ANALYSIS AND STRENGTHENING METHODS FOR HISTORICAL MASONRY STRUCTURES

Hafez Keypour1, Yasin M. Fahjan2, Ali Bayraktar3 ABSTRACT Analysing and strengthening historical structures are challenging task due to the geometrical complexity and lack of knowledge about the inner core material and due to the fact that the masonry material is not able to carry tensile stresses. A better understanding of both gravity load transfer mechanism and lateral resisting system of such structures is the key issue for a comprehensive structural analysis, interpretation of the analysis results and a proper intervention. Nowadays, with the development of computational methods, analyses of historical structures are mostly performed using Finite Element Analyses. Generating a finite element model of the structure require a good engineering experience to make a reasonable geometrical simplification of the complex geometry and a good assumption of unknown inner-core materials. In this study, the design and construction principles for seismic strengthening of historical masonry structures are described in details. The seismic damage characteristics for different elements (foundations, load-bearing walls, arches, domes) are discussed and appropriate seismic strengthening methods are proposed. The importance of minimal disturbance to the system and the crucial to devise a strengthening method that would preserve all historical and architectural attributes of the structure are emphasized. A detailed strengthening procedure are utilized to analyze and strengthening a historical building in Istanbul. Introduction Conservation of our historic heritage and restoration of historical structures necessitates a great effort of different expertise (structural, architectural, historical, etc). There are many challenges for understanding the behaviour of historical structures under gravity and seismic loads, usually of architectural importance. Historical structures have very complex load carrying behaviour due to the massive and continuous interaction of domes, vaults, arches and pillars. Typically, these structures are more massive than contemporary structures and that usually carry their actions primarily in compression. The restoration and sometime strengthen of these historical structures is vital in order to save them for the next generations. For proper intervention, understanding of the structural behaviour and good engineering judgment with sufficient experience of the old construction techniques and concepts and correct interpretation of the

1 Ph.D. Graduated, Kandilli Observatory and Earthquake Research Institute, Bogazici University, 34684 Cengelkoy, Istanbul, Turkey, keypour@boun.edu.tr 2 Assistant Professor, Department of Earthquake and Structural Science, Gebze Institute of Technology, 41400 Gebze, Kocaeli, Turkey, fahjan@gyte.edu.tr 3 General Manager, SGM, Seismic Strengthening Center, Inonu Cad. Sumer Sok. Zitas D1 Blok No:19/8, Kozyatagi, Kadikoy 34742, Istanbul, Turkey, destek@sismik-guclendirme.com.

analysis results of comprehensive structural analyses are needed. The great challenging of understanding the behaviour of these structures, is that the master historical structures was designed not only for complex load carrying system and continuous interaction of domes, vaults, arches and pillars, but also for architectural concepts and for enough light and proper acoustics. In other hand, even though the contemporary structures was designed to carry their actions primarily in compression, the tension stresses occur due to gravity and lateral forces in different structural components. To take care of these tensile stresses a special kind of tension element were introduced and located at the necessary locations. Generally these tension elements are composed of timber or iron. To prevent the deterioration of such elements in long term, the humidity resulted from underground water movement should be controlled. This leads to the most important issue to keep such structures alive for long time span. The key is the proper foundation and underground gallery/gate systems. The ignorance of the accumulation of master builder’s knowledge and construction technique may lead to incorrect intervention and long term harmful effects on the structure. Recently, in the literature some researchers studied the traditional design concepts of different types of historical structures (Huerta 2001, Boothby 2001, Bayraktar, 2006). In other hand, earthquakes are known to be the natural hazards that have affected tremendously historical structures. It has been noticed that in seismic areas, the master builders tried to introduce special techniques to make the structure withstand the lateral forces. For example, Ottoman historical structures in general had special lateral resisting systems depending on sufficient horizontal diaphragms and vertical frame resisting systems. Ottoman important historical structures (Suleymaniye, Selimiye, and Sultan Ahmet mosques, etc.) generally were constructed considering different horizontal layers strengthen by special tension elements and a vertical frame resisting systems. Elements of Historical Structures -Foundation and Galleries

For better understanding, it is worth to explain how foundations are constructed in ancient structures. The soil is excavated until a good ground is reached. In the middle of the area where the building is constructed two or three water well is made. Then 20 cm of mortar (special type of monumental mortar that is mixed of lime, crashed bricks) is placed. Afterward some timbers are put as grid, which works as tension members in the foundation. A mat foundation of about 70-120 cm thick is made by lime-soil-sand mixture. The grid timbers are mounted on top layer of foundation too.

Figure 1. The Galleries or underground gates below mat foundation (left). The right picture shows the gallery

that is filled with soil about 80% during two centuries. The water wells under the foundation is interconnected to each other with small gates (tunnels or galleries), then

opened to air outside the building construction area or connected to other water wells of neighbour’s lands, to make galleries system that later go to open to air. By this method, the humidity or moisture from ground is stopped and prevented to go to the walls or ground floors. This is crucial for all historical structure. There was no other way to stop moisture in such building in the past. Using this galleries system, enhance the durability of structure and help it to withstand for so many centuries otherwise the structure can have a lot of humidity problem that shorten its life. Unfortunately, the 21st century’s civil engineer or architects do not know this subject yet and there are so many problems for restoration of such buildings at the present. The size of small gates varies from 30-40cm for general building to 1-2 meters for big structures such as mosques and churches. They are generally constructed of mortar, stones or bricks. In some of them water current may exists and it was generally drinkable. Since in our century the water is within our access, the restoration experts close these wells or gates by concrete mat foundation. Closing such system decrease, the ability of the ground to infiltrate wet air from the ground therefore allow humidity penetrate through the walls and deteriorate the structure materials. An example of underground galleries below mat foundation is shown in Figure 1. Structural Elements for Ottoman Historical Structures Historical structures have very complex load carrying behaviour due to the massive and continuous interaction of domes, vaults, arches, pillars and walls. Typically, these structures are more massive than contemporary structures and that usually carry their actions primarily in compression. The structural resistance depends on the geometry of the structure, shape of the structural components and the characteristic strength and stiffness of the material used. The characteristic thickness of the masonry structural components should be able to resist compression, tension and shear stresses results from the structure’s own weights and those imposed by wind and earthquake. Major geometrical forms of Ottoman historical structural elements are arches, vaults, domes and walls. A detailed description of the load carrying mechanism of these elements can be found in (Unay, 2001), and a brief summery is given below:

Arches: Arches are the structural elements that span a horizontal distance carrying its own weight and other loads totally or mainly by internal compression. The most important characteristic of the arch is that as a part of its primary action, it does always thrust outwards on its abutments as well as weighing down vertically on them.

Vaults: The vault is a structural system that distributes loads by arch action through a single curved plane to continuous supports. The stresses within the vault are primarily compressive. It can be considered as a curved bearing wall enclosing a space. Lateral stability is developed within the plane of the vault, due to its continuous form.

Domes: The dome is a structural form, which distributes loads to supports through a doubly curved plane. It is a continuous geometric form, without corners or perpendicular changes in surface direction. It encloses the maximum volume with a minimum of surface area. The dome must be designed to resist compressive stresses along the meridian lines and to resolve circumferential tensile forces in the lower portion of hemispherical domes. The dome is an extremely stable structural form and resists lateral deformation through its geometry.

Tension Elements: Even though the historical structures was designed to carry their actions primarily in compression, the tension stresses occurs due to gravity and lateral forces in different structural components. To take care of these tensile stresses such special tension elements were introduced and located at the necessary locations. Generally, these tension elements are composed of different styles of timber or iron. Examples of different tension elements are shown in Figures 2-4. To prevent the deterioration of such elements in long term, the humidity resulted from underground should be controlled. This leads to the most important issue to keep such structures alive for long time, which is the proper foundation and underground gallery/gate systems. Lateral Supporting Systems for Historical Structures As it is described in the previous sections, historical structures have very complex load carrying system with continuous interaction of domes, vaults, arches, pillars and walls. Most of the previous studies dealing with the load transfer mechanisms of the structural systems under gravity static loads, For example, Ottoman dome structures (Mungan, 1988, Karaesmen and Unay, 1988). Therefore, the behaviour of historical structural systems under dynamic lateral loads are needed to be investigated to better understand overall load carrying mechanism.

The general concepts of modern earthquake engineering can be utilized to understand such complex lateral load transfer mechanisms. In conventional structures, the lateral resisting systems depend on sufficient horizontal diaphragms and vertical load resistant frames to withstand the earthquake dynamic forces. The same concept can be applied to understand the behaviour of historical structures. Horizontal diaphragms at different levels of Suleymaniye Mosque-Istanbul and the load resistant cross sections of Fatih Mosque-Istanbul are shown in Figures 5 and 6 respectively. Numerical Analysis of Historical Structures Nowadays, with the development of computational methods, analyses of historical structures are mostly performed using Finite Element Analysis. The analysis begins by generating a finite element model of entire structure or structural element. In the geometrical model, different elements can be employed to represent columns, arches, domes and vaults. Truss, beam, solid, membrane, plate and/or shell elements are examples. The selection takes into account the type of analysis to be performed, availability of “element geometry-material model” relation, the accuracy needed in terms of time and effort, complexity of the model, among others. Understanding the vertical and horizontal load transfer mechanism is necessary to generate reliable model of the structure and to make meaningful interpretation of the results. For dynamic lateral forces, proper cross section representing the lateral resisting system should be selected to demonstrate the analysis results. Strategies in the Modelling There are different strategies that can be used in the modelling (Lourenço, 2002). One modelling strategy cannot be preferred over the other because different application fields exist for each one, according to the complexity and detail requirements.

Macro modelling: The simplest strategy for the numerical modelling of masonry structures is the macro-modelling, where masonry units, mortar and mortar-unit interface are smeared out in a homogenous continuum material. Macro modelling does not distinguish between individual units and joints, but treats masonry as a homogenous anisotropic continuum. Macro models are more applicable when the structure has large dimensions and stresses are uniformly distributed along the macro-length. Specifically, macro-models are more practice-orientated when equilibrium between time and efficiency is required.

Simplified micro modelling: This methodology is more complex than macro modelling. Although mortar and masonry units are still represented as a continuum, the interface between them is modelled by discontinuous elements, known as interface elements. Masonry is thus considered as a set of elastic blocks bonded by potential fracture/slip lines at the joints. In this case, Poisson’s ratio of the mortar is not included.

Micro modelling: It is the most advanced modelling methodology. Both, mortar and masonry units are modelled independently. Inelastic properties for each one can be assigned. Additionally, discontinuous elements are used to model the interface between mortar and units. This type of modelling applies notably to structural details. Type of Analysis

Linear elastic analysis: The most common idealization of the behaviour used for the analysis of historical constructions is the linear elastic analysis, which needs information about the linear elastic properties and the maximum allowable stresses of the material. Linear elastic analysis assumes that the material obeys Hook’s law. The global stiffness matrix is constructed only once. Consequently, the linear analysis has a great advantage in saving computational time. The stress distribution resulting from this type of analysis represents with good accuracy the behavior of the structural components only if the values are lower, equal or a small percent above the allowable stress. For instance, the static dead load case is normally well described by a linear elastic analysis. Another application is the free vibration analysis of the structure in order to get the dynamic properties of the structure like natural periods and mode shapes. For complex structure, time history analyses can be performed in the linear elastic range to give an idea about the overall behaviour of the system.

Nonlinear analysis: On the contrary, nonlinear numerical analyses have demonstrated power in a different sense. The fact that softening behaviour, crack propagation and loss of strength can be followed through appropriate nonlinearities is a clue of preference by many researches. Nonlinear analyses distinguish two types of nonlinearity: material and geometrical). The material nonlinearity follows the behaviour of the material beyond the elastic range. The curve of the material can follow softening or hardening behaviour. Different types of plasticity models can be chosen depending on the material. In the geometric nonlinear analysis the point of load application changes with the increase of actions, the structure buckles due to instability. After every

increment of the load, the stiffness matrix of the structure is recalculated for the new geometric conditions obtained in the previous load step.

Figure 2. Timbers used as tension members in masonry walls (100 years and 3000 years-old walls).

Figure 3. The holes show the existence of timbers that vanished during centuries in Istanbul city walls.

Figure 4. Iron clamps in Edirnekapi Mihrimah Sultan Mosque- Istanbul and timbers with ropes in Vakil

Caravanserai, Kerman, Iran.

Figure 5: Horizontal diaphragms at different levels of Suleymaniye Mosque-Istanbul

Figure 6: Lateral load resistant cross sections of Fatih Mosque-Istanbul. Identification and Reasons of Damages in Historical Structures Generally, damages in masonry structures are due to excessive stresses or forces in the main body of the load carrying systems. Damages types in the walls are:

a) Compressive stress caused by vertical load, b) Shear stresses caused by lateral force, c) Tension force in horizontal within the walls that cause vertical cracks, d) Spoiling of timbers in the walls due to moisture or other problems, e) Corrosion of iron clamps due to moisture, f) Spilling of walls because of not having enough expansion joints and g) Propagation of cracks due to successive earthquakes during centuries.

Damage types in the foundations are: a) Ground movements, b) Raising and settling of underground water, c) Spoiling of foundations and d) Loss of functions of underground galleries and gates under the foundations.

Damage in other part of historical structures such as arches and domes are generally of the above-mentioned types. For any kind of restorations or strengthening works, these problems should be identified and proper solution provided. Proposed Strengthen Procedure for Historical Structures

Under the lights of previous discussion, it is recommend that prior to strengthening procedure for the historical structures and intervention process taking place, the following points may be taken into account: • Investigate the real reason of existing cracks and damages. There is a misleading judgment that most of the

cracks resulted from foundation settlements. A better understanding of the tension elements location, conditions and deterioration may give more insight understanding of the reasons.

• Identify the construction materials and the reason of using different materials in different structural elements (stone, brick, tree leafs walls, etc.)

• Examine the locations and conditions of tension members within the structure • Understand the reason for special construction techniques (double arches, tension ribs, etc. ) • Identify the Type and conditions of foundation system • Investigate the Existence of underground gallery/gate systems. In recent years, many methods have been improved for strengthening masonry walls or arches. In these methods, the most important problem is the compatibility between the new material and the original material of the structures. In this case, the restoration does not last a long life and generally disturbs the structure. The most common used restoration materials are concrete, steel bars and carbon fibres textures. If the new material rigidity differs from the original, during earthquake non-uniform behaviour may cause severe damage in walls. In other cases, protecting the steel bars from moisture is difficult task, and generally, they start to corrode and their functions lose accordingly. The carbon fibres textures, which are adhered on the dusty walls and is open to air is not a good choice. It practice, it has been noticed that after few years, the adhesive force is vanished. All existing methods are applied from outside of walls and no extensive work can be done inside the walls. The seismic strengthening technology proposed in this work is based on the principle of integrating especially tension-absorbing carbon-fibre-band reinforcements into unreinforced structures, eliminating their weakness under seismic loads. In addition to presenting a long-lasting engineering-based solution to the seismic strengthening for the structure, the proposed method is believed that it meets all technical, architectural, logistical, and financial criteria set by different Codes and Board of Trustees. It is advised that prior to strengthen implementation, a detailed finite element analysis to be carried out to find the tension zones. The analyses allow computing the amount of carbon-fibre-band necessary for strengthening. The bands is applied by inserting them about 10cm inside the wall from sides and refilled with the same kind of mortar. In Figure 7, a schematic view of the application of proposed strengthening method.

Figure 7. Schematic view of strengthening procedure. 1) Band, 2) mortar and 3) masonry wall. Analysis and Strengthen of a Historical Building in Istanbul As an example, the Kuleli military high school structure in Istanbul was taken into consideration for retrofitting and strengthening project. It is more than 170 years old and has undergone of several restorations in the past. As it can be seen from Figure 8, the building is consists of several blocks and each is analyzed individually. The amounts of tension stresses are found under several earthquake load conditions. After that, the amounts of high resistant carbon fibre bands are calculated per meter width. In application, the mortar is removed 7-10cm between the stones of the wall before the carbon fibre bands are inserted. It should be noted that the removed mortar should not be used again and new composite of the same kind of historical mortar can be reapplied.

Figure 8. Finite element modelling of Kuleli military high school in Istanbul. In general, many of restoration works does not include any strengthening projects. All cracks are filled with masonry or mortar and restoration works continue with architectural and painting details. Ignoring structural rehabilitation and strengthening process during a long period of time, lead to weaken the lateral structural capacity of building, therefore the structure is completely demolished during moderate and high seismic activities (In Iran about 120 historical monuments are lost every year). It is believed that the proposed method enhances the lateral capacity of the masonry buildings and if a better technology is found in the future, the bands are easily removed. Conclusions Analysing and strengthening historical structures are difficult tasks. A better understanding of the structural behaviour of such structures and good engineering judgment with sufficient experience of the old construction techniques and concepts and correct interpretation of the analysis results of comprehensive earthquake structural analyses are necessary for proper intervention of these structures. The general concepts of modern earthquake engineering should be used to better understanding the lateral resisting systems. A comprehensive investigation methodology of the reasons of the damages and cracks in historical structures prior to strengthening procedure is essential. If it is necessary, the tension capacities of the masonry members can be enhanced using the proposed strengthening technique. References Bayraktar, A, 2006. Analytical Study of Historical structures and of seismic strengthen methods. Beta

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Lourenco, P. Roca (Eds.), Guimaraes. Karaesmen, E. and Unay A. I. 1988. A Study of structural aspects of domed buildings with emphasis on

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