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The three-arched middle Bronze Age gate at Tel Dan - A structural investigation of an extraordinary archaeological site
Construction and Building Materials 41 (2013) 950–956
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
The three-arched middle Bronze Age gate at Tel Dan - A structural investigation of an extraordinary archaeological site Rosa Frances ⇑ Department of Architecture, The Neri Bloomfield WIZO School of Design and Education, Haifa, Israel Faculty of Architecture and Town Planning, Technion Israel Institute of Technology, Haifa, Israel
h i g h l i g h t s " The three sun-dried mud–brick arches of Tel Dan’s gatehouse are a unique structure. " These arches are the world’s oldest known complete and free-standing ‘true’ arches. " The arches of Tel Dan in a brief archaeological perspective. " Structural and technical investigation of the arches and gate’s untimely burying. " The construction technique as a perception of arches’ structural development.
a r t i c l e
i n f o
Article history: Available online 7 September 2012 Keywords: Bronze Age masonry Gatehouse Structural investigation Arch Sun-dried mud–brick structure
a b s t r a c t The three sun-dried mud–brick arches of Tel Dan spanning the gatehouse passageway are the world’s oldest known complete and free-standing monumental ‘true’ arches made of this material. This gatehouse is dated to the mid-18th century BC, and stands today more than 7 m high. The city gate was excavated during the 1978–1985 seasons, revealing a vast gatehouse in a remarkable state of preservation. All its three arches were soundly constructed in three concentric radial courses. The archway owes its extraordinary state of conservation to the fact it was filled and buried by the inhabitants after only some years of use. The paper presents the structural and technical investigation of the performance and failure of this unique historic building form, putting it in an archaeological context and discusses the research methods and findings giving a picture of the strength and limitations of the mud–brick arches – extraordinary representatives of historic building technologies. The article puts forward a hypothesis for the construction technique, which could provide a perception of the architectural development of arches and an aspect of the dynamic of learning – in progress in the building cultures. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The Canaanite city of Laish – later the biblical Dan – is an archaeological site in North Israel. Located at the foot of Mt Hermon and the Golan Heights and fed by one of the tributaries of the Jordan, the Dan Spring, Laish was a fertile station on the ancient caravan route from Egypt to Syria. The city is mentioned since the 19th century BC in ancient Egyptian and Mesopotamian texts. During the 18th century BC, Laish was fortified with huge manmade earthen embankments which created ramparts encircling the entire city. The ramparts of Canaanite Dan constitute one of the best examples of the defense systems common in that period.
⇑ Address: Department of Architecture, The Neri Bloomfield WIZO School of Design and Education, Haifa, Israel. Tel.: +972 48251790; fax: +972 48340053. E-mail address: [email protected] 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.07.015
Excavations led by Professor Abraham Biran from the Hebrew Union College-Jewish Institute of Religion’s archaeological expedition (HUC-JIR), have been carried out at Tel Dan (as the site is known today) since 1966. The impressive findings included sections of imposing walls and gates, as well as a ritual site dating back to the time of dramatic events recounted in the Bible (Genesis 14:14). A major finding was the uncovering of a complete multicolored mud–brick city gate on the eastern side of the city, dating from the Middle Bronze Age [1], (Figs. 1 and 2). The gate was excavated during the 1978–1985 seasons, revealing a vast gatehouse in a remarkable state of preservation. This gatehouse, 15.45 m wide and 13.5 m deep, is dated to the mid-18th century BC, and stands today more than 7 m high. Built entirely of sun-dried mud–bricks surviving today as high as 47 courses, the gate is the best-preserved mud brick Bronze Age gate in the Near East [2]. Three enormous intact arches, framing the entryway into the city, among the earliest known examples of an arched structure,
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Fig. 1. Left: Tel Dan’s gatehouse model, adapted from [3]. Right: proposed restoration model, adapted from [5].
Fig. 2. Left: gate complex showing the eastern arch and the towers – 1993, adapted from [6]. Right: the eastern façade – now protected under a modern shelter, adapted from [7], with descriptive overlay highlighting the arch – 2005.
Fig. 3. Left: gate showing the north tower and recessed archway with steps leading up to it from the east. Earlier steps are visible at the right – 1980. Right: the western façade; going down to the city (today covered); adapted from [3].
are the most remarkable elements of this gate. The arches of Tel Dan – built more than 1500 years before the Romans used arches in their construction – demonstrate expert ancient masonry engineering and fine aesthetic qualities. The gate is noteworthy not only because of its complete three arches and its historic significance, but also because of its excellent state of preservation. The Canaanite gate at Tel Dan preserves the earliest intact archway in the world, nearly 3700 years old. Since their discovery, researchers
have wondered whether these three monumental sun-dried mud– brick arches are not the world’s oldest known complete and free – standing true arches. A second gate structure, albeit less completely preserved, featuring a true arch of the same Canaanite period, has been excavated at the site of ancient Ashkelon – a seaport in the South Israel, by Lawrence Stager of Harvard University. The top of the arch collapsed in antiquity (which may explain the reason why
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Fig. 4. Left: voussoirs of east arch; note the horizontal wall bricks abutting the arch. Right: close-up to the bricks and mortar of east arch – 2005; adapted from [3].
Fig. 5. Left: section showing east arch and tower. Right: the west arch; adapted from [4].
Fig. 6. Left: the middle arch. Right: isometric view of the gatehouse (the roof is restored); adapted from [4].
the gate was filled in, thereby preserving it until now), but the dimensions of the ancient city entrance could be determined and its original appearance has been recently reconstructed [8]. The gateway of Tel Dan owes its remarkable state of preservation to the fact it was filled and buried to become part of the core structure of the ramparts under added earthen embankments after only some years of use. The arches survived the millennia covered in soil. As with all archaeological excavations, the site was susceptible to damage once exposed. Deterioration of the mud–brick began soon after the excavation in 1979 uncovered the eastern face of the gate. For protection, the Israel Antiquities Authority backfilled the central arch and western face of the gate and erected a roof
over still-visible surfaces. While this is a good preventive measure, there are still problems with slow erosion, insect, bird infestation, and deterioration as the result of other natural factors. These days the Israeli Nature and Park Authority and the Israeli Antiquities Authority are actively working for the restoration and conservation of Tel Dan’s ‘city gate’.
2. Description of the three arched-gate of Tel Dan According to Biran [9] the gate complex consists of two towers, each 5.15 m wide, flanking a recessed, arched gateway, also 5.15 m
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Fig. 7. Left: Creating the boundaries of the load path for a ‘true’ arch action. Right: possible natural lines of pressure, including the lower bound equilibrium state.
wide. The entire structure is thus 15.45 m wide. The Tel Dan is a four-chamber gate constructed of sun-dried multi-colored mud bricks (Fig. 6 on the right). It has three arched narrower entry-ways with four rooms. Stone steps lead to the first – eastern gate (Fig. 3 on the left). The middle gate is covered by a square building. The third western-gate leads out to a flight of steps down into a street (Fig. 3 on the right). While headway along the passageway is 2.5– 2.6 m high, the soffit of the eastern entry arch is some 3.1 m above the bottom threshold step. The middle and western arches offer 2.5 m of headroom. The three arches of this gatehouse are constructed of three superimposed concentric radial courses, formed what is known as a ‘basket’ arch (Figs. 5 and 6 on the left). All the three survived with significant portions of mortar, edging fill and white plaster (made of lime and calcite), still adhere to the joints between the courses (Fig. 4). The three courses and the mortar measure together roughly 1.25 m at the spring, but only ca. 0.95 m at the top of the arch. The vast majority of the exposed bricks of all three arches are rectilinear in section. The longest exposed arch brick is 45 cm long and the shortest 20 cm, the thickest is 18 cm thick and the thinnest is 8 cm [2]. The apparently smaller size of many of the visible bricks suggests they were both cut down and shaved to fit as needed during construction of the three arched courses, or that smaller bricks to a nearly triangular section were specially made for the arches. The tops of some of the third course bricks may have been shaved to create a more level line for the horizontal courses to follow. Wedge-shaped mortar joints and creative brick laying were artfully created to produce the spring for all three rings of the three arches. ‘These monumental arches exhibit expertise engineering, practical knowledge and aesthetic qualities, particularly in areas where timber or stone for trabeated construction would have been readily available’ [3] and [4].
3. The arches of Tel Dan in a brief archaeological perspective According to Mainstone [11] ‘all developments must have a beginning or perhaps more than one. . . The brick arch and the stone voussoir arch may be taken as the first fully developed forms. Neither exists as a direct natural prototype, nor do we find the monolithic natural arch in any of the areas where they first appeared’. Among the early man-made forms that may have contributed to their developments are: the accidentally-wedged boulder, the two long blocks of stone inclined inwards to meet as an inverted V, or alternatively two bundles similarly inclined and tied
together. Alongside these forms, we should also note the early and widespread construction of the ‘false’ arch, created by corbelling out the masonry blocks. Its distinguishing characteristic is that all the bricks are bedded horizontally on one another. At each course they project slightly beyond those of the course below, so as to narrow progressively the gap to be spanned. This method of construction was used for roofing internal chambers of the Egyptian Pyramids. According to Mainstone [11] ‘The chief merit of all these early forms was that they could be constructed with little or no centering or other temporary support. . . Their head shortcomings were the very limited spans.’ The false arch was the least efficient form evolving bending moments and therefore cracking tensile stresses. The simple true arch of mud–brick pointed the way to more efficient forms. Its direct further development seems to have been very closely linked with that of its longitudinal extension the barrel vault. That vault was constructed as a continuous barrel by first building an end wall and then inclining each successive ring of bricks back towards to it. Each brick was pitched on edge to give the maximum surface in contact with the previous ring. ‘To adapt this procedure to the construction of a free-standing arch called only for the provision of light centering to provide temporary support for the first rings of the bricks set on edge. When one or more superimposed concentric rings had been so constructed, additional stiffness could be imparted by adding further rings with the bricks set in what is now the normal way with their longer dimensions running radially and at right angles to the face of the arch’ [11]. A key question of the investigation of the unique historic building form of Tel Dan gate is the construction technique of the arches. This might provide an understanding of the developments in architectural and structural form of arches, as well as of the establishment of new building technologies and their lateral expansion. The paper puts forward the following hypothesis of the construction technique of Tel Dan’s arches. In Fig. 9 are sketched lines that follow the path of the mortar joints between the bricks. Extrapolating the lines one can find that they meet approximately at three different points, the first at the center and the others at the intersection between the intrados and the line passing through the spring of the arch. This kind of construction hints the building of a primordial three-centered basket arch – a round arch whose inner curve is drawn with circles having three centers. According to Mainstone [11] ‘in unreinforced masonry the construction becomes possible only with forms that can be built thought from a firm base provided by what has been built hitherto’. In the case of Tel Dan arches, the builders probably
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Fig. 8. Geometries of equilibrium. The actions of the concentring blocks are treated as lumped masses applied at their center of gravity. Left: the magnitudes of the forces are transferred to the force polygon. Right: (1) the lower bound equilibrium state i.e. the maximum trust state representing the most horizontal trust; (2) internal trust lines close to the minimum trust state – the extremity representing the least horizontal trust, the structure transfers to its abutments.
Fig. 9. Drawing lines between the bricks of the concentric rings. The lines meet at three different points, conceiving the idea for a primordial three-center arch construction. Left: the eastern arch. Right: the western arch.
put in place successive bricks in a radially pattern, using a geometric tool – perhaps a thread, at the right and left centers of the basket arch and subsequently rotating it at equal angles. The bricks held together by the boding action of the rapidly setting mortar, until gravitation load overcame the friction. From this position a light centering has probably been used, until all the remaining bricks, arranged with the help of the thread – posed this time at the center, have been set. When the first concentric ring had been constructed, two more rings were added with the bricks radially to the face of arch. While on the way to true arched construction, the Tel Dan arches remain within the realm of corbelled construction. They are among the first known evidences of the building of self standing arches, a ring between the continuous barrel vault, the ‘false’ – corbelled and the ‘real’ arch. Furthermore these arches offer a view of the development and expansion of historic building technologies as will be explained next. Despite our usual association of arches with large-scale Roman stone architecture, load-bearing arches are known and have in fact been in use in Mesopotamia and Egypt at least since the 3rd millennium BC. The shortage of timber was a fact of Mesopotamian life that probably dictated the adoption of the vaulting. According to Oates [12] ‘the construction of the vaults with the pattern of voussoirs laid radially was known in Mesopotamia since well be-
fore 3000BC, though usually in underground structures were the ground rather than the associated wall takes the thrust of the vault. . . In the 3rd millennium three techniques of vaulting were in simultaneous use – radial, pitched-brick and corbelling’. Earliest vaults are mentioned in archaeological reports from Ur, Mumbqat and Tell al-Rimah all in ancient Mesopotamia. All do exhibit some cantilevering – produced by the alternating long and short bricks of the first course. The pitched vaults – simple tunnels and low-profile pitched domes of the great temple at Tell al Rimah in northern Mesopotamia constructed late third and early second millennium BC, are the earliest known above-ground pitched vault structures. Aboveground freestanding arches are more unstable than vaults with their greater mass and support system. The length of the vault in relation to the width of the span affords the vault a far more massive support than that of an arch with the same span. An end wall, even when not carrying the weight of the vault, does provide closure and an element of protection against twisting and shearing. According to the archaeologist Gila Cook from HUC-JIR ‘freestanding aboveground monumental archways have been excavated at only three sites: Ashkelon, Mumbqat and Tel Dan. The Ashkelon gate, on the Mediterranean coast, is dated by the excavator to ca. 1780 and may be the earliest. Mumbqat, situated on the east bank of the great bow of the Euphrates in Mesopotamia, is dated by the excavator to the 16–15th century BC and is the
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latest. Tel Dan is located at the headwaters of the Dan and Jordan Rivers south of Mt. Hermon and is dated to the second half of the 18th century BC. This arch offers a view of the dynamic of learning – a work in progress’. According to Schaffer [10] ‘Israel’s historical building technology is not different or outstanding in any of its technological characteristics relative to the Mediterranean Sea region and the Near East culture. . . This is characterized by public buildings, city, gates, towers, walls and palaces, which are present in almost all the antique tells. . . The technology of mud bricks consists of mud bricks reinforced with straw and mud bricks reinforced with crushed stone-technologies that were used both in Egypt and in Mesopotamia. . . Since Israel is both the crossing point and passage for major historical events and cultures, it is not surprising that building technologies have been brought to Israel relative in a count period from their ‘‘official date’’ (original) appearance’. The mud–brick arches of Tel Dan are the surviving witnesses of the building technologies of the Middle Bronze Age in the Fertile Crescent, the region being often considered as the cradle of civilization, the birthplace of writing and the wheel, the scene of struggles and migrations of the earliest human civilizations. Some additional findings explained in next section, will enhance the picture of the adaptation and transfer of technologies in these historic building cultures.
4. Static investigation of the arched gate of Tel Dan In order to give a comprehensive assessment of the performance and abandonment of the gate some other questions are investigated. The first one inquires whether the arches of Tel Dan are really ‘true’ arches according to the structural definition, that is to say: these are compressive form-active shapes which carry purely axial load, without bending [17]. The second one investigates whether there was a structural reason for the construction of the arches in three courses, instead of one, or this was an aesthetic principle. Another question examines the reason of choice of an arch rather than the simpler and more economical post and beam system for these spans, inquiring why the builders build a mud–brick structure in an area blessed with excellent wood and stone. And finally what was the reason for the early burying of the gatehouse. For these aforementioned inquiries a simplified method has been chosen as satisfactory to give the appropriate answers. The absence of more accurate information required for advanced structural analysis has enhanced this decision. Essential missing data is the characterization of the mechanical properties of the masonry due to its extraordinary age, which has caused significant physical changes in the constitution, as well as due to the mixed use of different kinds of bricks with various physical properties and mortar. Nevertheless, the mechanical behavior of masonry customarily has one salient feature: a very low tensile strength. This property is so important that it has determined the forms of historical constructions introducing the funicular shapes of vaults and arches. According to Heyman [13] ‘stress requirements, such as needed for the design of steel or reinforced concrete, are transformed into geometrical requirements for the design of masonry. The shape of a masonry element must obey simple rules’. The grandness of the arch is its capacity to convert the gravitational loads into compressive stresses in the voussoirs and by doing so, the perfect harmony between form, load and material is achieved. Furthermore, according to Lourenço and Roca [14] ‘as a rule, the geometric idealization of historical constructions should be kept as simple as possible, as long as it can be considered adequate. If possible it is better to use two-dimensional models than three – dimensional idealizations’. Consequently a two-dimensional mod-
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el has been considered as adequate for this investigation. Moreover, the current analysis focuses on equilibrium equations – keeping in mind that equilibrium equations are most important when analyzing historical structures. ‘In essence, the internal forces in the element, which are subject to the laws of static equilibrium, must be within the boundaries of the masonry’ [13]. Given the abovementioned reasons, the current structural analysis was based on a generally accepted conservative simplification which assumes no tensile strength for the masonry. Furthermore the investigation was based on the thrust line analysis, using a twodimensional model and applying basic graphic statics. (The lines of trust are theoretical lines visualizing and representing the path of the resultants of the compressive forces on the voussoirs’ section.) The analysis makes use of the Safe Theorem for masonry structures, also known as the Lower Bound theorem according to which ‘if it is possible to demonstrate at least one possible equilibrium state, then the structure can also find at least one possible state’, [15,16]. Therefore in this analysis the range of the path of internal lines of trust – the natural lines of pressure – has been drawn on the geometric configuration of the Tel Dan gate. In Fig. 7 on the right and Fig. 8 some possible paths of resultant forces are demonstrated, all lying within the extrados and intrados of the arch and representing possible equilibrium states for the arches. This simple method has the power to prove that the monumental arches of Tel Dan gate are very early really ‘true’ arches according to the structural definition [17]. Moreover, the absence of cracks in the structure proves that any sliding of the arches’ support did not occur, and that the foundation was very stable, giving no structural reason for failure. In addition, the analysis puts forward a structural reason for the three courses’ construction: Fig. 8(2) on the right demonstrates a possible long path contained entirely within the arch. This path releases a low horizontal force which in turns causes a low thrust and a very low stress level, one more suitable for masonry. The three concentric rings therefore, by creating a wide cross-sectional shape, they decrease on one hand the magnitude of compressive stresses and on the other they permit vast load trajectories. Hence, the construction of the arches in three courses instead of one – besides aesthetic values – increases the safety factor of the structure which is dominated by the fragile nature of the unbaked brick. The three sun-dried mud–brick arches of Tel Dan stimulate the investigators to enquire the causes of choosing mud–brick sun dried arches rather than the simpler post and beam system, in an area blessed with excellent wood and stone. The paper suggests the following reasons: Traditional materials are divided into those that are combustible and those lacking tensile strength. Wood is a burnable material and therefore inappropriate for the city’s defense; post and beam stone would have been the perfect solution. However, due to beam’s low tensile strength, its capacity for efficiently bridging wide spans or carrying heavy loads from the upper structure is limited. This limitation would bear the idea for a stone arch, yet this kind of construction requests the labor of accurately pre-cutting the very strong volcano stone found at this area, and the need to provide much heavier centering than that for brick arches. According to Schaffer [10] ‘the stage of mud–brick technology existed in public construction of Israel for more than 1000 years, until it began to lose popularity and be replaced by local stone architecture and technology that had been influenced by the Phoenicians, the builders of the Jerusalem Temple from stone and wood, and by other Mediterranean Islands’ population invasions such as the Filisteas, who also brought the Iron Technology necessary for a good stone cut and dressing’. Therefore, earth bricks for building the gate of Tel Dan in the Middle Bronze Age, were an unexpected and surprising choice for us, yet it was the natural choice for the builders, who according to Oates [12] ‘displayed a familiarity with their techniques and material that could only derive from long tradition and knowledge at this kind of construction’.
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Stability rather than failure of the material is usually the dominant concern of the masonry. Collapse occurs when the load path can no longer be contained within the masonry, although this is not the case of Tel Dan arches. This fact makes the burying of the arches even more intriguing. Next the paper adopts the following structural cause for the gatehouse’s untimely burying: the failure of the material was the dominant concern of this gate. With its extreme humidity, the local environment in the Golan Heights is not particularly kind to monumental mud–brick architecture. The builders realized that the seepage of rain through the bricks of the inner core, in addition to local erosion do not permit such a structure to survive for long and therefore they filled in and buried it under added earthen embankments. This act may suggest that the builders were foreigners who came to Dan from Mesopotamia, where mud–brick was the default building material – since wood and stone were unavailable and where the climate was more conducive to mud–brick edifices (100 mm of yearly rainfall on average, versus ca. 600 mm at Tel Dan) [18,19]. Another gate, this time of stone and wooden beams, was constructed on the south side of the hill beneath this Iron Age gate visitors pass through today. 5. Concluding remarks For nearly 4000 years, a small gateway into a forgotten Canaanite city lay hidden beneath the dry soil of Tel Dan in northern Israel, until a team of archaeologists excavated the site in 1976. This year (2009), Tel Dan’s triple-arched gate is a candidate on the Tentative List for inscription in UNESCO’s manifest of World Heritage Site in the field of the History of Technology [20]. Its contribution derives from the discovery of the world’s oldest known free-standing structural form of arch, a form which has played a crucial role in the service of the humanity. The load-bearing arches of Tel Dan – built more than 1500 years before the Romans used arches in their construction – demonstrate expert ancient masonry engineering and fine aesthetic qualities. Moreover, the arches of Tel Dan constitute the connective vertebra in the bilateral development between the ‘false’ corbelled arch to the design of the ‘real’ arch. From a cultural point of view, Tel Dan offers an aspect of the dynamic of learning-in progress, being itself probably a direct, but certainly a lateral architectural development in the Ancient World. Furthermore, according to Dr. David Ilan [19] from HUC-JIR, Tel Dan reveals another story, perhaps one of an adaptation of technology
that not suited to a different environment from its original application. Acknowledgements The author wish to express her acknowledgement to the archaeologist Gila Cook – Head Surveyor, Tel Dan Excavations, Hebrew Union College (HUC-JIR) Jerusalem for the offer of important archaeological material, her advice and help. The financial support of the Neri Bloomfield WIZO School of Design and Education, Haifa, is greatly acknowledged. References [1] http://whc.unesco.org/en/tentativelists/1469/; 2009 [07.08.09]. [2] Biran A. The triple-arched gate of Laish at Tel Dan. Israel Explor J 1984;34:1–19. [3] Frances R, Cook G. The Tel Dan arches – early true arches, Part 1 – archaeological excavation. In: The 3rd symposium on the conservation of the built heritage – conservation of historic buildings and sites – from study to realization, the old city of Acre, Israel; January 4, 2007. [4] Frances R, Cook G. The Tel Dan arches – early true arches, Part 2 – structural investigation. In: The 3rd symposium on the conservation of the built heritage – conservation of historic buildings and sites – from Study to Realization, the old city of Acre, Israel; January 4, 2007. [5] http://www.jewishmag.com/59mag/dan/title.jpg; 2009 [07.08.09]. [6] http://www.graal.co.uk/kingdavid4.jpg; 2009 [07.08.09]. [7] http://www.bibleplaces.com/dan.htm; 2009 [07.08.09]. [8] Schloen D. Recent discoveries at Ashkelon. University of Chicago, The Oriental Institute; 2007. http://oi.uchicago.edu/research/pubs/nn/spr95_ash.html/; 2009 [07.08.09]. [9] Biran A. The discovery of the Middle Bronze Age gate at Dan. Biblic Archaeol 1981;44(3):139–44. [10] Shaffer Y. Changes in the building technology in Israel from ancient times until the 20th century and their influence on everyday life; 2000. http:// www.unesco.org/archi2000/pdf/schaffer.pdf/; 2009 [07.08.09]. [11] Mainstone R. Developments in structural form. Architectural Press; 1998. p. 76–77, 97–101. [12] Oates D. Innovations in mud–brick: decorative and structural techniques in ancient mesopotamia. World Archaeol, Archit Innov 1990;21(3):388–406. [13] Heyman J. Why ancient cathedrals stand up, the structural design of masonry; 2001, http://www.ingenia.org.uk/ingenia/issues/issue10/heyman.pdf/; 2009 [07.08.09]. [14] Lourenço P, Roca P. Analysis of historical constructions: from trust-lines to advanced simulations. Historical Constructions, Guimarães; 2001. p. 91–116. [15] Heyman J. The stone skeleton. Int J Solids Struct 1966;2:249–79. [16] Heyman J. The stone skeleton: structural engineering of masonry architecture. Cambridge University Press; 1997. [17] Macdonald A. Structure and architecture. Architectural Press; 2001. p. 37–47. [18] http://teldan.wordpress.com/mudbrick-gate; 2009 [07.08.09]. [19] http://huc.edu/newspubs/pressroom/2005/1/glueck.shtml; 2009 [07.08.09]. [20] http://whc.unesco.org/en/statesparties/il/; 2009 [07.08.09].
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
The three-arched middle Bronze Age gate at Tel Dan - A structural investigation of an extraordinary archaeological site Rosa Frances ⇑ Department of Architecture, The Neri Bloomfield WIZO School of Design and Education, Haifa, Israel Faculty of Architecture and Town Planning, Technion Israel Institute of Technology, Haifa, Israel
h i g h l i g h t s " The three sun-dried mud–brick arches of Tel Dan’s gatehouse are a unique structure. " These arches are the world’s oldest known complete and free-standing ‘true’ arches. " The arches of Tel Dan in a brief archaeological perspective. " Structural and technical investigation of the arches and gate’s untimely burying. " The construction technique as a perception of arches’ structural development.
a r t i c l e
i n f o
Article history: Available online 7 September 2012 Keywords: Bronze Age masonry Gatehouse Structural investigation Arch Sun-dried mud–brick structure
a b s t r a c t The three sun-dried mud–brick arches of Tel Dan spanning the gatehouse passageway are the world’s oldest known complete and free-standing monumental ‘true’ arches made of this material. This gatehouse is dated to the mid-18th century BC, and stands today more than 7 m high. The city gate was excavated during the 1978–1985 seasons, revealing a vast gatehouse in a remarkable state of preservation. All its three arches were soundly constructed in three concentric radial courses. The archway owes its extraordinary state of conservation to the fact it was filled and buried by the inhabitants after only some years of use. The paper presents the structural and technical investigation of the performance and failure of this unique historic building form, putting it in an archaeological context and discusses the research methods and findings giving a picture of the strength and limitations of the mud–brick arches – extraordinary representatives of historic building technologies. The article puts forward a hypothesis for the construction technique, which could provide a perception of the architectural development of arches and an aspect of the dynamic of learning – in progress in the building cultures. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The Canaanite city of Laish – later the biblical Dan – is an archaeological site in North Israel. Located at the foot of Mt Hermon and the Golan Heights and fed by one of the tributaries of the Jordan, the Dan Spring, Laish was a fertile station on the ancient caravan route from Egypt to Syria. The city is mentioned since the 19th century BC in ancient Egyptian and Mesopotamian texts. During the 18th century BC, Laish was fortified with huge manmade earthen embankments which created ramparts encircling the entire city. The ramparts of Canaanite Dan constitute one of the best examples of the defense systems common in that period.
⇑ Address: Department of Architecture, The Neri Bloomfield WIZO School of Design and Education, Haifa, Israel. Tel.: +972 48251790; fax: +972 48340053. E-mail address: [email protected] 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.07.015
Excavations led by Professor Abraham Biran from the Hebrew Union College-Jewish Institute of Religion’s archaeological expedition (HUC-JIR), have been carried out at Tel Dan (as the site is known today) since 1966. The impressive findings included sections of imposing walls and gates, as well as a ritual site dating back to the time of dramatic events recounted in the Bible (Genesis 14:14). A major finding was the uncovering of a complete multicolored mud–brick city gate on the eastern side of the city, dating from the Middle Bronze Age [1], (Figs. 1 and 2). The gate was excavated during the 1978–1985 seasons, revealing a vast gatehouse in a remarkable state of preservation. This gatehouse, 15.45 m wide and 13.5 m deep, is dated to the mid-18th century BC, and stands today more than 7 m high. Built entirely of sun-dried mud–bricks surviving today as high as 47 courses, the gate is the best-preserved mud brick Bronze Age gate in the Near East [2]. Three enormous intact arches, framing the entryway into the city, among the earliest known examples of an arched structure,
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Fig. 1. Left: Tel Dan’s gatehouse model, adapted from [3]. Right: proposed restoration model, adapted from [5].
Fig. 2. Left: gate complex showing the eastern arch and the towers – 1993, adapted from [6]. Right: the eastern façade – now protected under a modern shelter, adapted from [7], with descriptive overlay highlighting the arch – 2005.
Fig. 3. Left: gate showing the north tower and recessed archway with steps leading up to it from the east. Earlier steps are visible at the right – 1980. Right: the western façade; going down to the city (today covered); adapted from [3].
are the most remarkable elements of this gate. The arches of Tel Dan – built more than 1500 years before the Romans used arches in their construction – demonstrate expert ancient masonry engineering and fine aesthetic qualities. The gate is noteworthy not only because of its complete three arches and its historic significance, but also because of its excellent state of preservation. The Canaanite gate at Tel Dan preserves the earliest intact archway in the world, nearly 3700 years old. Since their discovery, researchers
have wondered whether these three monumental sun-dried mud– brick arches are not the world’s oldest known complete and free – standing true arches. A second gate structure, albeit less completely preserved, featuring a true arch of the same Canaanite period, has been excavated at the site of ancient Ashkelon – a seaport in the South Israel, by Lawrence Stager of Harvard University. The top of the arch collapsed in antiquity (which may explain the reason why
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Fig. 4. Left: voussoirs of east arch; note the horizontal wall bricks abutting the arch. Right: close-up to the bricks and mortar of east arch – 2005; adapted from [3].
Fig. 5. Left: section showing east arch and tower. Right: the west arch; adapted from [4].
Fig. 6. Left: the middle arch. Right: isometric view of the gatehouse (the roof is restored); adapted from [4].
the gate was filled in, thereby preserving it until now), but the dimensions of the ancient city entrance could be determined and its original appearance has been recently reconstructed [8]. The gateway of Tel Dan owes its remarkable state of preservation to the fact it was filled and buried to become part of the core structure of the ramparts under added earthen embankments after only some years of use. The arches survived the millennia covered in soil. As with all archaeological excavations, the site was susceptible to damage once exposed. Deterioration of the mud–brick began soon after the excavation in 1979 uncovered the eastern face of the gate. For protection, the Israel Antiquities Authority backfilled the central arch and western face of the gate and erected a roof
over still-visible surfaces. While this is a good preventive measure, there are still problems with slow erosion, insect, bird infestation, and deterioration as the result of other natural factors. These days the Israeli Nature and Park Authority and the Israeli Antiquities Authority are actively working for the restoration and conservation of Tel Dan’s ‘city gate’.
2. Description of the three arched-gate of Tel Dan According to Biran [9] the gate complex consists of two towers, each 5.15 m wide, flanking a recessed, arched gateway, also 5.15 m
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Fig. 7. Left: Creating the boundaries of the load path for a ‘true’ arch action. Right: possible natural lines of pressure, including the lower bound equilibrium state.
wide. The entire structure is thus 15.45 m wide. The Tel Dan is a four-chamber gate constructed of sun-dried multi-colored mud bricks (Fig. 6 on the right). It has three arched narrower entry-ways with four rooms. Stone steps lead to the first – eastern gate (Fig. 3 on the left). The middle gate is covered by a square building. The third western-gate leads out to a flight of steps down into a street (Fig. 3 on the right). While headway along the passageway is 2.5– 2.6 m high, the soffit of the eastern entry arch is some 3.1 m above the bottom threshold step. The middle and western arches offer 2.5 m of headroom. The three arches of this gatehouse are constructed of three superimposed concentric radial courses, formed what is known as a ‘basket’ arch (Figs. 5 and 6 on the left). All the three survived with significant portions of mortar, edging fill and white plaster (made of lime and calcite), still adhere to the joints between the courses (Fig. 4). The three courses and the mortar measure together roughly 1.25 m at the spring, but only ca. 0.95 m at the top of the arch. The vast majority of the exposed bricks of all three arches are rectilinear in section. The longest exposed arch brick is 45 cm long and the shortest 20 cm, the thickest is 18 cm thick and the thinnest is 8 cm [2]. The apparently smaller size of many of the visible bricks suggests they were both cut down and shaved to fit as needed during construction of the three arched courses, or that smaller bricks to a nearly triangular section were specially made for the arches. The tops of some of the third course bricks may have been shaved to create a more level line for the horizontal courses to follow. Wedge-shaped mortar joints and creative brick laying were artfully created to produce the spring for all three rings of the three arches. ‘These monumental arches exhibit expertise engineering, practical knowledge and aesthetic qualities, particularly in areas where timber or stone for trabeated construction would have been readily available’ [3] and [4].
3. The arches of Tel Dan in a brief archaeological perspective According to Mainstone [11] ‘all developments must have a beginning or perhaps more than one. . . The brick arch and the stone voussoir arch may be taken as the first fully developed forms. Neither exists as a direct natural prototype, nor do we find the monolithic natural arch in any of the areas where they first appeared’. Among the early man-made forms that may have contributed to their developments are: the accidentally-wedged boulder, the two long blocks of stone inclined inwards to meet as an inverted V, or alternatively two bundles similarly inclined and tied
together. Alongside these forms, we should also note the early and widespread construction of the ‘false’ arch, created by corbelling out the masonry blocks. Its distinguishing characteristic is that all the bricks are bedded horizontally on one another. At each course they project slightly beyond those of the course below, so as to narrow progressively the gap to be spanned. This method of construction was used for roofing internal chambers of the Egyptian Pyramids. According to Mainstone [11] ‘The chief merit of all these early forms was that they could be constructed with little or no centering or other temporary support. . . Their head shortcomings were the very limited spans.’ The false arch was the least efficient form evolving bending moments and therefore cracking tensile stresses. The simple true arch of mud–brick pointed the way to more efficient forms. Its direct further development seems to have been very closely linked with that of its longitudinal extension the barrel vault. That vault was constructed as a continuous barrel by first building an end wall and then inclining each successive ring of bricks back towards to it. Each brick was pitched on edge to give the maximum surface in contact with the previous ring. ‘To adapt this procedure to the construction of a free-standing arch called only for the provision of light centering to provide temporary support for the first rings of the bricks set on edge. When one or more superimposed concentric rings had been so constructed, additional stiffness could be imparted by adding further rings with the bricks set in what is now the normal way with their longer dimensions running radially and at right angles to the face of the arch’ [11]. A key question of the investigation of the unique historic building form of Tel Dan gate is the construction technique of the arches. This might provide an understanding of the developments in architectural and structural form of arches, as well as of the establishment of new building technologies and their lateral expansion. The paper puts forward the following hypothesis of the construction technique of Tel Dan’s arches. In Fig. 9 are sketched lines that follow the path of the mortar joints between the bricks. Extrapolating the lines one can find that they meet approximately at three different points, the first at the center and the others at the intersection between the intrados and the line passing through the spring of the arch. This kind of construction hints the building of a primordial three-centered basket arch – a round arch whose inner curve is drawn with circles having three centers. According to Mainstone [11] ‘in unreinforced masonry the construction becomes possible only with forms that can be built thought from a firm base provided by what has been built hitherto’. In the case of Tel Dan arches, the builders probably
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Fig. 8. Geometries of equilibrium. The actions of the concentring blocks are treated as lumped masses applied at their center of gravity. Left: the magnitudes of the forces are transferred to the force polygon. Right: (1) the lower bound equilibrium state i.e. the maximum trust state representing the most horizontal trust; (2) internal trust lines close to the minimum trust state – the extremity representing the least horizontal trust, the structure transfers to its abutments.
Fig. 9. Drawing lines between the bricks of the concentric rings. The lines meet at three different points, conceiving the idea for a primordial three-center arch construction. Left: the eastern arch. Right: the western arch.
put in place successive bricks in a radially pattern, using a geometric tool – perhaps a thread, at the right and left centers of the basket arch and subsequently rotating it at equal angles. The bricks held together by the boding action of the rapidly setting mortar, until gravitation load overcame the friction. From this position a light centering has probably been used, until all the remaining bricks, arranged with the help of the thread – posed this time at the center, have been set. When the first concentric ring had been constructed, two more rings were added with the bricks radially to the face of arch. While on the way to true arched construction, the Tel Dan arches remain within the realm of corbelled construction. They are among the first known evidences of the building of self standing arches, a ring between the continuous barrel vault, the ‘false’ – corbelled and the ‘real’ arch. Furthermore these arches offer a view of the development and expansion of historic building technologies as will be explained next. Despite our usual association of arches with large-scale Roman stone architecture, load-bearing arches are known and have in fact been in use in Mesopotamia and Egypt at least since the 3rd millennium BC. The shortage of timber was a fact of Mesopotamian life that probably dictated the adoption of the vaulting. According to Oates [12] ‘the construction of the vaults with the pattern of voussoirs laid radially was known in Mesopotamia since well be-
fore 3000BC, though usually in underground structures were the ground rather than the associated wall takes the thrust of the vault. . . In the 3rd millennium three techniques of vaulting were in simultaneous use – radial, pitched-brick and corbelling’. Earliest vaults are mentioned in archaeological reports from Ur, Mumbqat and Tell al-Rimah all in ancient Mesopotamia. All do exhibit some cantilevering – produced by the alternating long and short bricks of the first course. The pitched vaults – simple tunnels and low-profile pitched domes of the great temple at Tell al Rimah in northern Mesopotamia constructed late third and early second millennium BC, are the earliest known above-ground pitched vault structures. Aboveground freestanding arches are more unstable than vaults with their greater mass and support system. The length of the vault in relation to the width of the span affords the vault a far more massive support than that of an arch with the same span. An end wall, even when not carrying the weight of the vault, does provide closure and an element of protection against twisting and shearing. According to the archaeologist Gila Cook from HUC-JIR ‘freestanding aboveground monumental archways have been excavated at only three sites: Ashkelon, Mumbqat and Tel Dan. The Ashkelon gate, on the Mediterranean coast, is dated by the excavator to ca. 1780 and may be the earliest. Mumbqat, situated on the east bank of the great bow of the Euphrates in Mesopotamia, is dated by the excavator to the 16–15th century BC and is the
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latest. Tel Dan is located at the headwaters of the Dan and Jordan Rivers south of Mt. Hermon and is dated to the second half of the 18th century BC. This arch offers a view of the dynamic of learning – a work in progress’. According to Schaffer [10] ‘Israel’s historical building technology is not different or outstanding in any of its technological characteristics relative to the Mediterranean Sea region and the Near East culture. . . This is characterized by public buildings, city, gates, towers, walls and palaces, which are present in almost all the antique tells. . . The technology of mud bricks consists of mud bricks reinforced with straw and mud bricks reinforced with crushed stone-technologies that were used both in Egypt and in Mesopotamia. . . Since Israel is both the crossing point and passage for major historical events and cultures, it is not surprising that building technologies have been brought to Israel relative in a count period from their ‘‘official date’’ (original) appearance’. The mud–brick arches of Tel Dan are the surviving witnesses of the building technologies of the Middle Bronze Age in the Fertile Crescent, the region being often considered as the cradle of civilization, the birthplace of writing and the wheel, the scene of struggles and migrations of the earliest human civilizations. Some additional findings explained in next section, will enhance the picture of the adaptation and transfer of technologies in these historic building cultures.
4. Static investigation of the arched gate of Tel Dan In order to give a comprehensive assessment of the performance and abandonment of the gate some other questions are investigated. The first one inquires whether the arches of Tel Dan are really ‘true’ arches according to the structural definition, that is to say: these are compressive form-active shapes which carry purely axial load, without bending [17]. The second one investigates whether there was a structural reason for the construction of the arches in three courses, instead of one, or this was an aesthetic principle. Another question examines the reason of choice of an arch rather than the simpler and more economical post and beam system for these spans, inquiring why the builders build a mud–brick structure in an area blessed with excellent wood and stone. And finally what was the reason for the early burying of the gatehouse. For these aforementioned inquiries a simplified method has been chosen as satisfactory to give the appropriate answers. The absence of more accurate information required for advanced structural analysis has enhanced this decision. Essential missing data is the characterization of the mechanical properties of the masonry due to its extraordinary age, which has caused significant physical changes in the constitution, as well as due to the mixed use of different kinds of bricks with various physical properties and mortar. Nevertheless, the mechanical behavior of masonry customarily has one salient feature: a very low tensile strength. This property is so important that it has determined the forms of historical constructions introducing the funicular shapes of vaults and arches. According to Heyman [13] ‘stress requirements, such as needed for the design of steel or reinforced concrete, are transformed into geometrical requirements for the design of masonry. The shape of a masonry element must obey simple rules’. The grandness of the arch is its capacity to convert the gravitational loads into compressive stresses in the voussoirs and by doing so, the perfect harmony between form, load and material is achieved. Furthermore, according to Lourenço and Roca [14] ‘as a rule, the geometric idealization of historical constructions should be kept as simple as possible, as long as it can be considered adequate. If possible it is better to use two-dimensional models than three – dimensional idealizations’. Consequently a two-dimensional mod-
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el has been considered as adequate for this investigation. Moreover, the current analysis focuses on equilibrium equations – keeping in mind that equilibrium equations are most important when analyzing historical structures. ‘In essence, the internal forces in the element, which are subject to the laws of static equilibrium, must be within the boundaries of the masonry’ [13]. Given the abovementioned reasons, the current structural analysis was based on a generally accepted conservative simplification which assumes no tensile strength for the masonry. Furthermore the investigation was based on the thrust line analysis, using a twodimensional model and applying basic graphic statics. (The lines of trust are theoretical lines visualizing and representing the path of the resultants of the compressive forces on the voussoirs’ section.) The analysis makes use of the Safe Theorem for masonry structures, also known as the Lower Bound theorem according to which ‘if it is possible to demonstrate at least one possible equilibrium state, then the structure can also find at least one possible state’, [15,16]. Therefore in this analysis the range of the path of internal lines of trust – the natural lines of pressure – has been drawn on the geometric configuration of the Tel Dan gate. In Fig. 7 on the right and Fig. 8 some possible paths of resultant forces are demonstrated, all lying within the extrados and intrados of the arch and representing possible equilibrium states for the arches. This simple method has the power to prove that the monumental arches of Tel Dan gate are very early really ‘true’ arches according to the structural definition [17]. Moreover, the absence of cracks in the structure proves that any sliding of the arches’ support did not occur, and that the foundation was very stable, giving no structural reason for failure. In addition, the analysis puts forward a structural reason for the three courses’ construction: Fig. 8(2) on the right demonstrates a possible long path contained entirely within the arch. This path releases a low horizontal force which in turns causes a low thrust and a very low stress level, one more suitable for masonry. The three concentric rings therefore, by creating a wide cross-sectional shape, they decrease on one hand the magnitude of compressive stresses and on the other they permit vast load trajectories. Hence, the construction of the arches in three courses instead of one – besides aesthetic values – increases the safety factor of the structure which is dominated by the fragile nature of the unbaked brick. The three sun-dried mud–brick arches of Tel Dan stimulate the investigators to enquire the causes of choosing mud–brick sun dried arches rather than the simpler post and beam system, in an area blessed with excellent wood and stone. The paper suggests the following reasons: Traditional materials are divided into those that are combustible and those lacking tensile strength. Wood is a burnable material and therefore inappropriate for the city’s defense; post and beam stone would have been the perfect solution. However, due to beam’s low tensile strength, its capacity for efficiently bridging wide spans or carrying heavy loads from the upper structure is limited. This limitation would bear the idea for a stone arch, yet this kind of construction requests the labor of accurately pre-cutting the very strong volcano stone found at this area, and the need to provide much heavier centering than that for brick arches. According to Schaffer [10] ‘the stage of mud–brick technology existed in public construction of Israel for more than 1000 years, until it began to lose popularity and be replaced by local stone architecture and technology that had been influenced by the Phoenicians, the builders of the Jerusalem Temple from stone and wood, and by other Mediterranean Islands’ population invasions such as the Filisteas, who also brought the Iron Technology necessary for a good stone cut and dressing’. Therefore, earth bricks for building the gate of Tel Dan in the Middle Bronze Age, were an unexpected and surprising choice for us, yet it was the natural choice for the builders, who according to Oates [12] ‘displayed a familiarity with their techniques and material that could only derive from long tradition and knowledge at this kind of construction’.
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Stability rather than failure of the material is usually the dominant concern of the masonry. Collapse occurs when the load path can no longer be contained within the masonry, although this is not the case of Tel Dan arches. This fact makes the burying of the arches even more intriguing. Next the paper adopts the following structural cause for the gatehouse’s untimely burying: the failure of the material was the dominant concern of this gate. With its extreme humidity, the local environment in the Golan Heights is not particularly kind to monumental mud–brick architecture. The builders realized that the seepage of rain through the bricks of the inner core, in addition to local erosion do not permit such a structure to survive for long and therefore they filled in and buried it under added earthen embankments. This act may suggest that the builders were foreigners who came to Dan from Mesopotamia, where mud–brick was the default building material – since wood and stone were unavailable and where the climate was more conducive to mud–brick edifices (100 mm of yearly rainfall on average, versus ca. 600 mm at Tel Dan) [18,19]. Another gate, this time of stone and wooden beams, was constructed on the south side of the hill beneath this Iron Age gate visitors pass through today. 5. Concluding remarks For nearly 4000 years, a small gateway into a forgotten Canaanite city lay hidden beneath the dry soil of Tel Dan in northern Israel, until a team of archaeologists excavated the site in 1976. This year (2009), Tel Dan’s triple-arched gate is a candidate on the Tentative List for inscription in UNESCO’s manifest of World Heritage Site in the field of the History of Technology [20]. Its contribution derives from the discovery of the world’s oldest known free-standing structural form of arch, a form which has played a crucial role in the service of the humanity. The load-bearing arches of Tel Dan – built more than 1500 years before the Romans used arches in their construction – demonstrate expert ancient masonry engineering and fine aesthetic qualities. Moreover, the arches of Tel Dan constitute the connective vertebra in the bilateral development between the ‘false’ corbelled arch to the design of the ‘real’ arch. From a cultural point of view, Tel Dan offers an aspect of the dynamic of learning-in progress, being itself probably a direct, but certainly a lateral architectural development in the Ancient World. Furthermore, according to Dr. David Ilan [19] from HUC-JIR, Tel Dan reveals another story, perhaps one of an adaptation of technology
that not suited to a different environment from its original application. Acknowledgements The author wish to express her acknowledgement to the archaeologist Gila Cook – Head Surveyor, Tel Dan Excavations, Hebrew Union College (HUC-JIR) Jerusalem for the offer of important archaeological material, her advice and help. The financial support of the Neri Bloomfield WIZO School of Design and Education, Haifa, is greatly acknowledged. References [1] http://whc.unesco.org/en/tentativelists/1469/; 2009 [07.08.09]. [2] Biran A. The triple-arched gate of Laish at Tel Dan. Israel Explor J 1984;34:1–19. [3] Frances R, Cook G. The Tel Dan arches – early true arches, Part 1 – archaeological excavation. In: The 3rd symposium on the conservation of the built heritage – conservation of historic buildings and sites – from study to realization, the old city of Acre, Israel; January 4, 2007. [4] Frances R, Cook G. The Tel Dan arches – early true arches, Part 2 – structural investigation. In: The 3rd symposium on the conservation of the built heritage – conservation of historic buildings and sites – from Study to Realization, the old city of Acre, Israel; January 4, 2007. [5] http://www.jewishmag.com/59mag/dan/title.jpg; 2009 [07.08.09]. [6] http://www.graal.co.uk/kingdavid4.jpg; 2009 [07.08.09]. [7] http://www.bibleplaces.com/dan.htm; 2009 [07.08.09]. [8] Schloen D. Recent discoveries at Ashkelon. University of Chicago, The Oriental Institute; 2007. http://oi.uchicago.edu/research/pubs/nn/spr95_ash.html/; 2009 [07.08.09]. [9] Biran A. The discovery of the Middle Bronze Age gate at Dan. Biblic Archaeol 1981;44(3):139–44. [10] Shaffer Y. Changes in the building technology in Israel from ancient times until the 20th century and their influence on everyday life; 2000. http:// www.unesco.org/archi2000/pdf/schaffer.pdf/; 2009 [07.08.09]. [11] Mainstone R. Developments in structural form. Architectural Press; 1998. p. 76–77, 97–101. [12] Oates D. Innovations in mud–brick: decorative and structural techniques in ancient mesopotamia. World Archaeol, Archit Innov 1990;21(3):388–406. [13] Heyman J. Why ancient cathedrals stand up, the structural design of masonry; 2001, http://www.ingenia.org.uk/ingenia/issues/issue10/heyman.pdf/; 2009 [07.08.09]. [14] Lourenço P, Roca P. Analysis of historical constructions: from trust-lines to advanced simulations. Historical Constructions, Guimarães; 2001. p. 91–116. [15] Heyman J. The stone skeleton. Int J Solids Struct 1966;2:249–79. [16] Heyman J. The stone skeleton: structural engineering of masonry architecture. Cambridge University Press; 1997. [17] Macdonald A. Structure and architecture. Architectural Press; 2001. p. 37–47. [18] http://teldan.wordpress.com/mudbrick-gate; 2009 [07.08.09]. [19] http://huc.edu/newspubs/pressroom/2005/1/glueck.shtml; 2009 [07.08.09]. [20] http://whc.unesco.org/en/statesparties/il/; 2009 [07.08.09].