scheme consists of decoupling a building structure above the plinth levei (i.e. superstructure) from the foundation in a simple and effective way. This category of isolation systems utilizes friction, allowing some parts of the structure to slide relative to the others in course of an earthquak:e attack.

In practice, sliding movements of the structure could be provided for by utilizing principies of the traditional masonry construction. Plinth levei masonry and superstructure are often joined together through a damp-proof layer of cement-sand mortar, the lower wall below the plinth levei being at least a half-brick thicker than the upper one. Such a projection could provide unconstrained sliding of the superstructure over the lower plinth wall.

The following classifications can be made concerning the different possibilities of achieving P-F isolation concept [3]:

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• By the shape of a sliding surface : - Continuous systems (Figure la), and - Discrete systems (Figure I b) .

• By the location of isolation devices : - Base-leveI isolation (Figure 2a), - Floor-Ievel isolation (Figure 2b), and - Multiple-Ievel isolation, i.e. base and floor isolation (Figure 2c).

L ~ Figure 1. Classification of P-F isolation systems by the shape of a sliding surface:

a) Continuous system, and b) Discrete system.

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a. b. c. Figure 2. Classification of P-F isolation systems by the location of isolation devices:

a) Base leveI isolation; b) Floor leveI isolation, and c) Multiple leveI isolation.

Continuous sliding systems consist of the "sliding joint" provided throughout the structure at the base and/or the floor leveI. They are appropriate for application when buIk material such as sand or graphite powder is used as a sliding layer (Figure 3a). However, P-F sliding concept can be also achieved by placing sliding bearings at certain discrete points of a structure, for example at the wall comers and junctions. This solution is feasible when compact bearing materiaIs, such as Teflon/Steel sliding couple, are used (Figure 3b) ..

Although P-F scheme could be easily applied both at the base (plinth) leveI, and at any other floor leveI, prior to the present research there was no published evidence of the

276

studies on multiple-Ievel P-F isolation systems. However, the idea of utilizing sliding friction as a seismic isolation concept is a simple one and it has long since attracted attention of the research community. Thus, numerous publications are available reporting on the continuous theoretical and experimental research efforts directed to devising an optimal base isolation system utilizing pure-friction [4]-[7].

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Figure 3. Construction detail of the P-F isolation system: a) Continuous, and b) Discrete system.

In the previous stuclies on the topic, research effons have been devoted mainly to application of P-F systems to single-storey masonry buildings. When subjected to the earthquake-type excitation, such structures demonstrate behaviour characteristic of rigid mass systems. However, multi-storey masonry buildings subjected to the same excitation behave in a different way, due to their inverted mechanical and dynarnic characteristics. Large inventory of existing multi-storey masonry buildings, as well as the need for an innovative strategy for seismic protection of the new ones, necessitate a study on optimal seismic isolation scheme for multi-storey masonry buildings. As an altemative to the base isolation scheme, the two-Ievel (i.e. base- and floor-) isolation scheme is explored.

Study on existing P-F isolation systems applied to masonry builclings reveals their beneficial performance. Therefore, this isolation concept is chosen for the further study. The proposed scheme is verified both experimentally and analytically. Essential experimental results of the study are outlined in the following , whereas the detailed analysis is presented elsewhere [3] .

4. EXPERIMENTAL VERlFICATION OF THE SEISMIC ISOLATION SCHEME

4.1 The Test Model

To evaluate the effectiveness of proposed isolation scheme in dirninishing damage of multi-storey masonry buildings during major earthquakes, performance of both a

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conventional and isolated test model structure is investigated by shak:e-table testing. A typical two-room, three-storey high residential building is selected as the prototype structure. The adopted aspect ratio (height/length/width) is typicaI for the Indian housing practice. One door opening in each outer cross-wall, and four window openings, two by two in each longitudinal walI, are provided.

The model structure is obtained from the prototype one by scaling down all dimensions by a factor of three (1:3 scale), Figure 4. All materials and their characteristics are kept the same. Outside dirnensions of the model structure are: length 2.13 m, width 1.65 m, and height 3.31 m. Total model weight is 5.3 tono For the purpose of model construction, specially manufactured 1:3 scale burnt clay bricks (80/40/25 mm) are used. Obtained class of bricks is 250 (25 N/mm2

). For the masonry work, 1:6 cementlsand mortar is applied.

The test structure is designed according to the Indian Code of Practice for Earthquak:e Resistant Design and Construction of Buildings [1]. The structure is confined by means of the vertical reinforcement (18 rnild steel bars), which are provided at corners and junctions of the walls, as well as around the doors and windows, Figure 4. Reinforcement diameter is 5 mm, and rnild steel quality of fy = 250 N/mm2 is used. For grouting pockets in masonry around the bars, 1:3 cementlsand mortar ratio is applied.

Seisrnic isolation systems are implemented both at the plinth leveI and at the second storey floor leveI. A discrete isolation system in the form of Teflon/Stainless Steel sliders is applied at the plinth leveI (Figure 3b), and a continuous isolation system in the form of "sliding joint" at the second storey fIoor leveI (Figure 3a).

To determine the frictional characteristics of sliding materials/couples to be used, several series of tests on the individual sliding bearings are carried out. Static frictional coefficient values of Teflon/Steel sliding couple obtained experirnentally are in the range from 0.1 to 0.12, depending on the contact pressure. Another series of tests has been performed on the cementlsand mortar and concrete tiles, with grease as a sliding membrane. The experimentally obtained frictional coefficient value for this sliding couple is approximately 0.4.

Technically, a "sliding joint" at the second storey levei is constructed by providing a smoothly finished layer of cementlsand mortar over the floor-slab. Sliding film in the form of grease is applied over the previous layer. Finally, a reinforced concrete ring beam-bond beam, is constructed under all internal and external walls to ensure integral movement of the superstructure above the second storey leveI. From this point on, masonry construction proceeds in a conventionaI manner.

A discrete isolation system at the plinth leveI is constructed by placing sliding bearings at the wall corners and junctions. Upper and Iower sliding plates are made separately. Lower sliding surface comprises a stainless steel sheet bonded to the rnild steel plate, and it is adhered to surface of the plinth wall by epoxy-based resin "Araldite". Upper sliding surface comprises a Teflon pad adhered to the rnild steel plate. The upper plate is at the same time used as anchoring point for a vertical reinforcement bar.

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213

Figure 4. Test model structure

4.2 Experimental Program

Artificial earthquake acceleration records were used for the simulation of earthquake excitation in course of the experimento The target spectrum curve was determined as a smooth broadened envelope curve of both the average spectra on rock and stiff soil sites obtained in the USA, and of the Koyna earthquake (India, 1967) response spectrum. Besides the horizontal component, the vertical component of the earthquake excitation was introduced in some cases. Zero period acceleration value of the vertical excitation target spectrum curve was taken as two/thirds of that of the horizontal one.

Total number of thirteen test runs was carried out on a computer-controlled shake-table facility of the Department of Earthquake Engineering, University of Roorkee, lndia. Out of them, the conventional model was tested in five runs, and the sliding one in eight runs . The vertical component of the earthquake excitation was introduced in six runs. The model structure was subjected to the excitation with the maximum recorded peak ground acceleration (PGA) of O.379g during the experimento

279

4.3 Results of Experimental Investigations

Following completion of the experiment, most relevant parameters regarding performance of the model structure were evaluated.

Acceleration amplification represents the ratio between response acceleration at a certain levei of the structure (usually the upperrnost levei) and the peak ground acceleration (PGA). Experimentally obtained values of the horizontal acceleration amplification ratio were lying in the range from 1.31 to 2.51. Comparison was made between obtained amplification ratios for the conventional system (run No.3) and the sliding one (run No.7) subjected to excitations of the same PGA levei (approximately O.2g). Figure 5 shows that a considerably higher amplification ratio was obtained for the conventional structure (run No.3) than for the sliding one (run No.7). It has to be admitted that reduction in the acceleration response of the isolated structure as compared to the conventional one was not as pronounced in this case as expected. However, it was observed that the minimum amplification ratio of 1.31 is obtained in the run No.13, when sliding model was subjected to the highest ever excitation of approximately O.4g. Further comparative study on the system responses during test runs with the various excitation leveis was carried out. It has been shown that efficiency of the proposed isolation scheme in terrns of reduction in acceleration responses was more pronounced at the higher excitations.

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Figure 5. Acceleration amplification ratios at each storey levei, runs No. 3 and 7

Another relevant parameter for evaluation of the isolation effect is input energy transmitted to the structure in course of the experiment. Relations between the input energy and the PGA for each test run were derived, and the results are presented in Figure 6. For the conventional system, amount of input energy was increasing with the increase of PGA levei, whereas for the sliding system an opposite trend was observed. This is in support of the previous observation conceming the pronounced efficiency of P-F isolation scheme at higher excitation leveis.

During the experiment, the model was subjected to combined horizontal and vertical excitation for several times. To identify the influence of vertical component of earthquake excitation to the response of a sliding structure, results obtained in the runs No.7 and 10 are considered. In the run No.7, the recorded horizontal PGA value was O.201g, which was almost the same as the one obtained in the run No. lO (O .208g). Although the vertical

280

component of the excitation was introduced in the run No. lO (vertical PGA was O.148g), horizontal amplification ratio was almost the same in both the cases (1. 81 in the run No. 7, and 1.86 in the run No.lO) . This clearly shows that vertical component of excitation does not represent a predominam parameter influencing response of the sliding structure.

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Figure 6. Input energy versus PGA: a) Conventional systems, and b) Sliding systems.

In one of the test runs, the sliding model structure was subjected to the excitation with a recorded horizontal PGA of approximately OAg. Even at such a high excitation levei there were no signs of any structural damage observed. At the same time, conventional structure has been subjected to the maximurn excitation of approximately 0.21g without any signs of damage.

5. CONCLUSIONS

As a result of the research presented herein, the following conclusions can be drawn:

(i) Seisrnic isolation technique can be used to achieve "damage-free" behaviour of masonry buildings even during the severe earthquakes. Study on the presently available seisrnic isolation schemes revealed that a feasible isolation scheme for masonry buildings lies in the category of "pure-friction" (P-F) isolation systems.

(ii) Experiments have shown that the proposed P-F isolation scheme is capable of reducing response acceleration levei of multi-storey masonry buildings appreciably as compared to the conventional structure response. This trend is particularly pronounced at the higher excitations.

(iii) A comparative analysis on the amount of simulated earthquake energy input to lhe conventional and sliding system is carried out. The study reveals that, for a conventional system, the amount of input energy is constantly increasing with the increase of horizontal excitation leveI, whereas for a sliding system an opposite trend is observed. This finding suggests that the effectiveness of the proposed isolation scheme is more pronounced at higher excitation leveIs.

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(iv) Model structure has been exposed to horizontal and vertical excitation simultaneously in several shake-table test runs. Test results show that vertical component of the excitation does not represent a predominant parameter influencing the isolated structure response.

Based on the results of this research it can be concluded that the proposed P-F seismic isolation scheme is capable of reducing dynamic response of multi-storey masonry buildings to high intensity earthquakes appreciably, as compared with the conventional system response. This scheme is therefore recommended for adoption as a feasible practical means for seismic protection of masonry buildings up to about four storeys.

6. ACKNOWLEDGEMENTS

The research reported in this paper has been sponsored by the Indian Council for Cultural Relations, Ministry of Human Resources, Govemment of India, and by the Department of Earthquake Engineering, University of Roorkee, India, and their support is gratefully acknowledged. The author is especially indebted to her Ph.D. Thesis supervisors, Df. S.Basu, Professor, Df. A.S.Arya, Professor Emeritus, and Dr.S.K.Thakkar, Professor and Head, Department of Earthquake Engineering, University of Roorkee, India, for their expert and inspiring guidance in course of the reported work.

7. REFERENCES

1. Indian Standard Code of Practice for Earthquake Resistant Design and Construction of Buildings (IS :4326-1976)

2. Earthquake Engineering Research Institute, "Loma Prieta Earthquake Reconnaissance Report", Earthquake Spectra, Supp1ement to Vo1.6, 1990, pp.127-149.

3. Nikolic-Brzev, S., "Seismic Protection of Mu1ti-storey Brick Buildings by Seismic Isolation Technique", Thesis submitted in the fulfilment of the degree of Doctor of Philosophy, Departrnent of Earthquake Engineering, University of Roorkee, India, 1993, pp. 235.

4. Qamaruddin,M., Arya,A.,S., and Chandra, B., "Seismic Response of Masonry Building with Sliding Superstructure", Journal of Structural Engineering, 112, 2001 -2011, 1986.

5. Qamaruddin, M., Chandra, B., and Arya,A.,S., "Dynarnic Testing of Brick Building Models", Proceedings, The Institution of Civil Engineers, 77(2), 1984, pp. 353-365.

6. Qamaruddin, M., "Development of brick building systems for improved earthquake performance", Thesis submitted in the fulfllment of the degree of Doctor of Philosophy, Department of Earthquake Engineering, University of Roorkee, India, 1978.

7. Li Li, " Base Isolation Measure for Aseismic Buildings in China", Proceedings of the Eight World Conference on Earthquake Engineering, San Francisco, 6, 1984, pp.791-198.

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