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StructuralUtilisationofWoodenBeamsasAnti-SeismicandStabilisingTechniquesinStoneMasonryinQasrel-Bint,Petra,Jordan

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Structural utilization of wooden beams as anti-seismic and stabilisingtechniques in stone masonry in Qasr el-Bint, Petra, Jordan

Shaher Rababeh a,⇑, Husam Al Qablan b, Shatha Abu-Khafajah a, Mohammad El-Mashaleh b

a Architectural Engineering Department, Hashemite University, P.O. Box 150 459, Zarka 13115, Jordanb Civil Engineering Department, Hashemite University, P.O. Box 150 459, Zarka 13115, Jordan

h i g h l i g h t s

�Wood has been used to strengthen stone ashlar buildings in the ancient monuments, such as Qasr el-Bint, Petra, Jordan.� Wood was used in a form of flexible string-courses that held the brittle stone ashlars together.� Use of wooden beams imbedded in stone ashlars increase the strength of buildings and function as an anti-seismic device.� Study proved the role of the imbedded wooden beams in reducing the shear stress of the structure by up to 50%.� Study shows that monument has survived moderate earthquakes because of these beams that functioned as anti-seismic devices.

a r t i c l e i n f o

Article history:Received 11 August 2013Received in revised form 1 December 2013Accepted 2 December 2013Available online 10 January 2014

Keywords:Seismic loadsFinite elementSurface frictionWooden beamsQasr el-BintPetra

a b s t r a c t

Wood has clearly been used to strengthen stone ashlar buildings in the ancient monuments in Petra, Jor-dan. However, to date, no analysis has been made of how wood works from structural engineering pointof view, especially during earthquakes. Wood was used in a form of flexible string-courses that held thebrittle stone ashlars together. This study analyses how this technique increases the strength of buildingsand to what extent it functions as an anti-seismic device. Conducting such study requires the multidis-ciplinary collaboration of scholars with knowledge of architecture, architectural history and structuralengineering. The result of this analysis proved the role of the imbedded wooden beams as anti-seismicdevice in reducing the shear stress of the structure by up to 50%. Repairing and preserving these beamsare to strengthen the structure against possible earthquakes. The simulation results showed that themonument under study; which is Qasr el-Bint, has survived moderate earthquakes because of thesebeams that functioned as anti-seismic devices, and can continue to do so if these beams are retrofittedand strengthened.

� 2013 Elsevier Ltd. All rights reserved.

1. Aims and approaches

In determining the required work needed to repair andstrengthen the monument of Qasr el-Bint against possible earth-quakes, Bani-Hani and Barakat [1] specify the type and locationof the required work without mentioning the presence of the woo-den string-courses in their analysis, or taking them into consider-ation in their recommendations. This paper attempts to identifythe use of the imbedded wooden beams in sandstone masonry inQasr el-Bint as a Nabataean technique and anti-seismic device usedin Petra monuments. To achieve this objective, after introducingthe origin of the use of wood string-courses, emphasis will beplaced on the structural system and the engineering configuration

of the monument. Analysing how this technique increases thestrength of buildings and functions as an anti-seismic device toprotect against earthquake damage will be conducted by simula-tion work which includes a three-dimensional finite element dy-namic model. The results of this study will conclude somerecommendations for the conservation of historical buildings inseismically-active regions, based on a thorough understanding oftheir structure and construction features and materials, whichmay help to prevent earthquake induced damage to buildings, con-sidering traditional and modern materials and techniques.

2. Introduction and literature review

One of the most important features of Nabataean constructionis the use of wood string-courses. This technique has aroused theinterest of a number of scholars [2–6], but no study had yet beenable to explain how these wooden string-courses contributed to

0950-0618/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.12.018

⇑ Corresponding author. Tel.: +962 799055312; fax: +962 (0) 5 3826348.E-mail addresses: srababeh@hu.edu.jo (S. Rababeh), hqablan@hu.edu.jo (H. Al

Qablan), s.a.khafajah@hu.edu.jo (S. Abu-Khafajah), mashaleh@hu.edu.jo (M. El-Mashaleh).

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the structural function during earthquakes. Hammond [4,5] re-ports that the origin of this technique is not clear, it does not seemto have been used in Roman engineering, and reported that ‘‘thisdevice would appear to be a Nabataean innovation.’’ However,the Old Testament mentions the use of cedar beams in the con-struction of the temple where the beams alternated with stoneas ‘‘three rows of hewed stones, and a row of cedar beams’’ [7]were used in construction.

Evidences of using wood to strengthen ancient mud and ma-sonry buildings in different parts of the world are reported bymany scholars. Thomson [8] observes that in the second

millennium BC wood was extensively employed to strengthenthe mud-brick upper portions of walls in Anatolia, and in Babylonin the Mesopotamian plains. Kienzle [9] observes, in his investiga-tion of Knossos palaces, that wood lacing was used in construction.Therefore, ‘‘the date of what can be reasonably described as tim-ber-laced masonry construction [is taken] back to as early as1500 to 2000 B.C.’’ [10]. A similar practice is found in Minoan Creteand also in Bronze Age Greek architecture [11]. In the Levant, longnarrow gaps, 6–10 cm were recovered between the masonrycourses (both fieldstones and ashlars) in Iron Age buildings at Ha-zor, Samaria and Megiddo [12,39] which indicates decayed woo-den beams. This technique was also used in Coptic Churches ofthe fifth and sixth centuries AD at Abu Mina, Barbara [13,19] andat Baouit [14] in Egypt, as well as in late antique in Balkan [13].It was used also in southern Arabia [15] and Afghanistan duringthe tenth and the twelfth centuries, as well as in Islamic buildingsin Cairo [16]. Wooden beams were embedded in the wall coursesin intervals, each of which is about 1.2 m high, in the nineteenthcentury mud-brick houses at Sariköy and Hacilar, near Aphrodisias[11]. According to Wright [17] ‘‘wooden stringer beams inset intomud brick and dressed stone masonry were almost universal inareas subject to earthquakes in order to tie the construction to-gether when subject to lateral stresses (Naumann, pp. 91–108)’’.

The earliest examples of the use of dressed wood in masonrybuildings of Petra were first studied by Rababeh [11,18] who re-corded and examined examples built from the first century BC tothe second century AD. In these examples wood, as a constructionmaterial, was probably an anti-seismic device that was conservedby the dry climate of Petra for nearly two millennia [11]. In otherclimates wood weathered away leaving grooves, thus Petra’sexamples provide an explanation for the grooves elsewhere. Basedon this, Rababeh [11] demonstrates that the earliest survivingexample of this use of wood comes from the foundations of Qasrel-Abd in Iraq el-Amir, built in the early second century BC.Grooves, 10 cm wide and 10 cm deep, appeared in the secondcourse of the interior walls of Qasr el-Abd [11] that could be ofAlexandrian influence [20].

The wood used for string-courses in Qasr el-Bint was identifiedas cedar [20]. The cedars of Lebanon were the most important woodresource of the eastern Mediterranean lands. The good qualities ofcedar trees that made it durable were mentioned by Vitruvius[21] in the first century BC. In Vitruvius’ words: ‘‘Owing to the fierce

Fig. 1. Plan of Qasr el-Bint (after Zayadine et al. [6]).

Fig. 2. East–West section view, showing the string wooden beams location (after Zayadine et al. [6]).

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bitterness of its sap, [cedar] is not injured by dry rot or the wormFurther, it does not admit flame from fire, nor can it burn of itselfetc. It has no open pores by which the fire can penetrate, and repelsits force and prevents injury being quickly done to itself by fire’’[21]. The ancient cedar trees still exist, and the literary sourcesprove the availability of the cedars in Lebanon in antiquity [22].

A study of the literature shows different classical and advancedapproaches used for structural analysis of historical buildings. Thiswas thoroughly examined by Roca et al. [41]. The finite elementmodeling (FEM) is one of these approaches, which is extensivelyused to examine the structural behavior of buildings under seismicforces [42]. FEM is used in this study to understand how the woo-den string-courses in Qasr el-Bint helped the structure to resistseismic forces in the past. This understanding is expected to con-tribute to designing comprehensive and adequate conservationmethods for Qasr el-Bint and similar structures.

3. Monument structural configuration and seismic resistance inQasr el-Bint

One of the examples for using wooden string-courses techniqueis Qasr el-Bint, Petra, dated to the last quarter of the first century

BC. It is located on a low steep rocky slope, where the differencein height for the horizontal distance is only c. 4.0 m. The sandstonebedrock makes a firm foundation [11,18,23]. It was built on a po-dium 4 m high that extends 3.7 m beyond the outer walls excepton the northern side [24]. It consists of a rubble core retained byashlar masonry laid in courses 40–60 cm high [3,6,24,25] to bethe foundation for the columns, each 70 cm diameter, surroundingthe temple [25] from east, south, and west sides.

Qasr el-Bint is square in plan, (31.5 � 31.5 m), with a pronaos, abroad naos and a tripartite adyton facing north (Fig. 1). The thick-nesses of inner and exterior walls range from 1.3 m to 2.7 m. Thecross wall, which separates the pronaos from the naos, has a door-way (c. 6 m wide) and a row of small arches along the top of thewall. Behind this the adyton is divided into three compartments;the central one measures c. 8.3 � 5.6 m. The side compartmentsare distyle in antis, and measure 8.3 � 5.6 m [10,22]. The interco-lumniations at the front of them were spanned by arches (Fig. 2) [3].

Three types of sandstone were used to construct the walls inQasr el-Bint; the smooth, the tear, and the honeycomb. The cours-ing in the building is neither isodomic nor pseudoisodomic and thejoints of the beds are usually continuous along each stretch of wall-ing. The height varies on an average from c. 40 to c. 60 cm, and thelength of blocks also varies from c. 30 cm to c. 1.00 m. One excep-tion is the massive cross wall, c. 2.70 m thick, which separates thenoas from the pronaos. The jambs of the huge central door waywere built in pseudoisodomic masonry. This part has been restoredwith the original sandstones in 1961–1962 [26]. The lower part ofthe outer walls of the Qasr has a higher first course made of ortho-states, c. 1.6 m high, which were not squared off properly, mor-tared into position, and separated by narrower stacks of blocks[27].

Walls with two-skins and a core (1.3–2.7 m thick) are found inQasr el-Bint [3,27]. Wall thickness relates to its proportion includ-ing its height and length, and to its position in the structure. A tal-ler wall is thicker as evident from Qasr el-Bint walls. There, thewalls are 23 m high and their thickness ranges from 1.4 to 2.7 m.Also, wall thickness relates to the need for insulation. The exteriorwalls were normally made of three layers, connected with mortar[28] to prevent the penetration of moisture, and to provide goodprotection from the summer heat.

Fig. 3. General view for the southwest corner (after the authors).

Fig. 4. Frictional and tied surfaces of Qasr el-Bint (after the authors).

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Sandstone ashlars masonry is relatively heavy at 2600 kg/cu m.It has a very small tensile strength, and thus its flexibility per unitweight is small. Thus, ashlar masonry is highly vulnerable to dam-age by the ground vibration during an earthquake. The design andthe configuration of Qasr el-Bint can be considered as a good seis-mic shape, which has the desirable attributes of symmetrical plan.The plan varies in details, but the basic symmetrical form isremarkably constant. Although this symmetry is around one axis,not two, as a pyramid, it helps to reduce the moments of torsion.It is difficult to suppose that the architects adopted this form of de-sign to give their buildings the ability to sustain tremors. But theirintention was mainly to achieve balance and beauty in both planand facades. However, it can be concluded from this that the designof the composition of the building mass is of major importance asan anti-seismic device. Symmetrical plan achieves the coinci-dences, geometrically, between the centre of mass and all possibleearthquake directions, by which the torsion moments would be re-duced through the structure. As a result, the building will not tendto rotate around the centre of resistance. Thus, the choice of struc-tural material and the way in which it was used in the constructionof Qasr el-Bint improved seismic performance.

4. Seismic risk in Petra and the construction material anddesign

Petra is subject to occasional earthquakes, some of whichbrought down most of the freestanding buildings, and created var-ious cracks in the rock-cut monuments [5]. Three major earth-quakes occurred in the region; one in 31 BC [5], and the othertwo in AD 114 and AD 363 [5,29–34]. The one dated to the 363AD, measured 7 on the Richter scale [37], produced vertical andhorizontal movements [35], and was probably responsible for Pet-ra’s destruction [5,29,30,36]. The main cause of these earthquakesis the presence of the rift in the Jordan Valley-Wadi Arabah-Gulf ofAqaba, which is part of the Great Rift Valley extending from NorthSyria to Mozambique in South Africa. The moving plates of theearth’s surface, or the so-called plate tectonic [35], provides anexplanation for much of the seismic activity in Petra. Such move-ment could have been either a slow slip, which does not causeshaking ground, or sudden earthquakes. In the latter, the earth isgradually distorted about the rift, in response to distant forces.Therefore, the earthquakes may have followed the rift line along

Fig. 5. Exponential decay friction model (after the authors).

Table 1Material properties of Qasr el-Bint.

Mechanical properties Stone walls Wooden beams

Modulus of elasticity, E 20.0 GPa 7.0 GPaPoisson ratio, m 0.2 0.3Mass density, q 2600.0 kg/m3 600.0 kg/m3

Damping 0.003 0.003

Fig. 6. Three-dimensional model of Qasr el-Bint (after the authors).

Fig. 7. Time domain of earthquake simulation. (a) Acceleration in the transversedirection, and (b) in the vertical direction (after the authors).

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the Wadi Arabah in its southern route. Although the earthquakesmentioned caused damage to many of the freestanding buildingsin Petra, some of them are still standing. Qasr el-Bint is one of thesebuildings.

4.1. The use of wood

The first illustration of the wooden beams used in Qasr el-Bentwas made by Kohl [2], followed by Wright [3], and then Zyadine[26]. The cedar wooden beams, some of which have survived, wereimbedded lengthwise along the walls (Fig. 3). The beams (c.15 � 20 cm in section) were positioned with a small gap betweenthem, approximately 20 cm wide, which was filled up with rubbleand mortar to form a stable course. Flat ties were placed at inter-vals across this course. When the wood decayed it left a hollowgroove along the wall. As reported in [2] the first course of woodenbeams in the east facade was embedded only c. 3 m above theground. It is difficult for current scholars to notice the existenceof the first course of wooden beam because of the weathering ofthe stone since 1910 when Kohl [2] first observed it. The secondand the third courses are still visible as shown in Fig. 3.

Similar devices have been found on most of the freestandingbuildings at Petra. Also, the technique was widely used in therealm of the Nabataeans, such as in the Negev desert in buildingI at Mampsis [38], at Wadi Ramm, at Khirbet edh-Dharih, and pos-sibly in Qasrawat [5]. Different accounts have been suggestedregarding the structural function of this use of wood, basically re-lated to engineering reasons and reinforcement purposes [3,5,8].However, Rababeh [11,18] suggests that the wooden beams couldhave been used as contraction and expansion joints that enable themonument to accommodate with movement. Structural move-ment occurs all the time, and usually its magnitude is too smallto be noticed. This movement is inevitable and can be caused bysettlement, wind, moisture, and change in temperature throughseasons.

Thus, stone blocks are subject to physical changes in length,width, height, and volume of their mass because of environmentalchanges and mechanical conditions. The effects may be permanentcontraction in form of drying or shrinkage, creep, or abnormal

changes from chemical reactions of sulphate attack. As movementof blocks occurs, they can relieve the internal stress by cracking,forming a new joint. In other words, as the temperature rises theblocks lengthen, and unless sufficient spaces are left between theblocks to allow for expansion, the blocks are likely to crack. Tominimize this, wooden joints might have been used betweenblocks to accommodate movement without losing structureintegrity.

5. Structural modeling

The question that arises is: To what degree did the use of woo-den string-courses help with seismic resistance? As has been notedpreviously, the main cause of damage to structures during anearthquake is their response to motion of the ground. During anearthquake, the waves of energy/movements reach the foundationof the walls and are transmitted to reach every element in thebuilding generating new types of stress. It is well known that ma-sonry materials are strong and can sustain the purely compressionforces of the static situation, but not lateral ones, which result fromthe new dynamic situation. These seismic loads will damage struc-tures due to the fact that the masonry materials are very weak inresisting tensile stress. However, wooden beams contribute forkeeping the masonry integrity by resorting to friction forces, whichenhances the energy dissipation capacity of the structure.

Herein, the nonlinear finite element package ABAQUS was em-ployed to study the structural behavior of Qasr el-Bint. Generatinga nonlinear finite element model of such a structure needs extraattention, and good engineering experience to make a reasonablegeometrical simplification of the complex geometry and a goodassumption of unknown materials. In this study, two finite elementmodels were prepared for Qasr el-Bint to investigate the effect ofwooden strings on reducing the seismic loads effects. The firstmodel was created based on the absence of wooden beams fromthe structure, while the second model assumed that there areembedded wooden strings between the stone walls. These woodenbeams will connect the stone walls using tied and frictional sur-faces. In contact simulations, the pairs of surfaces that may contact

Fig. 8. Shear stress (Pascal) distribution for the eastern wall. (a) Time = 5 s, and (b) time = 10 s (after the authors).

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each other and their constitutive models governing their interac-tions must be identified. Herein, a master and slave surfaces weredefined. These surfaces must be carefully selected to achieve thebest possible contact simulation. The stone surfaces were selectedto be the master surfaces while the wood surfaces, which are softerthan the stone surfaces, were selected to be the slave surfaces asshown in Fig. 4. In this formulation, the penetrations of the slavenodes into the master surface are prevented. Penetrations of mas-ter nodes into the slave surface can go undetected unless the meshon the slave surface is adequately refined. In this simulation, thelower faces of the wooden beams shown in Fig. 4 were connectedto the stone surfaces using the tie constraints while the upper

surfaces of the wooden beams were connected to the stone sur-faces using friction surfaces to avoid the instability in the modelsimulation. The tie constraint prevents surfaces, initially in contact,from penetrating, separating, or sliding, relative to one another. Itis very important to adjust the nodes to ensure that the two sur-faces are exactly in contact at the start of the analysis. Any gapsthat exist between the two surfaces, however small, will result innodes that are not tied to the opposite surface.

Simulation of the frictional behavior between wood and stonesurfaces is a very sophisticated task. The frictional properties ofsuch contacting surfaces and the dynamic characteristic of thebodies involved (mass, stiffness, damping) are thus required to

Fig. 9. Shear stress (MPa) at points A, B, C, D,. . ., I and J, on the eastern wall of Qasr el-Bint shown in Fig. 2. (—) Without wooden beams and (—) with wooden beams (after theauthors).

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take into account the stick–slip motion. Stick–slip motion may be amanifestation of dynamic instabilities inherent in the coupling ofnormal and tangential relative motions of contacting bodies. Ingeneral, the friction coefficient that opposes the initiation of slip-ping from a sticking condition (static friction coefficient) is differ-ent from the friction coefficient during established sliding (kineticfriction coefficient). As a matter of fact, the static friction coeffi-cient is higher than the kinetic friction coefficient. These coeffi-cients depend on large variety of parameters, including slidingvelocity, acceleration, contact pressure, surface roughness, temper-ature, humidity, type of lubricant between the contact surfaces,and, of course, material combination. This is the reason that thedata found in the many reference tables shows a large variation.

When two surfaces in contact slide relative to each other, inter-mittent vibration of the relaxation type can often be observed. The

discontinuity between the two states—sticking or slipping— can re-sult in convergence problems during the simulation. The slip ratewhich is the speed with which one surface of the connected sur-faces moves with respect to the other is an important factor indefining the frictional forces which decrease with increasing slid-ing speeds. Since there is no available data in the literatures, tothe extent of the authors’ knowledge, regarding these frictionalsurfaces, which needs an extra experimental investigation, anexponential decay friction model was assumed to represent thefrictional behavior between the wooden beams and the stone wallsas shown in Fig. 5. The friction model assumed that the frictioncoefficient decays exponentially from the static value to the kineticvalue according to Eq. (1). This model has the capability to explainthe complex relationship between the friction force and the slidingvelocity between the contacting surfaces.

Fig. 10. Lateral stress (MPa) at points A, B, C, D,. . ., I and J, on the eastern wall of Qasr el-Bint shown in Fig. 2. (—) Without wooden beams and (—) with wooden beams (afterthe authors).

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l ¼ lk þ ðls � lkÞe�dc c

:

eq ð1Þ

where lk is the kinetic friction coefficient, ls is the static frictioncoefficient, dc is a user defined decay coefficient, and c

:

eqis the slip

rate [40]. The static, kinetic and decay friction coefficients were as-sumed to be 0.35, 0.21, and 0.16 respectively [43]. Due to the lack ofdefining the frictional coefficient between wood and stone surfacesdescribed above, an extra investigations and experiment worksneed to be done in order to identify and characterize these coeffi-cients exactly.

In this study, the stone walls and the wooden beams were as-sumed to be behaving as linear elastic isotropic materials. The solesource of non-linearity is conveyed by wooden–stone elementshear friction forces resorting to an exponential decay law of thefriction coefficient as a function of the relative sliding displacementbetween those elements. The material properties for the stonewalls and wooden beams are assumed based on similar materialsproperties [44–46] and shown in Table 1. The exact values of thesematerial properties need an extra experimental investigation.

There is much that needs to be done to accurately assess theload carrying capacity of this structures under both static and dy-namic loading. Herein, the nonlinear finite element model is com-posed of mainly eight noded quadrilaterals, stress/displacementsolid elements with large-strain formulation (C3D8) that havethree degrees of freedom per node [40]. This type of analysis is veryexpensive in terms of computational time and memory needed. Inorder to simplify the finite element model and to avoid unneces-sary convergence problems due to frictional surfaces, the widthof the wooden beams were assumed to have the stone walls widthas shown in Fig. 4. The masonry walls were represented as ahomogenous continuum, without making any distinction betweenelements and joints. These approximations and assumptions areappropriate when a compromise between accuracy and efficiencyis needed, as in the case of large or complex computations[47,48]. The geometry of the structure is divided into sufficientnumber of elements to allow for free development of displace-ments. Some trial simulations were also carried out to study theconvergence of the results. In order to perform an accurate contactanalysis, a reasonably smooth contact surface and a uniform meshwith sufficient number of nodes over the contact surfaces were ap-plied. Based on these simulations, the finite element mesh used forthe analysis is shown in Fig. 6. A total of 11940 solid elements wereused to represent the stone geometry while 786 solid elementswere used to represent the embedded wooden beams in Qasr el-Bint. Two artificial simulated ground accelerations were used inthis study, one of them is vertical acceleration while the otherone is transverse acceleration as shown in Fig. 7, which simulatethe two components of earthquake vibrations. Herein, the horizon-tal ground acceleration was considered in just one direction in or-der to simplify the results. These ground accelerations wherechosen such that earthquake magnitude is about 6.5 on the Richterscale [40]. Prior to the earthquake excitation, Qasr el-Bint is sub-jected to gravity loading due to itself weight. In ABAQUS analysisthese loads were specified in two consecutive steps: static and dy-namic. Initially, the gravity load was specified in the first staticstep, while for the dynamic analysis in the second step, the trans-verse and vertical components of the ground accelerations shownin Fig. 7 were applied to all nodes at the base of Qasr el-Bint.

5.1. Analysis of three-dimensional finite element model (FEM) of Qasrel-Bint

This study addresses the use of embedded wooden beams inQasr el-Bint as a means of providing seismic resistant ability towithstand earthquake effects. Fig. 8 shows the shear stressdistribution on the eastern wall for the two cases described in

the previous section; without and with using the wooden beams,respectively. It can be clearly seen from the figure that the embed-ded wooden beams works as anti-seismic device and reduces theshear stress. In order to study these phenomena in detail, the shearand normal stress were studied at different locations in the easternwall as shown in Figs. 6, 9 and 10. It is clear that the shear and nor-mal stress are reduced when the wooden beams are used. The per-centage reduction of these types of stress depends on the locationof the points on the stone wall and the time period. For example,the reduction in the maximum shear stress for the points A andG is almost negligible, while for the maximum shear stress forthe points B, C, D, E, F, H, I and J shown in Fig. 9 have been reducedby 52.3%, 15.2%, 35.5%, 55.9%, 56.8% , 38.2%, 34.0%, and 24.89%,

Fig. 11. Shear stress (MPa) at point A (—) and point E (—)on the eastern wall of Qasrel-Bint shown in Fig. 5. (a) Without wooden beam (b) with wooden beams (after theauthors).

Fig. 12. Displacement at point K on the eastern wall. (a) Horizontal and (b) verticaldisplacements. (—) Without wooden beams and (—) with wooden beams (after theauthors).

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respectively. The reduction in the maximum normal stress for thepoints A, B, C, D, E, F, G, H, I and J shown in Fig. 10 have been re-duced by 54.5%, 17.4%, 15.4%, 48.6%, 45%, 41.4%, 43.2%, 41.4%,30.3%, and 6.8%, respectively. It is also worth mentioning that thevariation between the negative and positive stress is smootherwhen the embedded wooden beams are used which reduces thedamage effect of the earthquake loads on the structure. It is wellknown that when structures are subjected to seismic loads theshear stress induced at the base are larger than shear stress in-

duced at the top of the structure. Fig. 11 demonstrates this factat point A (top of the structure) and point E (base of the structure)for both cases (with and without using wooden beams).

Fig. 12 shows the vertical and transverse displacement of pointK. It can be clearly seen from this figure that the transverse dis-placement is smoother when the wooden beams are not used. Thisis mainly due to the frictional surfaces between the wooden beamsand the stone walls. The frictional shear stress in the transverseand lateral directions between the wooden beam and stone surfaceat the same point is shown in Fig. 13. The relative tangential mo-tions in the transverse and lateral directions between the woodenbeam and stone wall at point K are shown in Fig. 14. This figureprovides a good estimation of how far the frictional surfaces havemoved during the excitation of the structure with respect to eachother.

From the above results it can be easily concluded that struc-tures with embedded wooden beams exhibit markedly higher levelof energy dissipation when compared with structures withoutembedded beams. When lateral forces are exerted betweencourses the maximum shearing force to be withstood is friction.This leads to a significant reduction of both translational and tor-sional responses. As a result of this, the structure remains stretchyand intact during the earthquake, preventing the walls from fallingapart.

In view of this, wooden beams function as structural isolators,which can reduce the seismic response of the structure. At thepresent, seismic base isolation is an effective method used to pro-tect structures against earthquakes. Structural engineers use dif-ferent modern materials such as rubber bearings to reduce theseismic response of structures.

6. Conclusion

The use of wood in Qasr el-Bint was intended to provide earth-quake protection. It is probable that a strong earthquake in the Jor-dan Valley in the first century BC have awakened the Nabataeanarchitects to the need of using this technique. It is, therefore, fairto point out that the Nabataean builders probably had seismic pro-tection in mind when they used it. The techniques they chose ordeveloped were those most suited to the available materials, tak-ing into consideration the qualities of those materials, the cost,the topography of the site, and other local conditions. The Nabata-ean builders did not only aim to achieve a high degree of physicalprotection from the external environment, but also to obtain firmwalls and columns which can carry the different loads imposedby the roofs and ceilings.

The reason that Qasr el-Bint monument, the most impressivesandstone building at Petra, is still standing to its full height inparts is apparently because the wooden-string courses, at regularintervals in its walls, held it together. It can be easily concludedthat structures with embedded wooden beams exhibit distinctlyhigher level of energy dissipation when compared with structureswithout embedded wooden-strings. These wooden-strings con-tribute for keeping the masonry integrity resorting to frictionforces, which enhances the energy dissipation capacity of thestructure during earthquakes, and thus prevents the structure fromcollapsing. As a result, the structure remains flexible and integral.In view of this, wooden strings function as an anti-seismic device,which can reduce the effect of the seismic loads on the structure.One could imagine that the building was dancing in response tothe vibrations of the earthquake.

This study represents a step forward to understand the ancientengineering of Qasr el-Bint. Any future restoration and preservationworks should include the repair of the string wooden beams tomaintain the strength and to enhance the stability of the building.

Fig. 13. The frictional shear stress between the wooden beam and stone surface atthe point K in the (a) transverse direction and (b) lateral directions (after theauthors).

Fig. 14. The relative tangential motions between the wooden beam and stone wallat point K in the (a) transverse direction and (b) lateral directions (after theauthors).

68 S. Rababeh et al. / Construction and Building Materials 54 (2014) 60–69

Author's personal copy

This will give the historical monument of Qasr el-Bint the ability towithstand future earthquakes without excessive damage.

Acknowledgements

As the research required, we have seen many of the chief build-ings in Jordan. Our travels enabled us to examine buildings and totake many of photographs and to make drawings. This fieldworkwould not have been possible without travel grants from the Hash-emite University, which we have highly appreciated. It is a plea-sure to acknowledge the first author great debt to Dr. J.J. Coultonand Dr. J. McKenzie who have greatly influenced the method of thisstudy, and deepened his understanding of Classical constructiontechniques. We give special thanks to Dr. J. McKenzie who readmost of the first draft and we are very grateful for her help in edit-ing the paper. The authors also acknowledge the anonymous re-viewer for their constructive comments that significantlyimproved the final presentation of the paper. The major templein Petra, Qasr el-Bint, was the subject of an excellent and detailedstudy by Dr. F. Zayadine, Dr. F. Larché, and Dr. J. Dentzer, whichwas of great help. Our great thanks are also due to Nawal Rababehand Shereen Alyousef for their assistance in developing some of thefigures.

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