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EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICSEarthquake Engng Struct. Dyn. 2012; 41:1939Published online 1 April 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/eqe.1115

Minaret behavior under earthquake loading: The caseof historical Istanbul

C. S. Oliveira1, E. akt2, D. Stengel2,3 and M. Branco1,,

1Instituto Superior Tcnico, ICIST, Lisbon, Portugal2Bogazii University, Istanbul, Turkey

3Karlsruhe University, Karlsruhe, Germany

SUMMARY

Minarets are very slender structures with an old existence. The historical ones are made of cut-stone-blockmasonry assembled in peripheral cylindrical wall with an interior helicoidal stair supported on a central

core and on the wall. They are spread throughout the Islamic world and constitute an important heritagenot only of religious value, but also of great cultural interest. Throughout the times, these structures aspart of a mosque, have suffered significant damage during the earthquakes. Istanbul presents interestingcharacteristics to evaluate their dynamic behavior, as they are in great number, in an area where a largeevent in the next 30 years has been predicted.

In this paper, we performed a series of in situ ambient vibration tests to old minarets of various sizes andcompared results of frequencies with numerical modeling of the same structures. For the low-amplitudemotion, the frequency values of the first modes can be obtained from an empirical formulae function ofthe inertia of the cross-section and of the height of the main body. Damping ratios for these amplitudesare of the order of 0.51.0%.

Dynamic linear analyses of these structures indicate that for most cases very high stresses develop forPGA above 0.5 g, an input with a reasonable chance of occurring in the next 30 years. These high stressesare expected to cause the toppling of the minarets in the form that has been observed in the recent pastevents. Copyright 2011 John Wiley & Sons, Ltd.

Received 5 July 2010; Revised 23 January 2011; Accepted 26 January 2011

KEY WORDS: minarets; masonry structures; seismic performance; in situ frequency measurements;analytical modeling

1. INTRODUCTION

The Historical Peninsula of Istanbul is full of very ancient mosques with one or more tall andslender minarets. These masonry minarets stand almost free or with the lowest section as part ofthe adjacent structure, the main mosque building, above ground. They have very different heightsranging from 15 to 70 m and different designs, with one, two, or three balconies. Along the

several centuries of their existence, they have gone through a few earthquakes. For example, duringthe last earthquakes of 1999 (Kocaeli and Duzce) many minarets, with main structural systemof either reinforced concrete or masonry, suffered extensive damage, while others survived withminor damage [1, 2]. Studies revealed high odds for a strong earthquake event to occur in the next30 years [3]. Therefore, it is important to better understand how these structures behave duringmoderate to strong shaking, defining the levels of seismic action for which no harm may resultand to propose some retrofitting policies, for the case when these levels are exceeded.

Correspondence to: M. Branco, Instituto Superior Tcnico, ICIST, Lisbon, Portugal.E-mail: [email protected]

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20 C. S. OLIVEIRA ET AL.

Figure 1. Location of mosques with studied minarets in Istanbul (number of minarets between brackets):1Hagia Sophia (#4); 2Yeni Cami (#1); 3Rstem Pasa (#1); 4Sleymaniye (#2); 5Sehzade (#1);

6Akbyk (#1); and 7Mihrimah (#1) (Google Earth).

Several papers have already dealt with the topic [46], but the collection of minarets in Istanbul

is so huge in number and variable in character that a comprehensive study using analytical andambient vibration testing cannot be extrapolated from previous studies. We selected a group of11 minarets (Figure 1) spanning heights from 23 to 67 m. For all of them we performed in situtesting for identification of at least the first eight modes and corresponding frequencies. For sevenof these minarets we conducted linear numerical dynamic modeling (Figure 2).

This selection was made with the aim of analyzing different geometric types and to evaluatehow different levels of slenderness affect the seismic behavior of a minaret. The models werecalibrated through the comparison of modal frequency values obtained by in situ measurementswith the numerical values.

We then proceeded to determine the structural response in terms of maximum displacementsand stresses for the site-specific ground motion that can be expected in each location.

2. STRUCTURAL DESCRIPTION

Classical Ottoman minarets have a standardized assembly of components or segments as shownin Figure 3. Minarets are slender structures, usually of cylindrical form and they can stand aloneor be contiguous and integral with the mosque structure. There is essentially a masonry wall tubeand an inner core surrounded by a helicoidal stairway going up counter-clock wise, made of singlesteps spanning from the inner core to the wall. These pieces penetrate half the thickness of theperipheral cylindrical wall, in the bodyshaft section. In some cases there are two parallel stairsas it happens in Hagia Sophia #1. The basic elements of the minaret are: footing, boot/pulpit(kaide), transition segment (kp), cylindrical or polygonal bodyshaft, stairs, balcony ( serefe), upperpart of the minaret body (petek), spire/cap (klah), and end ornament (alem). They may be builtin cut-stone, brick, or a mixture of both. The top is usually a 3-D timber structure covered by5-mm-thick lead sheets. Iron clamps hold wall blocks together.

Above the upper balcony, the helicoidal stairway stops as well as the stone core. A woodencylindrical column with slightly smaller diameter gives vertical continuity to the inner core untilthe base of the cap, also serving as a support to a rudimentary vertical wooden stair [7].

3. DAMAGE IN PAST EARTHQUAKES

Looking into the past, there were several occasions where minarets in Istanbul suffered from groundshaking [8, 9]. In the 10th September 1509 earthquake, the Hagia Sophia Minarets collapsed [8].

Copyright 2011 John Wiley & Sons, Ltd. Earthquake Engng Struct. Dyn. 2012; 41:1939

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MINARET BEHAVIOR UNDER EARTHQUAKE LOADING 21

Figure 2. Minarets modeled: (a) Hagia Sophia #2; (b) Hagia Sophia #3; (c) Hagia Sophia #4; (d) YeniCami; (e) Sleymaniye #3; (f) Akbyk; and (g) Mihrimah.

In the 22nd May 1766 and 10th July 1894 earthquakes the minaret of Mihrimah fell [10]. Thedamage to 75 minarets resulting from the August 17 and November 12, 1999 earthquakes wasdocumented in [6]. The damage distribution reported for 45 minarets in Dzce, Bolu, and Kaynalgives an indication of the extent of damage to minarets in the region. The 19 out of 35 (54%)reinforced concrete (RC) minarets surveyed in Dzce, Bolu, and Kaynal , sustained damage ofintensity severe to collapse. Seven out of ten (70%) masonry minarets surveyed in these three cities,sustained higher levels of damage including collapse. The location of the failure in the minaretsthat collapsed during the 1999 earthquakes was found to be at the region near the bottom of thecylinder, where a transition was made from a circular to a square section. In RC minarets, thereinforcing bars in this section were spliced. Inspection of the failure regions revealed no neckingor fracture in the bars.

The old masonry minarets were also observed to fail near the bottom of the cylinder, wherethe lateral stiffness and strength are smaller compared with those of the transition regionor the minaret base and where the minaret connects to the adjacent building or is part of itat the lower section. Few cases of minor damage were also observed, such as the collapse ofparts of the balcony. However, in a few places where large damage was present in buildings,minarets performed quite well as in the case of Glck during the 1999 Kocaeli earthquake(Figure 4).


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Figure 3. Minaret component description (elevation) of a typical classical Ottoman minaret.

Figure 4. No damage to minaret in heavily damaged area in Glck during the 1999 Kocaeliearthquake (in http://earthquake.usgs.gov).

According to the inventory made by Frat [6] for damage assessment of the historical Istanbulminarets during the Kocaeli 1999 event, none of the selected minarets for this study withstoodany particular damage, although two of the mosques presented vertical cracking in the inner core(Sleymaniye #3 and Sehzade). Historical structures, such as the mosques, with heavy and stiffwalls are subjected to larger lateral earthquake forces due to their short periods of vibration, whencompared with the minarets which are less stiff and have longer periods of vibration [11].


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From many observations and studies [12] for tall minarets of either masonry or RC the mostcritical zones are the ones above referred. Analytical studies made on a few representative minaretshapes [1] for both masonry and RC structures got essentially to the same conclusions.

4. SURVEY FOR GEOMETRIC AND MATERIAL PROPERTIES

The geometric characterization of the minarets under analysis is a very difficult task due to thelack of detailed information, especially for the absence of drawings for most of them. In one singlecase, the Mihrimah minaret (Figure 2(g)) we were offered the detailed AutoCad drawings, dueto the recent reconstruction works undergone.

For Hagia Sophia (Figure 2(a)(c)) and Sleymaniye (Figure 2(e)) there were old drawings withgeneral outer dimensions of their minarets. For all the others we did not have any information.To proceed in an approximate way, we analyzed pictures taken by ourselves, and looked atold engravings and all possible material available. To avoid parallax errors we used picturestaken from the top of other minarets. However, in all cases, during the in situ tests we collectedthe most easily available geometric data such as the number of steps to important locations,step size, thickness of walls, step width, inner and outer diameters at the balcony area, etc. Allthe gathered information was confronted in order to reduce the errors in the evaluations. Onlywith a detailed surveying campaign it would have been possible to reduce geometric errors to aminimum.

In this work, all but the minarets with complete drawings have some geometric errors. Forcertain parameters, such as heights and dimensions of the pulpit, errors could be larger than 10%.In fact, the geometric characteristics of the pulpit changed quite significantly from case to case.In a few cases, the pulpit is part of the main building and it is well confined. This is the case ofSleymaniye #4 and Rstem Pasa. In all the cases, the pulpit functions structurally as a heavyfoundation, with not much deformation capacity. The vibration records made in parts till the top ofthe pulpit show almost no movement as compared with the ones obtained at higher levels, indicatingthat the flexibility of the structure starts at the transition zone. According to the parameterizationpresented in Figure 5, a database with the geometric characteristics of each minaret was created. InTable I the main characteristics are presented. Most of the minarets were made of stone masonry

Figure 5. Minaret geometrical parameterization.


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Table I. Main geometric characteristics of the studied minarets (in m) following the parameterization ofFigure 5. The seven minarets highlighted in gray are the ones that were modeled (some of the values

presented may be subjected to about 10% error).

Hagia Hagia Hagia Yeni RstemSophia Sophia Sophia Sleym. Cami Akbyk Mihrimah Pasa Sleym. Sehzade

Minaret 12 3 4 3 1 1 1 1 1 1

Total height (m) 66.55 63.20 44.96 74.40 54.90 23.02 38.65 41.6 51.7 48.7Balconies 1 1 1 3 3 1 1 1 2 2Body height(h2+h3+h4) (m) 37.90 39.70 33.60 54.80 38.00 15.10 30.74 22.00 34.80 30.90Wall thickness(wallth 3) (m) 1.00 0.55 0.85 1.13 0.65 0.43 0.30 0.45 0.83 0.60Body diameter(ext 3) (m) 4.70 3.10 3.30 4.06 3.00 2.33 2.30 2.48 3.20 2.90

(stone blocks cut to measure and assembled on site). However, the Hagia Sophia #4 was madeof brick masonry (brick and mortar), with wooden stair steps in some parts. The Akb yk minarethas a mixed system. It has a core of stone masonry and stone stairs. The exterior wall is of brickmasonry.

Material properties are very difficult to estimate, due to the anisotropy of the masonry andits dependence on the region providing the materials and the construction techniques. While thegeneral weight (=2225kN/m3) did not cause great controversy, the modulus of elasticity (E)and the stress limits () in both compression and tension were very difficult to assess. Directmeasurement of the mechanical properties is a difficult task due to the impossibility to performintrusive techniques in historical structures. Only tests such as seismic tomography, radar, etc.can be used. They will produce some local indications as far as the modulus of elasticity ofthe material is concerned, but overall information is almost impossible to obtain. On ultimatestresses and strains (compression and traction) the problem is even more complex. We proceedin a different way: the modulus of elasticity was calibrated for each case, based upon the resultsof the dynamic characterization tests. And, based on some of the most recent values published in

the literature [13, 14] we considered the compression strength as 26 MPa and the tensile strengthwas assumed as 10% of that value (2.6 MPa). These values are only indicative and were used toevaluate qualitatively the results obtained through the computer model. The values for the modulusof elasticity present a wide range of values from case to case. We considered as reference for thisstudy, the values used in studies on the Sleymaniye [15, 16] and Hagia Sophia mosques [17, 18].In these cases, the modulus of elasticity was considered as 3.55.0 GPa for brick masonry and9.014.0 GPa for stone masonry. The concentrated mass for a balcony such as in Hagia Sophia #2would be around 38 tons, adding up to the wall weight. The spire mass with a 5-mm-thick leadwould be in general of the order of 23 tons.

5. SOIL CLASSIFICATION

Although the analyzed minarets were located in the same area with inter separations of not morethan a few kilometers (Figure 1), the soil characterization according to the latest studies [19, 20]indicates that there are a few differences as far as geotechnical properties are concerned. In termsof the NEHRP soil classification [21], the minarets analyzed are founded in soils of types C (shear-wave velocity between 750 and 350m/s) and D (shear-wave velocity between 350 and 200m/s)as presented in Table II.

We are aware that in the case of relative soft soil (CD in Table II) and for tall and heavystructures, soilstructure interaction may be very important to define the rotation at the foundationlevel. Further studies should be developed to analyze this effect.


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Table II. Soil classification of the sites of the minaretsunder study (courtesy of Zulfikar [20]).

NEHRP EC8 equivalentNum Minaret soil class soil class

1 Hagia Sophia C B2 Yeni Cami D C

3 Rstem Pasa C B4 Sleymaniye C B5 Sehzade C B6 Akbyk C B7 Mihrimah D C

6. AMBIENT VIBRATION SURVEY

A series of in situ ambient vibration tests have already been carried out in different types ofminarets [2225] for frequency as well as for mode identification studies. This alternative tech-nique to using other excitation sources has been applied in many different situations and, forflexible structures such as minarets, the results are very robust [26, 27]. The main idea of usingambient vibration tests to determine the dynamic characteristics of a structure is essentially basedon the principle that this vibration acts at the foundation level as white noise being filteredby the structure along the height. So, the input signal is amplified at the frequencies of thestructure at locations where modal shapes exhibit higher expression. By locating instrumenta-tion in several places and recording the response simultaneously in these places, it is possibleto identify both the frequencies and the corresponding mode shapes [28]. For slender structures,the response measured only at the top is enough to indicate their fundamental frequency. Theresults from ambient vibration tests compare very well with alternative techniques which useother excitation sources, such as earthquake ground motion recorded by permanent monitoringnetworks [23].

In the study we performed identification of both frequencies and mode shapes using threeaccelerometric high-resolution instruments with three components and common timing. It

was possible to obtain for all 11 minarets the first nine mode shapes, their frequencies anddamping ratios through the use of ARTeMIS Extractor software [29]. We will illustrate theused method on the Mihrimah minaret (Figure 2(g)). The others were treated in a similarmanner.

6.1. The Mihrimah minaret

We used several techniques to characterize the dynamic properties of the minarets applying theARTeMIS extractor software, already engaged in similar studies in the past [22]. Frequencydomain decomposition (FDD), enhanced frequency domain decomposition (EFDD), and stochasticsubspace identification (SSI) were used in the estimation of natural frequencies and damping ratiosThe tests on all minarets were carried out with three Gralp Systems CMG-6TD instruments withthree channels each, two for orthogonal horizontal directions, and one for the vertical direction, and100 Hz sampling frequency. The accelerometers were distributed along the height of the minaret.The first instrument was at the ground level, the second one was at approximately mid-heightbetween the balcony and ground, and the third instrument was at the highest balcony.

In parallel to the ambient vibration recordings, instruments were calibrated during the Mihrimahminaret testing in order to compare the recordings of the three instruments used and to detect anyabnormality among them. For the peak of resonance corresponding to the frequencies of vibrationof the structure, the three recordings presented the same amplitude, confirming that the threeinstruments were well calibrated.

The test on the Mihrimah minaret, done on February 6, 2009, was the best in terms of qualitycontrol with almost 30 min of simultaneous and uninterrupted data from three stations (ground,


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Figure 6. Example of the SVD lines of Mihrimah minaret with the picked mode technique.

Figure 7. SSI stabilization diagrams of Mihrimah minaret.

stair case, and balcony), which led to very stable results in terms of mode shape identification.Straight lines were used to connect the locations of the instruments in drawing the mode shapes.The data are decimated by 5, which results in a decreased Nyquist frequency to 10 Hz. 1024frequency lines were chosen.

In Figure 6 singular values of the signals, and an average of all the signals obtained from theEFDD technique, are shown. It is clearly seen from the figure that natural frequencies are close toeach other, and the frequency span for the first eight modes is 09 Hz.

In Figure 7, the stabilization diagram of estimated state space models obtained from the SSItechnique is shown. Frequency values obtained from the SSI technique are nearly the same asthose found in the EFDD technique.

As seen in the peak picking window (Figures 6 and 7), there are usually two coupled peaks,probably indicating two orthogonal modes. In the case of the first very dominant modes they arefound at two different spectral density lines. There are three pairs of peaks to find before a fourthsingle peak appears; most likely indicating torsion. That is not in accordance with previouslyreported results [1], where higher frequencies were obtained for torsional modes.

The first two modes are picked on two different frequency lines, although they are reallyclose. Nevertheless, they are definitely first modes, as they present the same shape, but in almost


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Figure 8. First two mode shapes: (a) f1=0.801Hz and (b) f2=0.840Hz.

Table III. Comparison of frequencies and damping ratio obtained from differentmethods (Mihrimah minaret).

EFDD SSIFDD

Mode Description f (Hz) f (Hz) Damp. (%) f (Hz) Damp. (%)

1 First-mode NW direction 0.80 0.81 0.81 0.932 First-mode NE direction 0.84 0.84 1.14 0.84 0.683 Second-mode E direction 3.22 3.21 1.09 3.27 1.884 Second-mode S direction 3.57 3.57 1.82 3.59 3.455 Third-mode E direction 4.26 4.25 1.24 4.25 1.306 Third-mode S direction 4.53 4.53 0.99 4.53 1.257 Torsion 5.57 5.57 0.57 5.57 0.59

orthogonal directions. The first two mode shapes of the minaret are shown in Figure 8. The smalldisplacement at about mid-height (point 2) is the result of the inclusion of the minaret into thestructure of the mosque up to some height between points 1 and 2. The first two peaks could beverified using the stochastic subspace identification method. The selected model showed the sameresults for the first two mode shapes, but also, the stabilization diagram revealed that there are no

stable modes, for some peaks obtained from FDD.Now, both estimators agree with each other and the estimation of the first seven modes can

be compared regarding frequencies and damping ratios (Table III). Actually, it was possible toextract two more modes, #8 and #9 for most of the minarets. For more details see [23]. Modalparameters (frequencies and damping ratios) obtained from both vibration tests and analyticalmodels (see Section 7) are close to each other, which confirm the reliability and robustness of thevibration tests.

6.2. Other minarets

The other minarets were subjected to a similar in situ ambient vibration and subsequent dataanalyses. We placed the instruments at different locations along the height, sometimes placingtwo at the same balcony (opposite sides) to enhance the torsion modes. From top to bottom the

accelerometers were positioned at the upper most balcony, at about mid-height between the groundlevel and at the ground level. The results are summarized in Table IV.

An analysis of Table IV shows that the frequencies obtained are similar in all three techniquesused. It can be said that the results in terms of frequency identification seem quite robust, althoughdamping estimates vary considerably. On average, the damping coefficient is around 1% for ambientvibration amplitudes of the order of 2 mg at the top of taller minarets.

6.3. Empirical formula for computing the fundamental frequency of minarets

Based on the data collected we developed a simple formula to estimate the first frequency ofvibration known the inertia of a general cross-section and the height of the minaret. According to


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Table IV. Frequencies and damping ratios of the first seven modes using different approaches.

EFDD SSI EFDD SSIFDD FDD

Mode f (Hz) f (Hz) Damp. (%) f (Hz) Damp. (%) Mode f (Hz) f (Hz) Damp. (%) f (Hz) Damp. (%)

Hagia Sophia 1 Hagia Sophia 21 1.32 1.27 1.44 1.24 1.24 1 1.17 1.17 1.94 1.18 1.22

2 1.37 1.38 2.38 1.38 0.88 2 1.27 1.28 1.51 1.28 0.723 3.81 3.79 0.98 3 3.37 3.46 0.95 3.38 1.634 4.00 4.09 7.30 3.99 1.21 4 4.05 3.75 1.57 3.75 1.415 8.50 8.77 2.22 8.54 2.57 5 9.64 9.58 3.47 9.64 1.686 8.89 8.85 6.25 9.02 3.70 6 10.64 10.66 0.13 10.7 1.317 12.6 12.6 0.70 12.58 3.21 7 12.79 12.78 0.47 12.8 1.85

Hagia Sophia 3 Hagia Sophia 41 1.02 1.03 1.41 1.03 0.75 1 1.03 1.04 1.56 1.05 0.932 1.05 1.04 1.58 1.04 0.74 2 1.05 1.06 0.18 1.08 1.003 2.71 2.76 3.51 2.94 2.41 3 3.55 3.48 0.66 3.5 1.604 3.08 3.01 0.71 3.13 4.93 4 3.81 3.75 0.42 3.76 0.695 4.32 4.35 0.95 4.30 4.20 5 5.57 5.48 1.63 5.51 2.466 4.59 4.58 1.51 4.62 1.99 6 5.80 5.80 0.43 5.8 1.017 7.37 7.44 0.27 7.21 2.21 7 7.46 7.47 0.20 7.73 3.68

Rstem Pasa Sehzade1 1.37 1.36 0.67 1.35 0.77 1 1.18 1.18 0.73 1.18 0.932 1.38 1.38 0.58 1.38 0.64 2 1.31 1.31 1.19 1.31 1.163 2.98 2.97 0.64 2.94 4.69 3 3.31 3.31 1.00 3.33 1.324 3.41 3.41 1.08 3.48 4.30 4 3.48 3.47 0.86 3.45 1.745 4.93 4.94 1.27 4.96 2.83 5 5.79 5.86 0.91 5.84 2.156 5.24 5.24 0.99 5.35 2.01 6 6.11 6.12 0.68 6.18 1.017 7.80 7.76 0.86 7.75 0.91 7 7.73 7.73 0.41 7.72 1.34

Sleymaniye 1 Sleymaniye 31 0.95 0.96 1.85 0.99 2.23 1 0.83 0.84 0.212 1.17 1.32 1.66 1.18 1.01 2 0.84 0.85 1.203 4.91 4.91 0.41 4.90 1.43 3 3.68 3.68 0.30 3.7 0.914 5.18 5.16 0.73 5.10 1.01 4 3.84 3.83 0.29 3.86 1.75 6.88 6.90 0.40 6.95 1.59 5 5.49 5.49 0.23 5.5 1.78

6 7.06 7.14 0.48 7.13 0.92 6 5.80 5.80 0.23 5.83 0.337 10.86 10.91 1.74 10.96 2.34 7 6.41 6.41 0.16 6.42 1.83

Akbyk Yeni Cami1 1.68 1.69 0.75 1.68 0.91 1 0.63 0.65 2.67 2 1.74 1.73 0.63 1.77 2.39 2 0.66 0.66 2.52 0.65 0.853 3.47 3.46 0.37 3.45 1.34 3 2.98 2.97 0.91 2.97 1.254 3.49 3.48 0.43 3.59 0.43 4 3.08 3.08 0.69 3.07 1.485 5.14 5.14 0.13 5.12 0.69 5 5.79 5.77 0.89 5.81 2.906 5.15 5.16 1.09 5.20 0.75 6 6.05 5.95 0.56 6.26 4.217 6.97 6.98 0.50 6.86 0.94 7 7.54 7.53 0.55 7.44 3.69

Note: FDDFrequency Domain Decomposition; EFDDEnhanced Frequency Domain Decomposition; SSIStochastic Subspace Identification.

Clough and Penzien [30] the first frequency of vibration for a cantilever is given by

f=1

2(1.875)2

E I

m L4(1)

where E is the modulus of elasticity, I the second moment of area, m the mass per unit of length,and L the total height of the cantilever.

Therefore, the fundamental frequency of the minaret is expected to be a function of the secondmoment of area and the height of the minaret (E and m are material properties). The formulationherein considered that the height of the minaret, H relevant for the first frequency of vibration, was


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Figure 9. Fundamental frequency of vibration (experimental versus empirical) for all the minarets analyzed.

given by the length from the top of the pulpit to the base of the spire (h2+h3+h4 in Figure 5). Therelevant cylindrical cross-section was the base of the shaft (Table I). Based upon these parametersa new equation (Equation (2)) was developed to account for this parameterization.

f=.A0.5.I.H (2)

where A is the area of the cylindrical cross-section and I the second moment of area as definedby Equations (3) and (4). The constants were defined as =38, =0.7, and =1.1. Thesevalues were determined through an iterative process that minimizes the difference between theexperimental results and the empirical equation, considering an average density of 2ton/m3 andan elasticity modulus E=10GPa, as referred in previous studies [1518].

A=

4 [

2

ext3

(ext3

2

wallth3.1)

2

] (3)

I=

ext3

2

4

ext3

2wallth3.1

4(4)

Note: wallth3.1 is the wall thickness at the base of the cylindrical body.In Figure 9 we compare the results obtained with Equation (2) and the experimental in situ

values, and errors range to a maximum of 30%. A similar error was observed for the Fatih minaret,which has a height (H) of around 55 m and an ambient vibration frequency of 0.50.55Hz [31].The natural frequency calculated by Equation (2) was 0.72 Hz.

If we have more data points from measured minarets, the parameter in Equation (2) couldbe disaggregated into two other parameters, reflecting the mass and modulus of elasticity, and theerrors certainly would reduce. That is what we visualize with Mihrimah and also with MustafaPasha which are minarets made of a high-quality block masonry (E higher than average). For theseminarets, the empirical formula would lead to higher frequency values, reducing the distance tothe experimental in situ values (Equation (2) applied to the geometry of Mustafa Pasha Minaretleads to f=0.68Hz against 1.04 Hz from ambient vibration).

7. NUMERICAL MODELING

We performed linear dynamic analysis of seven minarets selected from the 11 under study,which presented the most distinctive geometry (Figure 2). SAP2000 [32] was the software to


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Table V. Modulus of elasticity used for each minaret.

Hagia Sophia Hagia Sophia Hagia Sophia Yeni Sleymaniye#2 #3 #4 Cami #3 Akbyk Mihrimah

E (GPa) 9.5 11.5 4.0 4.5 20.0 2.5 8.5

1st mode 2nd mode 3rd mode Torsion mode Vertical mode

Figure 11. Typical analytical mode shapes.

through the model, such as the degradation of the masonry, the elasticity of the foundations, or theeffect of the adjacent main mosque building, among others. In this way, the process of calibration is

simplified, as there are no more experimental data to be accounted for. The validity of the followinganalyses is not an issue, as only a linear analysis is performed and it is directly dependent on themodal analysis, whose results were calibrated. Nevertheless, the values obtained are close to theones considered in studies in similar structures, as referred in Section 4 [1518], with only a fewexceptions. Just as a note we should mention that the difference in modulus of elasticity of HagiaSophia minarets is due to the fact that minaret #4 is made of brick masonry with wooden stairs, asopposed to the stone masonry in the other two minarets. The only two minarets whose modulusof elasticity does not follow the common trend are Yeni Cami and Sleymaniye #3, which couldbe related to the interrelation with the mosque. Further studies should be conducted to clarify thissituation.

7.2. Modes of vibration

Mode shapes are more or less similar in all modeled minarets (Figure 11). They are in pairs(orthogonal in the plane XY), and torsion appears only in higher frequencies. Sometimes, wealso obtained the vertical mode, also at a higher frequency. The model frequencies obtained forthe minarets under study are presented in Table VI, where the comparison with the experimentalresults is also made.

Each model produces pairs of orthogonal modes with almost the same frequency. In fact, theonly source of non-axi-symmetry is the stairway, which causes very little effect. Only torsion andvertical modes appear alone. This simple behavior is not exactly the same in the real minarets, dueessentially to the connections or links present at the pulpit level. For higher modes, the deformationof the pulpit is not negligible, as well as in torsion and vertical vibration.


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Table VI. Comparison between experimental analytical frequencies of vibration, in Hz.

Mode of Hagia Hagia Hagia Yeni SleymaniyeVibration Sophia #2 Sophia #3 Sophia #4 Cami #3 Akbyk Mihrimah

First Model 1.18 1.00 1.07 0.66 0.86 1.70 0.78Exp/mod 0.99 1.03 0.97 0.96 0.96 0.99 1.03

Second Model 1.21 1.00 1.07 0.66 0.86 1.70 0.78

Exp/mod 1.05 1.05 0.99 1.00 0.99 1.02 1.08Third Model 3.96 2.79 4.34

Exp/mod 0.85 1.07 0.85 Fourth Model 4.01 2.79 4.34

Exp/mod 1.01 1.07 0.89 Fifth Model 8.63 4.66 5.21 6.15 7.69 4.52

Exp/mod 1.12 0.93 1.07 0.94 0.91 0.94Sixth Model 9.25 4.66 5.21 6.15 7.69 4.52

Exp/mod 1.15 0.99 1.11 0.98 1.02 1.00Seventh Model 9.42 9.49 8.07 7.23 10.45 10.71 9.58

Exp/mod 1.36 0.78 0.92 1.04 0.72 0.88 0.58

In the cases of Hagia Sophia #2 and Yeni Cami, the vertical modes occur at 11.49 and 9.70 Hz,

respectively. This mode of vibration was not detected experimentally as the vertical motion wasnot analyzed.

During the stage of calibration of the model, several hypotheses were tested to evaluate thecontribution of each structural element. The removal of the stairs has little effect on the modesof vibration dealing with translation. When the pulpit was part of the mosque, the restrain of thepulpit until the height of the building was tested. Nevertheless, the effects are also negligible, asthe pulpit has little movement. Especially, in the first pair of frequencies (mode shape), the pulpitalmost does not move, as it was also observed in the experimental records. It is important to pointout the existence of some pairs of frequencies which were detected in the experimental campaign,but the model did not detect them. This is happening in the cases of Hagia Sophia #3 and #4,Akbyk and Mihrimah, between the first and the second pairs of modes in the numerical model.Structural interaction with the building might be responsible for in situ modes, which do not appear

in the model. The frequencies measured in the Mihrimah mosque support this hypothesis [25].The modal mass participation ratio was compared for the Akb yk, Hagia Sophia #2, andYeni Cami. It was observed that the modal mass participation of the first mode was 33, 26, and22%, respectively. The cumulative sum of the modal mass participation ratio reaches 70% forthe 5, 6, and 10th modes, for each of the abovementioned minarets. It is possible to concludethat for slender minarets with low frequencies, the higher modes have a greater participa-tion in the dynamic behavior.

8. STRUCTURAL RESPONSE

Istanbul is located in an area with intense seismic activity, where numerous destructive earthquakeshave occurred due to its proximity to the North Anatolian Fault.

The structural response of the minarets to seismic loads was evaluated in two stages. First, alinear response spectrum analysis was conducted, based upon recent studies on regional earthquakehazard. Then, seven ground motions were used as input for a linear time-history analysis. The firsttwo ground motions are real records from the Kocaeli earthquake, one representing a near-faultsource and the other a distant source. The remaining five time histories are synthetic, i.e. simulatedfor five rupture scenarios involving segments of the North Anatolian fault in the Sea of Marmara.These analyses were performed in Hagia Sophia #2, Yeni Cami, and Akb yk, three minaretspresenting the most dissimilar geometries.

The effects of wind and temperature are not considered, neither soil nor foundation problems.The live load is considered negligible for these analyses.


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Table VII. Probabilistic earthquake hazard for a represen-tative site in the Old City of Istanbul for different RPs.

PGA (g) SA(0.2s ) (g) SA(1.0s ) (g)

RP=72yr 0.214 0.460 0.193RP=475yr 0.438 0.926 0.437RP=2475yr 0.611 1.408 0.687

Figure 12. Response spectrum for a 475-yr RP probabilistic seismic hazard in Istanbul co-plotted withthe spectra of Yarmca and Fatih records.

Figure 13. Ground motions records from the 1999 Kocaeli earthquake: (a) Yarmca and (b) Fatih stations(Cosmos, KOERI 1999, [20]).

8.1. Seismic loads

The recent destructive events in the Marmara region of Turkey demanded for a proper reassessmentof the earthquake hazard in Istanbul. Probabilistic time-independent and time-dependent studieswere conducted in this area to define the peak ground acceleration (PGA) and spectral accelerations(SA) at 0.2 and 1 s periods, for 50, 10, and 2% probabilities of exceedance in 50 years (returnperiods (RP) of 72, 475, and 2475yr, respectively) [19]. According to Erdik [33], the seismicmotion that can affect the sites of these minarets is summarized in Table VII. The linear responsespectrum considered is presented in Figure 12, for an RP of 475 yr and a soil type B according toEuroCode 8 [34, 35].

Two time-history analyses were made with ground motions from the 17th August 1999 Kocaeliearthquake: the Yarmca record (near the fault) with a PGA of 0.322 g; and the Fatih record,obtained in the vicinity of the minaret sites, far from the fault and with a PGA of 0.189 g. Bothsignals are shown in Figure 13, for the more severe component, as retrieved from the Cosmosdatabase and recorded by the Kandilli Observatory and Earthquake Research Institute (KOERI).In Figure 12, the response spectra of the abovementioned records are plotted together with to thecode defined response spectrum for a 475-yr RP.


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Additionally, five simulated ground motion time histories were used as the input. They arebroadband hybrid simulations due to five rupture scenarios to take place on the central Marmara(two ruptures of M=7.4) and northern boundary segments of the North Anatolian Fault (threeruptures of M=7.0) in the Marmara Sea. The simulations are for a representative site in thehistorical peninsula of Istanbul in close vicinity of the minarets. The details of the simulationapproach and of physical properties related to tectonic and geological setting can be found in [29].

The largest simulated PGA is 0.39 g.

8.2. Results

We subjected the three models to the seismic loads described in the previous section. The actionswere applied only in one direction, as the minarets have radial symmetry. Both the responsespectrum analysis and the time-history analyses used a linear modal superposition method. Whena linear elastic model is being analyzed both the linear modal superposition method and the directintegration method give out approximately the same results. Nevertheless, the first one is faster asless computation is involved. For simplification, modal damping of 5% (critical) was consideredfor all modes, a value much higher than the one determined through the dynamic characterizationtests, but which could represent the influence of amplitude input motion.

Stresses and horizontal displacements for vertical loads and for earthquake loads are shown in

Table VIII. The total vertical load at each section requires the integration of all values around thecircle. The average values presented consider the integration in for the interval [45,+45],centered at the point of peak value. In Figure 14, the maximum tensile stresses are shown forresponse spectrum analysis and time-history analyses of Yarmca and Fatih records.

This structural analysis allowed us to make a first evaluation of the effect of an earthquake ona minaret according to its dynamic properties. The preliminary conclusion is that the main focusof concentration of stresses is next to the transition section, where a reduction in section area andthe deviation in the load path occur. Near the balconies, an increase in forces is also observed, dueto the increase in mass.

Hagia Sophia #2 is the minaret with the highest mass. Its frequency is higher than the YeniCami, due to its much higher stiffness. This has a direct effect on the results of response spectrumanalysis. The Hagia Sophia and the Akbyk are subjected to a pseudo-acceleration close to the

maximum and considerably higher than Yeni Cami (Figure 14).As a note for comparison, Tankut and Pnarbas [35] when monitoring of a leaning minaret

35 m high found that the top of minaret moves during the year due to temperature changes around30 cm without causing a critical situation (4 MPa compression stresses). Doubling these values,according to these authors, the collapse might be imminent.

In relation to the ground motion analysis, it is important to mention that the real recordsconsidered present different properties, as one is near the fault (Yarmca) and the other is distant(Fatih). This way, the stresses in Hagia Sophia are higher than the ones in Yeni Cami for theFatih ground motion and the opposite for the Yarmca. Nevertheless, the Yarmca ground motionpresents higher stresses in both cases, as it has a higher PGA.

The Akbyk minaret presents contrasting results, as the Fatih ground motion causes higherstresses than Yarmca. Although, for Akbyk the results are similar for all the analysis considered,the response spectrum analysis leads always to higher values in the other minarets. Anotherimportant point that needs to be mentioned is that the compression level is at least 2.5 times lowerthan the reference value for stones (26 MPa), but the tensile stresses are much higher than thereference value (2.6MPa). The stresses in the stairs are considerably smaller than in the exteriorwalls, almost 10 times lower.

The time-history analysis results of simulated accelerations are essentially parallel to those ofreal records. The Yeni Cami minaret experiences similar top displacements when analyzed undersimulated accelerations and under Yarmca record. The top displacements of Hagia Sophia minaretare in the same order under response spectrum analysis and when analyzed using the Yar mcarecord. The Akbyk minaret on the other hand experiences largest displacements in the responsespectrum analysis. In terms of maximum tensile stresses, the Yeni Cami minaret experiences


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Table VIII. Structural analysis results for the four load cases (dead-load and three input ground motions)for three minaret models (shaded is the equivalent seismic coefficient).

Maximum Maximum Average AverageTop Vertical Horizontal Equivalent compression tensile compression tensile

displacement reaction reaction seismic vertical vertical vertical vertical(m) (kN) (kN) coefficient (kPa) (kPa) (kPa) (kPa)

YeniCami Dead

LoadNA 9916.9 NA NA 705 NA 705 NA

Response 0.398 9919.2 2548.8 0.26 6417 5011 5579 4172spectrumGround 0.10 9917.3 1154.4 0.12 2373 1074 2121 816motion 1(Fatih)Ground 0.68 9917.5 2373.9 0.24 10 595 8811 9138 7403motion 2(Yarimca)

Hagia Dead NA 45305.0 NA NA 764 NA 652 NASophia Load#2 Response 0.254 45594.0 15389.1 0.34 9084 7701 7347 6103

spectrumGround 0.14 45 340.2 8534.7 0.19 5241 3910 4246 3133motion 1(Fatih)Ground 0.30 45 348.6 11336.4 0.25 9991 8607 8009 6763motion 2(Yarimca)

Akbyk Dead NA 2048.0 NA NA 321 NA 321 NALoadResponse 0.131 2048.0 848.0 0.41 3755 3386 3362 2880spectrumGround 0.13 2048.0 666.0 0.33 4019 3465 3483 2933motion 1(Fatih)Ground 0.11 2048.0 737.0 0.36 3048 2823 2444 2368motion 2(Yarimca)

similar levels of stresses under simulated accelerations and Yarimca record accelerations. For HagiaSophia minaret as well simulated and Yarimca accelerations are more critical. For the Akb ykminaret similar levels of stresses are obtained from response spectrum analysis and from simulatedaccelerations.

The Yeni Cami and Hagia Minarets are more sensitive to near-field ground motions. Theyexperience considerable top displacements and compressive and tensile stresses. The Akb ykminaret presents a higher response for ground motion in the far field. It appears whether a destructivefuture earthquake will take near the city or far away from it will lead to different damage patternsin the minarets in Istanbul. The observation made for the Yarmca and Fatih records regarding the

stresses that they cause in the minarets depending on whether they are recorded in the near fieldor far field, holds for the simulated records as well.

9. CONCLUSIONS AND FINAL CONSIDERATIONS

An extensive dynamic characterization campaign was performed in 11 minarets in Istanbul, whichallowed for the determination of frequencies, modes of vibration and damping. The finite elementmodeling and analysis of seven of these minarets were performed. Linear dynamic structuralanalyses were conducted to access their earthquake risk level.


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Figure 14. Maximum tension stresses in the exterior wall. The presented values are theextreme values of the colored scale, in kPa.

Ambient vibration tests were conducted in all minarets recording the natural excitation at threelevels. Dynamic characteristics were determined through the FDD, EFDD, and SSI techniques,which permitted the identification of the first seven frequencies.

Using all the data gathered, an empirical formula was developed to estimate the first frequencyof vibration. This formula considers that the frequency is proportional to the square root of thesecond moment of area and almost inversely proportional to the square root of the height of theminaret. The formula leads to an upper limit with an error of 30%. Performing in situ testingin other minarets will contribute to validate the empirical formula, probably reducing the errorsassociated with it by considering the modulus of elasticity as well.

This formula has the importance to offer an expedite tool for the structural engineer to have afirst estimate of the minaret response to a given earthquake input.


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By calibrating the modulus of elasticity of the exterior walls, seven finite element models ofthe minarets were created, obtaining modal results similar to the experimental ones. Translation,torsion, and vertical modes of vibration were obtained. Effects of the connection to adjacent build-ings (mosque) or flexibility of the foundations should be accounted for in further studies. After themodels calibration, the seismic behavior three minarets, that presented the most distinguishinggeometry, was investigated. For this purpose, linear analyses were conducted. A response spectrum

analysis, based upon the most recent studies in earthquake hazard levels, was conducted. Addi-tionally, two ground motion records, one representing a near-fault site and the other a distant siteand five simulated accelerations were used in the time-history analysis. Owing to each minaretsdistinctive dynamic characteristics, the seismic response is different from case to case. Neverthe-less, the compression stresses obtained in all studied cases were below the reference maximumvalue. The tensile stresses however were considerable, as the minarets do not have high dead-loadsto balance the tensile forces created by earthquake loads. In a real seismic event, such high valuesof tensile stresses would probably not be achieved, as cracking would lead to some energy dissi-pation and eventually it would cause the minaret to collapse. This can only be further investigatedif nonlinear models are used. But with the linear modeling we can see that the main areas of stressconcentration are at the end of the transition phase, where a reduction in wall thickness occurs,and at the balconies, due to the increase in mass.

Several retrofit techniques can be considered to correct the observed deficiencies. One possibilityis to increase the resistance of the masonry through the confinement with exterior steel rings, as inchimneys. Owing to the low level of compression forces, a post-tension rod could be placed in thecentral core, increasing its stability. Another solution to increase the minaret resistance would beto use a helicoidal-spiral (wire-mesh or carbon-fibre) in the inner side of the outer walls. One lastidea would be to place tuned mass dampers inside the spire. For each case the effects of retrofitmust be carefully evaluated, in terms of stresses (or bending moments, shear, compression, andtension) and displacements developed.

Further studies would include analysis under site-specific ground motion and consequently anonlinear modeling of the mechanical properties of the masonry components. Also, the effect ofsoilstructure interaction should be analyzed.

Permanent instrumentation is desirable in at least one minaret consisting of strong motionaccelerometers and displacement transducers [36]. The construction materials should be charac-

terized by ultrasonic testing and other tests, to determine their mechanical, physical, and chemicalproperties. Also, the use of shaking table to calibrate the numerical model could be of great interest.This way the performance of proposed retrofit methods could be theoretically assessed and exper-imentally verified.

ACKNOWLEDGEMENTS

CSO acknowledges the partial financial support given by Fundao para a Cincia e a Tecnologia, Portugal(FCT) through its Pluri-Annual Programme. Ana Mateus made a few initial tests on the numerical modelingof the minarets. This work was made under the State Planning Organization through the Project 2003K120250 Development Emergency Retrofitting Techniques for Historical Structures.

To Mr Nafiz Kafadar, Ahmet Korkmaz, Gkhan Kesti and Orhan Ersah for handling the instrumentationand pre-processing the instrumental data.

To GeneralDirectorate of Foundations, Mr Yucel Sezek, Istanbul RegionalDirectorate, IstanbulDirectorate for Monuments and Reconstruction of Turkish Ministry of Tourism and Culture, for allowingthe field work in the minarets.

To Prof. M. Erdik for all support in this research.

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