Historical Earthquake Damages to Domed Structures in Istanbul

Historical Earthquake Damages to Domed Structures in Istanbul İhsan E. Bal Asst. Prof., Istanbul Technical University, Institute of Earthquake Engineering and Disaster Management, Maslak Campus, Istanbul, Turkey F. Gülten Gülay Prof., Istanbul Technical University, Department of Civil Engineering, Maslak Campus, Istanbul, Turkey Meltem Vatan Asst. Prof., Istanbul Aydin University, Department of Architecture, Florya Campus, Istanbul, Turkey Eleni Smyrou Asst. Prof., Istanbul Technical University, Department of Civil Engineering, Maslak Campus, Istanbul, Turkey Keywords: earthquake damages, historical records, domed structures ABSTRACT Istanbul, the capital of Eastern Rome, Byzantine Empire and Ottoman Empire, has always been an important city, decorated with emblematic buildings. The seismicity of the city and the surrounding area, however, has been one of the most challenging points the designer of these daring historical structures had to face. Very strong tremors, recurring in every one and a half century in average, hit the city leaving a tragic mark in the history. The legendry dome of Hagia Sophia, the most important structure of the city, for instance, collapsed in 1509 due to a strong shaking. The dome of Beyazıt Mosque, commissioned by the Sultan Beyazıt, collapsed 3 years after its completion during the 1509 Earthquake as well. Fatih Mosque, commissioned by the conqueror of the city, Mehmet the 2nd, collapsed during the 1766 Earthquake to such an extend that the bearing system of the structure had to be redesigned during the reconstruction works. Atik Ali Paşa Mosque in Beyazıt Square, experienced a severe damage during the 1766 Earthquake thus the load bearing system and the dome had to be repaired and even altered. This chapter discusses the domed structures in Istanbul, which are reported damaged during strong historical earthquakes. The attention is focused mostly to their domes, the most important component of the Byzantine and the Ottoman architecture. The significant shakings, together with their estimated epicenters and magnitudes, have been defined and the spatial 1 distribution of the reported damages in the domed structures has been examined. It is found that the Historical Peninsula, which is where once Constantinople was located, has several vulnerable structures and high seismic hazard level at the same time. Certain structures have been found to be quite vulnerable to strong shakings and received significant damages multiple times. The chapter discusses the possible effects of the future seismic events on the historical buildings in Istanbul, based on the recorded damages occurred during the past seismic events. 1. Motivation of the Research The main element of the religious buildings has been the main dome as far as Byzantine and Ottoman periods are concerned. Keeping the dome standing and constructing larger and larger domes were the main challenging issues the engineers of the old times had to face. Istanbul has been the capital of three empires leading thus to a very rich collection of historical structures. The concentration of domed heritage structures is higher than any other place. The main challenge, for the case of Istanbul, has always been the seismic safety of these structures because of the very active seismicity of the region, with more than 70 earthquakes with magnitudes 6 and above in the last 2 millennia (Figure 1). There are four structures considered in this Chapter, all placed within the old Constantinople, or the “Historical Peninsula” as the modern name calls, behind the 2nd Theodosian Walls (Figure 2). The structures considered span from the 6th century to the 18th as far as their construction periods are concerned. Figure 1. Known seismicity of the Marmara Region (modified from Ambraseys et al, 1991 ) 1.1. The Case Study Structures The four case study structures examined here represent the vast majority of the religious and emblematic structures found in Istanbul (Figure 2). Hagia Sophia (Megali Eklesia) is the most fascinating domed monumental building of the history, located in Istanbul, Turkey, with its features both from Christian and Islamic influences. The structure is estimated to resisted more than ten very strong (i.e. magnitude above 7) shakings in close by distance, a record that is a record on its own regarding the size and the age of similar structures. 2 Figure 2. Historical Peninsula and the four structures considered in this Chapter Atik Ali Paşa Mosque is a small structure built at the end of 15th century by the Grand Vezir of the Sultan. It is in a central point, on the main road ending in Topkapı Palace. Beyazıt Mosque is a mid-size Royal Mosque (Selatin Camisi), built in 1506. The Royal Mosques are funded by Sultan himself, traditionally following an important military victory where Sultan had also taken part in the battle field. These are large mosques that are supposed to be open 24 hours and they are generally surrounded by other facilities creating thus a campus. The plan of Beyazıt Mosque resembles that of Hagia Sophia in many aspects. Fatih Mosque was commissioned by Mehmed the 2nd, the conqueror of the city, and was completed in 1470. A strong earthquake destroyed the structure in 1766, thus the structure was rebuilt in 1771, not necessarily by sticking to the original design. 1.2. Hagia Sophia Hagia Sophia is one of the oldest magnificent domed monuments with a unique resemblance of architectural and engineering talent of human. It was built originally as a church in the Byzantine Age, then was transformed to a mosque after Ottomans captured Istanbul in 1453. It was selected as a world heritage site by UNESCO in 1985. The historians of architecture often explain the construction of Hagia Sophia as a technological design revolution (Çamlıbel, 1998). Hagia Sophia served as a model for many of the Ottoman mosques such as Beyazıt Mosque, Blue Mosque, Şehzade Mosque, Süleymaniye Mosque and some more others (Figure 3, Figure 4 and Figure 5). 3 Figure 3. Hagia Sophia Figure 4. Hagia Sophia Dome, Semi-Dome and the mosaics of Cherubim 1.2.1. Definition of the structural system and the history of the structure Hagia Sophia is located in Historical Peninsula of Istanbul, constructed less than 20km close to a major fault line. The foundation of Hagia Sophia lays on a soil profile including a thin top soil, fill and bedrock. As for the history of the structure is concerned, Hagia Sophia was initially constructed and named as Megali Ekleisia (“Great Church” in Greek) in 360 AD by Emperor Constantine, symbolizing the power of religion on temporal affairs in addition being an exceptional engineering and architectural masterpiece with its shape and building techniques. The original structure was destroyed and reconstructed in 415 with stone and a wooden roof. However, during Nika revolt in January 532 A.D. the existing Hagia Sophia church was fired and 4 partially destroyed. Although most of the brick and stone basilica was probably left standing, Justinian decided to create a new cathedral and the first stone was placed just 40 days after the event. The reconstruction of the existing structure was accomplished in 5 years, between 532-537 during Justinian reign by the scholars, designed by a mathematician called Anthemius of Tralles and an architect called Isidorus of Miletus. The church was opened in 537 with a ceremony. 1.2.2. Structural System The centrally-planned church has a rectangular plan with dimensions about 75mx95m (70mx92m) externally, including the two nartexes, without the atrium in the west direction (Freely and Cakmak, 2004). The structural system consists of a main dome, the half dome and small semi-domes on both sides of the East-side and the West-side, vaults, arches, pendentives, the buttresses and four massive piers that bear the main dome. The height of the dome from the ground is 55 m (Eyice, 1994). The dome is not completely a hemisphere because of the structural deformations pushing the two main frames apart. The dome has lost its perfect circular base and has become somewhat elliptical with a diameter varying between 31.24 and 30.86 m. The four piers bearing the dome have a height of 23 m (Kleinbauer et al., 2004). Hagia Sophia has 40 closely spaced windows at the bottom of the main dome, asserting that the base of the dome is insubstantial and is hardly touching the building itself. There are four thick arches bound by pendentives, spanning between these piers, as shown in Figure 5. The dome and the walls were constructed with brick and mortar, whereas stone brought from different regions was used for the piers. The columns were made with marbel. Iron was used for different aims such as the cramps between adjacent blocks of stone in the cornices and for long tie bars spanning across the springings of arches and vaults. Lead sheets were used to protect the outer surfaces of the vaults and dome (İlki et al, 2006). After its transformation to a mosque, the red brick minaret at southwest was built, then later on the other three were built from white marble; of which the slender one at northeast was erected by Sultan Bayezid II while the two larger minarets at west were erected by Sultan Selim II and designed by the famous Ottoman architect Sinan. It is believed that the reason for the varying dimensions and mass of the minarets was to counterweight the mass of the main structure and to distribute the weight uniformly. This application performed by Sinan which was considered as one of the earliest seismic and geotechnical engineering efforts. Latest research shows that without the counterweight of the minarets, the main structure would tend to open up in the level of the main arches faster (Mungan, 2007). 5 Figure 5. Plan view of Hagia Sophia 1.2.3. Structural Deficiencies, Reported Damages and Repair Works Hagia Sophia experienced many failures, reconstructions, restorations and interventions due to seismic actions, fires and other external effects throughout the history. In order to understand its present state and the structural behavior of the building, it is necessary to investigate the sequence of events the structure experienced in the history before applying any retrofitting or reinforcing procedure to protect this extra ordinary monument against future external actions. The sophisticated roof of the structure consisted of a large central dome resisting on four arches and spherical triangular surface structures called pendentives, supported by only four main piers and eight secondary piers creating an irregular octagon in plan. The whole complex having only 0.70m wall thickness has two primary and five secondary semi-domes with somewhat weaker arches between the main dome and primary semi-domes than the other two in the perpendicular direction. The pier system has different rigidities in longitudinal and transversal directions. It means that the structure was quite weak in NorthSouth directions originally, before the addition of the two buttresses by Sinan. There are studies (Mungan, 1988; Mungan and Turkmen, 1995; Şahin et al., 2005; Mungan, 2007) showing that one of the reasons for the collapse of the central dome and parts of the eastern semi-dome after the earthquake of December 557, only twenty years after the inauguration of the church, was the difference in stiffness of the perpendicular domes. Additionally, the 40m high main piers experienced about 1.0 m lateral displacements towards outside at that seismic activity (Mungan, 2007). The reason why the main piers rotate under torsional forces is that the main arches are sitting on the main piers eccentrically creating additional torsional moments. Due to the high seismicity of the Marmara Region, Hagia Sophia has suffered from numerous earthquakes during its long history. There are two major dome collapses reported, one in 989 and the other in 1346. 6 During the earthquakes occurred in 553 and in 557, many cracks in the main dome and the eastern semi-dome occurred. After the main dome collapsed completely during an earthquake in 557, the emperor ordered the nephew of the architect Isodorus to make the restoration work. He used lighter materials and elevated the dome by 6.25 meters without changing its original form, to reduce the weight of the dome, thus giving the building its current interior height of 55.60 meters. This reconstruction, giving the church its present sixth-century form, was completed in 562. This form of the second dome remains basically unchanged despite its partial collapse in 10th and 14th centuries (see Figure 6). The inclined east and west tympana were rebuilt around the year 800. Then, the structure experienced partial damage with the earthquake of 859 following the fire in 869. With these events, one of the half domes collapsed, but it was repaired by the order of Emperor Basil I. In 989, the dome of the basilica was ruined again by an earthquake. This time the Emperor was Basil II, and he ordered the architect Trdat for the restoration. He repaired the western arch and a portion of the dome and thus, the reconstruction lasted six years. The church was re-opened in 994. In 989, the collapse of the west main arch and part of domes was experienced. After the collapse in 989, the gallery arches between the main piers and staircase towers were underpinned and the staircase towers together with their connecting walls were heightened in order to increase the resistance of the main piers to the thrust of the west and east arches. All these measures were not sufficient apparently, because additional external buttresses were also built in 1317, only nearly three decades before the collapse in 1346, where the east main arch and the part of domes collapsed. It is reported in the literature that the structure passed through a series of serious maintenance and strengthening works, the most serious of which was conducted by Sinan. He built the two additional large minarets at the western end of the building. In 1509, 1556, 1754 and 1766 Earthquakes partial damage of the dome and in 1894, light damage at the dome of the structure were also recorded. A campaign of restoration of Hagia Sophia was initiated by the Sultan. The restoration work carried out by Gaspare and Giuseppe Fosatti brothers, Swiss-Italian architects during the period 1847-1849. They consolidated the dome and vaults, straightened the columns, and revised the decoration of the exterior and the interior of the building. They also closed some of the windows of the dome. 7 Figure 6. Different construction periods of the dome of Hagia Sophia (modified from Sato and Hidaka, 2001) Existing information regarding historical partial collapses, as well as locations of stress concentrations from linear analysis; and yield propagation and numerical collapse scenarios from the non-linear transient dynamic analysis provide relevant evidence that the detachment of the eastern and western semi-domes from the eastern and western main arches constitute the most important collapse mechanism in Hagia Sophia in the event of a large earthquake. One possible preventive action against this mechanism is to provide a continous tension tie system surrounding the base of the dome at a certain level to ensure a uniform dynamic behavior of semi-domes and the arches. Durukal et al. (2003) reported, by using the strong ground motion network installed on Hagia Sophia, that significant vertical vibrations at the crowns of the east and west main arches in Hagia Sophia probably indicate parts of the structure where most of the damage is to be expected during a major earthquake close to Istanbul. As a conclusion for Hagia Sophia, the historical records and modern analyses as well as imstrumental measurments indicate that there exists a differential movement of the arches, translated into a differential settlement at the base of the dome. This issue is the main problem of the dome of Hagia Sophia. 1.1. Atik Ali Paşa Mosque 1.1.1. Definition of the structural system and history of the structure The mosque was constructed in 1496, commissioned by one of the grand viziers of Sultan Beyazıt II. It is a small mosque with a main dome with 13.3m internal 8 diameter supported by a single semi-dome and with two smaller sized secondary domes on each side of the main dome to create a T-shaped plan (see Figure 7, Figure 8 and Figure 9). The importance of the mosque comes from the fact that it was built in very early times after the fall of the city to Ottomans. It is the oldest mosque in the city with distinct properties of a small-size classical Ottoman mosque. Despite the fact that the mosque possesses all properties of the classical type Ottoman mosques, the main piers, as shown in Figure 8, have Baroque style, a technique that was introduced to Ottoman architecture in the 18th century. There are available ambient vibration and material test results for this mosque in the literature (Thaskov and Krstevska, 2006). Figure 7. Atik Ali Paşa Mosque Figure 8. Interior of Atik Ali Paşa Mosque (see the middle pier with square Baroque cross sections) 9 Figure 9. Plan view of Atik Ali Paşa Mosque 1.1.2. Structural system The structural system consists of a main dome sitting on 3 main arches. One of the main arches is connected to the single semi-dome of the structure while the other two are supported by two smaller arches beneath jointly sitting on a single pier (Figure 9). Architecturally, and according to the tendencies followed by the architects of the time, these two main piers in the middle of the open area of the mosque should have been marble or granite columns. However what is seen outside today is a masonry pier with a square shape. The outer walls are three-leaf walls with two outer leafs being lime stone. The thickness of the outer walls is 1.8m in average. Most of the load of the dome is carried by the outer walls, a 3D analyses conducted by Bal et al. (Bal et al, 2010), however, shows that the vertical stresses on the piers is double of the outer walls because they have a smaller sectional area as compared to the tributary load they carry. 1.1.3. Structural Deficiencies and Reported Damages The structural details as well as reported damages for this building are rather limited in the existing literature, most probably due to the smaller size of the mosque as compared to other mosques and structures in the surrounding area. It is noted, however (Yüksel, 1983), that the dome was severly damaged during 1766 earthquake in which most of the structures in the area were severly damaged, and was retrofitted by replacing the main columns with brickwork piers. The dome, despite its small diameter, is quite sensitive to the movements below its supports, which are the main arches. The retrofitting due to a strong shaking also explains the Baroque style of the main piers, because the style dictates that the retrofitting operation must have been done in the 18th century. 1.2. Beyazıt Mosque 1.2.1. Definition of the structural system and the history of the structure Built between 1501 and 1506, Beyazıt is the oldest Royal Mosque in Istanbul 10 that follows the pattern of the Byzantine monument Hagia Sophia. The main structural shape of the mosque, consisting of two semi-domes and a main dome, springing on four massive piers, reminds the innovative layout of Hagia Sophia (see Figure 10, Figure 11 and Figure 12). Beyazit mosque was constructed just 15km away from Marmara Sea segment of the North Anatolian Fault, one of the most active faults in the world. In the last five centuries, in addition to many other smaller ones, the mosque had suffered 6 major earthquakes that occurred at a distance between 30 and 150km away from the structure and had varying surface magnitudes between 7.0 and 7.8. The mosque experienced a strong earthquake and repaired in 1509. 62 years after that earthquake, it was retrofitted by Sinan. Lav (2001) reports that the mosque lies on 48m thick soil layer, namely 6m infill, 17m green clay, 9m silty clay, 16m green clay and cracked greywacke as the bedrock material. Clay and silty clay layers have high level of plasticity, leading thus to amplification of earthquake waves even for the high amplitudes of the ground acceleration. Figure 10. Beyazıt Mosque Figure 11. Interior view of Beyazıt Mosque 1.2.2. Structural system 11 A classical Ottoman mosque, such as Beyazıt, consists of primary and secondary elements. Primary elements can be listed as a central dome, four or eight main arches, semi-domes (if any), secondary domes, thick outer walls and central columns. Secondary elements are pendentives, central dome supporters (if any), weighing towers (if any), circular secondary columns (granite or marble) and circumferential belt of the central dome. As opposed to the Byzantine churches, the circumferential belt is kept short and inclined accordingly with the dome. Pendentives may be the most interesting parts of these kind of structures and they can be defined as the extension of the main dome between the supporting arches as it has the shape of a curved equilateral triangle with its apex at the top of the main pier. Therefore, the arches have to carry not only the main dome but also most of the load acting on the pendentives directly. The mosque contains four great brick and cut-stone composed arches, springing from four stone piers that offer primary support to a central dome with 16.8m diameter and 36.5m height and to two semi-domes. The main arches under semi-domes initially had 90cm depth, however, the section depth was increased to 180cm at the crown level during retrofitting. There are available ambient vibration and material test results for this mosque in the literature (Thaskov and Krstevska, 2006). Original : Weak Arch (Brick) Existing : Strong Arch (Brick + Stone) Storng Arch (Brick+Stone) Central Dome Column Extension S6 43.50 m Semi Dome N 45.40 m Figure 12. Plan view of Beyazıt Mosque (after retrofitting by Sinan in 1574) The properties of the piers, which are constructed by traditional limestone material (küfeki), have been obtained from experimental results (TUBITAK, 2006). Brick masonry properties are not available for the mosque; however, the average properties of Hagia Sophia and Süleymaniye Mosques have been applied since the construction technique and the used material are close enough. The granite columns are red granite, mostly found in central Turkey, namely as Aksaray Red Granite. The characteristics are provided by the producers. Finally, the iron ties are assumed to be close to cast iron and to low quality steel. The details regarding the material properties of teh structure, ambient vibration tests results and their incorporation with an elastic 3D FE model can be found in. The results presented here, 12 regarding the safety of the dome before and after strengthening by Sinan, can be also found in (Bal et al., 2007a and 2007b; Sadan et al, 2007). Hagia Sophia, Beyazit Mosque and Suleymaniye Mosque can be considered as continuity of each other. The most pronounced similarity among these three structures is the structural truss as four main piers, two semi domes settled on arches and two perpendicular arches. It can be claimed that the designer of Suleymaniye Mosque, Sinan, had studied the deficiencies of the structure of the two aforementioned monuments and interestingly enough, he organized their retrofitting at the same time, between 1571 and 1574. The measures taken were also quite interesting and conceptually similar, since the deficiency which he was trying to diminish was originated from the same reason, i.e. the presence of weaker frames and arches, causing the swelling and differential movement of the main dome. He hesitated to apply the same intervention of adding arches and extending the columns inside Hagia Sophia, most probably because of the monumental importance of the structure in his era, avoiding thus altering the architectural characteristics and ratios. As distinct from Hagia Sophia and Beyazit Mosque, the stiffness of the four arches carrying the central dome of Suleymaniye Mosque and the rigidity of the frames supporting the arches are equal at each corner and each direction. Ambient vibration tests of three structures reveal that the stiffest among three is Suleymaniye Mosque, in line with the limited damage history in the past. The oldest among three, Hagia Sophia, has suffered a lot from the past destructive earthquakes, starting from 557. As a result, piers are inclined today, dome is stretched out, and the deterioration is clearly indicated by the ambient vibration measurements of modes. 1.2.3. Structural Deficiencies, Reported Damages and Repair Works Being guided by Hagia Sophia, the designer of Beyazıt Mosque repeated the most important drawback of it. Several studies (Mungan, 1988; Mungan and Turkmen, 1995; Şahin et al., 2005; Mungan, 2007) supported that deficiencies in the substructure of the main dome of Hagia Sophia could not be eliminated. The detrimental effect of these deficiencies could be reduced, if not eliminated completely, if the east and west arches between the main dome and the semi-domes were at least as strong as the arches in the perpendicular direction (5). Damage history of Beyazit Mosque reveals the weakness of the arches in one direction, even after retrofitting. The damage level is more pronounced for smaller angles between the dominant earthquake direction and the retrofitted arches. It should be noted that the earthquake direction is not the determining parameter of the damage; however, a distinction of the effect of the direction is easily made as shown in Figure 13. 13 Increasing Damage 1999, Ms=7.8 115 km, Damage : 2 1754, Ms=7.0 1766, Ms=7.2 30 km, Damage : 3 147 km, Damage : 2 1719, Ms=7.6 1894, Ms=7.0 175 km, Damage : 2 105 km, Damage : 3 1509, Ms=7.6 47 km, Damage : 4 N Direction of the weak arch Figure 13. Historical major earthquakes (larger than magnitude 7.0) structure so far having hit the In retrofitting by Sinan, a steeper arch was added below the two main arches rather than a perfectly circular arch. This may be explained in many ways, claiming that the reason was either architectural or structural, or even both. The effect of the added arch has been investigated in detail by researchers (Bal et al., 2007a and 2007b; Sadan et al, 2007). It has been reported that the stress distribution and the crack propagation along the piers and the arches obtained by the linear elastic analysis do not differentiate the three alternatives substantially. The vertical deflections, however, decreased susbtantially, decreasing the differential vertical displacement at the base of the dome in the order of magnitude, leading thus to much smaller tensile stress concentration on the dome. It can be clearly seen from Figure 14 that the possible tensile failure zones are much bigger in the original case, before Sinan’s retrofitting, leading to a possible collapse mechanism of the dome. On the contrary, Figure 15 represents the tensile zones almost parallel to the earthquake loading direction; however, tensile stresses do not penetrate below the dome, meaning that the partial collapse of the dome is unlikely. The strengthening by Sinan was done by using pointed steep arches. The use of pointed arches instead of circular ones, as well as the analyses presented here, show that the dome of the mosque is in danger when the drum and the base of the dome is not sitting on a uniformly moving support system. 14 Earthquake Direction (a) (b) Figure 14. Max. principal stresses of the dome before retrofitting by Sinan (a)view from bottom and (b) view from top (gray zones of the dome represent the possible tensile failure zones) Earthquake Direction (a) (b) Figure 15. Max. principal stresses of the dome after retrofitting by Sinan (a) view from bottom and (b) view from top (gray zones of the dome represent the possible tensile failure zones) 1.3. Fatih Mosque 1.3.1. Definition of the structural system and the history of the structure Fatih Mosque and its building complex (külliye) are one of the most important historical monuments in Istanbul. It was built by Atik Sinan during 1463 – 1470 and the mosque was the center of the first biggest building complex after conquest of Istanbul in 1453. Building complex has been consisted of 16 madrasah, hospital (darüşşifa, tabhane), public kitchen (imarethane) and Turkish bath (hamam). The main building was the mosque itself. The mosque collapsed severely during a strong shaking in 1766. Repair was not an easy task, apparently, thus the entire bearing system had to be changed and reconstructed. The single semi-dome asymmetric plan was transformed into a 4 semi-dome fully symmetric plan and the diameter of the dome was decreased around 25%. The plans of the original and the reconstructed versions can be seen in Figure 16. 15 Figure 16. Fatih Mosque – the original plan (left), the existing plan (right) The existing building, the new Fatih Mosque, has a square plan with a main dome (19 m diameter) in the center supported by the four semi domes. The domed system is supported by four arches standing on four pillars that are forming the main place (Figure 17 and Figure 18). The wall thickness for the outer walls is 1.50 m. Figure 17. Fatih Mosque 16 Figure 18. Fatih Mosque - interior Fatih Mosque, which was constructed in the period between Edirne Üç Şerefeli Mosque, Beyazid and Süleymaniye Mosques, with its 26 m diameter dome, has an important role in the evolution of Turkish mosque architecture. Although Fatih Mosque had been damaged and repaired after 1509, 1557 and 1754 earthquakes, it had subjected to devastating damages such as collapse of the main dome and collapse of the main walls after the biggest 1766 earthquake and totally collapsed. The existing building was started to be built in 1767 and was opened to service in 1771. 1.3.2. Structural System The structure of the first Fatih Mosque, which was constructed in 15th Century, was consisted of a main dome and a single semi-dome in the same elevation and three little domes in the lower level. The structure of this first building was described almost with all its details by Mehmet Ağaoğlu, Ali Saim Ülgen, Ekrem Hakkı Ayverdi and Robert Anhegger in spite of some disagreements of small details. Façade and the plan of the building by its 11m diameter dome were described in the illustration of Istanbul panorama and map of watercourse made by artist Lorichs from Flensburg. According to the available data, it is possible to say that the first structure of Fatih mosque is very similar with Atik Ali Paşa – Çemberlitaş Mosque (see Figure 16). According to the studies of M. M. Berilgen (Berilgen, 2007), the first Fatih Mosque (1463 and 1470) had been subjected to nine strong earthquakes and suffered various degrees of structural damage at every case, including the latest August 17, 1999 Kocaeli Earthquake, Mw = 7.4, with epicentral distance of approximately 100 km. Recently, a project has been initiated first to study the possible causes of earthquake damage and then develop retrofitting and strengthening techniques to protect this invaluable monument from further damages in the future earthquakes. As part of this investigation, local site soil conditions had been determined and site behavior during earthquakes had been studied in detail. The results of 1-D site response analysis, which included convolution and deconvolution analyses utilizing the strong ground motions recorded during the August 17, 1999 Kocaeli Earthquake are presented in. The results of the analyses had demonstrated the considerable degree of site amplification, compatible with the recorded motions and the 17 damage suffered. The expected site behavior during a probable future earthquake is also studied using a site specific simulated bedrock motion, and earthquake parameters to be used in dynamic structural analysis are estimated (Berilgen, 2007). 1.3.3. Structural Deficiencies, Reported Damages and Repair Works Building complex of Fatih Mosque passed through significant damages until the 22 May, 1766 Earthquake. There are lots of documents regarding repair works of Fatih building complex, particularly about construction of the new mosque. Most of documents include payment information, cost, type and amount of the used materials rather than repair techniques (Bir et al., 2013). nd Madrasah buildings and the mosque were heavily damaged and there is some information particularly about repair works done on madrasah buildings. Although Hospital building – (darrüşifa), which does not exist today, and was one of the important buildings of Fatih mosque building complex had been heavily damaged after the devastating 1766 earthquake, there is no any information about repair works and repair techniques of the building. As a result it is possible to say that all these facts and information regarding damages and repair works of Fatih Mosque building complex – külliye- are the evidences of observed damages today. The most detailed information about repair works after the earthquake is accessible on the Office of the Prime Minister Ottoman Archives. 2. Conclusions and Lessons Learnt Most of the important historical structures, not only in Istanbul but also in most of the historical cities of the world, possess a dome that is carried by an unreinforced masonry system beneath. The domes are spectacular in most of the cases, which has a pay-off of need for constructing a safe structure below. The dome covers the most of the closed space in a heritage building, and it is the most important element of a domed structure. The safety of the dome is not important only in terms of protecting lives but also keeping these marvelous buildings standing. Dome is a very strong structural element that has been used over centuries all around the world. A masonry dome is quite resistant to its own weight, creating a balanced compression-tension stress distribution scheme. The detrimental action for an unreinforced masonry dome, however, is differential deformations of the base. Whatever the reason is for that type of deformation pattern, the result is partial or total collapse of the dome. The four structures presented here, but also other examples known, prove that the base of the dome has to be “kept together”. The strengthening or preservation schemes to be used for kind of structures presented here should be consolidating the base of the dome not to allow differential deformations. Another conclusion the authors were able to withdraw from this study is that these structures, as presented here, have already resisted several strong earthquakes. Their design and way of responding to extreme actions should be respected and preserved as part of their heritage. Drastic changes in load bearing system would cause severe results. Acknowledgements The authors have always been enlightened by Prof. İhsan Mungan, before his untimely loss, in the area of behavior of historical structures, and in understanding domes in general. His 18 contribution is greatly appreciated and this chapter is dedicated to him. References Ambraseys, N. N. and C. F. Finkel (1991), “Long-term seismicity of Istanbul and of the Marmara Sea region”, Terra Nova, 3, pp: 527-539, 1991. Bal, İ. E., Sadan, O.B., Smyrou, E., and Gulay, F.G. (2007a) “Earthquake Resistance of Beyazit II Mosque, Istanbul”, IASS 2007 Conference, Shell and Spatial Structures: Structural Architecture - Towards the future looking to the past, December, Venice, Italy. Bal, İ. E., Sadan, O.B., Smyrou, E., and Gulay, F.G. (2007b) “Beyazit II: A Retrofitted Mosque Five Centuries Ago”, SEWC 2007: 3rd Structural Engineers World Conference, November, Bangalore, India. Bal İ. E., Gülay F. G., and Smyrou, E. (2010) “A Nonlinear Analysis Approach for Defining the Seismic Behaviour of Historical Masonry Buildings: Case Study of Atik Ali Paşa Mosque in Istanbul”, Proceeding of the Turkish-Saudi Workshop on Structural and Earthquake Engineering, (Ilki et al. Eds), ITU, Istanbul. Berilgen M.M. (2007), “Evaluation of local site effects on earthquake damages of Fatih Mosque”, Engineering Geology, Volume 91, Issues 2–4, 22 May 2007, Pages 240–253 Bir A., Barutçu B., Kaçar M. 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