Engineering Structures 30 (2008) 2253–2264www.elsevier.com/locate/engstruct

Dynamic analysis and seismic performance of reinforced concrete minarets

Halil Sezena,∗, Ramazan Acarb, Adem Dogangunb, Ramazan Livaogluc

a Civil & Environmental Engineering and Geodetic Science, The Ohio State University, 470 Hitchcock Hall, 2070 Neil Ave., Columbus,OH 43210-1275, United States

b Department of Civil Engineering, Karadeniz Technical University, 61080, Trabzon, Turkeyc Department of Civil Engineering, Gumushane Engineering Faculty, 2900, Gumushane, Turkey

Available online 20 February 2008

Abstract

An unusually large number of minarets, which are slender tower structures, collapsed during the 1999 Kocaeli and Duzce, Turkey earthquakeswith resulting damage to surrounding buildings and loss of life. The potential effects of the subsequently observed poor reinforcement detailingon the dynamic response is discussed. The probable cause of the extensive damage to reinforced concrete minarets is investigated by studying theobserved failure modes and their seismic performance, and through the dynamic analysis of a representative minaret. The effects of spiral stairs,door openings, and balconies on the dynamic behavior are examined. The maximum dynamic internal force demands were compared with thecalculated capacities. The locations of the maximum axial, shear, and flexural demands predicted from the finite element analysis of the minaretmodel were consistent with the earthquake damage observed at those critical locations.c© 2007 Elsevier Ltd. All rights reserved.

Keywords: Minaret; Reinforced concrete; Finite element analysis; Earthquake damage

1. Introduction

A minaret is a slender tower built next to a mosque. Whilemost historical minarets were constructed using reinforced orunreinforced stone or brick masonry, the majority of minaretsrecently constructed in Turkey are reinforced concrete (RC)structures. As shown in Fig. 1, a typical minaret structurecomprises a base or boot on top of its foundation, a taperedtransition segment, a circular body or shaft with one or morebalconies, and a spire at the top. The base or boot is usuallysquare or polygonal, and is sometimes called the pulpit byarchitects. The minaret can be free standing or the boot may beattached to the mosque structure. The minaret contains interiorspiral stairs running all the way up to the highest balcony levelwhich are not externally visible. Historically the balconies arebuilt so that someone could climb up the stairs and call forprayer. With the advent of loudspeakers, these balconies arenot needed; however, one or more balconies are built in each

∗ Corresponding author. Tel.: +1 614 292 1338; fax: +1 614 292 3780.E-mail addresses: sezen.l@osu.edu (H. Sezen), racar@ktu.edu.tr

(R. Acar), adem@ktu.edu.tr (A. Dogangun), rliva@ktu.edu.tr (R. Livaoglu).

0141-0296/$ - see front matter c© 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.engstruct.2007.11.005

minaret mainly for architectural reasons. Balconies create massconcentrations along the minaret’s height and affect its dynamicstructural response.

Currently, there are no structural code requirements orguidelines for the design of reinforced concrete minarets, orminarets in general, in Turkey. As a result, these slenderstructures have been built, for the most part, by experiencedcontractors and construction workers with no engineeringknowledge. In most cases, each contractor constructs a typicalminaret with the same structural and architectural featuresregardless of the local soil conditions or seismicity of theregion.

Turkey is located in one of the most seismically activeregions of the world. Fifty-seven destructive earthquakeshave struck Turkey in the twentieth century, resulting inthe destruction of infrastructure and more than 90 000deaths. During these earthquakes, many minarets weredamaged or have collapsed. Sezen et al. [14] documentsand discusses vulnerabilities and damages to 64 masonryand RC minarets after the 1999 Kocaeli (Mw7.4) and Duzce(Mw7.2) earthquakes. As a result of these two earthquakes,the collapse of 115 minarets in the city of Duzce alone was

2254 H. Sezen et al. / Engineering Structures 30 (2008) 2253–2264

Fig. 1. Typical reinforced concrete minarets in Turkey.

reported [6]. Sezen et al. reports that approximately 70% of theRC and masonry minarets surveyed in Duzce sustained severedamage or collapsed. Even though the minarets are hardlyever occupied, they are located mostly in residential areas orshopping districts, and their collapse sometimes causes lossof life. It is extremely important to regulate the constructionand design of these slender structures for safety reasons inanticipation of future earthquakes. This study attempts toidentify the structural vulnerabilities of minarets based on theirpast seismic performance.

In addition to widespread earthquake damage and collapses,some reported failures of minarets due to wind loadingindicate that most of these tower structures are vulnerableto lateral loads. A large number of research studiesinvestigating the seismic response of historical masonryminarets and towers are available [3,7,11,15–17]. However,there are only a few studies investigating the lateral responseof RC minarets [8,14]. Dogangun et al. [4,5] investigatethe architectural and structural properties of these slenderstructures. The description of each minaret segment and theassociated observed damage are presented below.

2. Observed damage and implications

The type and distribution of damage in a structure variesgreatly depending on many factors, including the detailing andproperties of the structure and its components, soil properties,and the magnitude of the earthquake. The effect of localsoil conditions on the seismic response of RC minarets isinvestigated by Acar et al. [1]. Observations from recentearthquakes suggest that the damage in the minarets is usuallyconcentrated in a few specific locations. These observed localdamage concentrations and vulnerabilities of minarets arepresented here.

Fig. 2. Damage to the transition segment (photos by (a) Firat [8] and (b)Scawthorn [13]).

The relatively stiff boot or base of the minaret normallysuffers no damage. The stiffness and strength of the minaret arereduced over the height of the tapered transition segment witha larger square or polygonal shape near its bottom and circularshape near the top. In a few cases, damage over the transitionsegment was observed. Fig. 2 shows two such cases wherethe concrete cracking or spalling was either spread over thesegment or concentrated near the top just below the cylindricalbody.

Horizontal circumferential cracks and concrete spalling nearthe bottom of the minaret cylinder or body were the mostcommon types of damage, leading to the collapse of RCminarets (Fig. 3). There are two main reasons for this type offailure. First, the cross section size becomes smaller, whichresults in reduced lateral and flexural strength. Second, asshown in Fig. 3 in most cases at that location all longitudinalsteel bars were lap spliced, creating a discontinuity. Prior to1999, smooth reinforcing bars were commonly used in Turkeybecause they are less expensive, more readily available thanribbed bars, and easier to bend and cut on site compared withribbed bars. Considering that the anchorage length required forthe smooth longitudinal bars is significantly larger than that ofdeformed bars, it is most likely that the lap spliced longitudinalbars failed before the full flexural strength could be developed.However, many other minaret collapses, e.g., top two picturesin Fig. 3, suggest that failure may have occurred simply becauseof insufficient flexural strength near the bottom of the cylinder.

The minaret shown in Fig. 4 survived after the August 17,1999 Kocaeli earthquake with some apparent distress causinglight cracks and concrete spalling near the cylinder base. Afternearly three months, during the November 12 Duzce event,the minaret collapsed at the section near the bottom of thecylinder where the smooth longitudinal reinforcing bars hadbeen spliced. The lap splice length was approximately 800 mm(Fig. 4b). The ends of the longitudinal bars had 180◦ hooks(Fig. 4c). It appears that the combination of smooth barswith 180◦ end hooks, and the existence of short lap splices,

H. Sezen et al. / Engineering Structures 30 (2008) 2253–2264 2255

Fig. 3. Minaret failures near the bottom of the cylinder bodies.

Fig. 4. Minaret in Kocyazi after the August 17 earthquake (minor cracks, [8]) and November 12 earthquake (collapse).

created a vulnerable region near the bottom of the cylinder.At that location, light or insignificant damage was observedafter the first earthquake, and collapse occurred after the secondevent. Anecdotal evidence and the picture of the survivedminaret (Fig. 4a) indicate no significant damage or permanentdeformation after the Kocaeli event. This shows that the minaretprobably stayed elastic during that earthquake. The typicalfailure mode (as in Fig. 3) after the second event suggests thatthe minaret was vulnerable near the bottom of its cylindricalbody and had very little or no inelastic strength and deformationcapacity to resist strong lateral forces during the latter event.

Many similar post-earthquake reconnaissance observationsprovided evidence for the probable cause of failure, whichis typically a result of sudden lateral and flexural strength

reduction due to a combination of several factors, including theuse of smooth rebar leading to weaker bond between concreteand steel, transverse hoops with hooks rather than continuousspiral reinforcement, short longitudinal lap splices, and thechoice of lap splice location where the cross section is reducedto a circle with a smaller size. Furthermore, discontinuedlongitudinal rebar with 180◦ end hooks seemed to contributeto the sudden stiffness and strength change near the bottom ofthe cylinder.

The collapsed minaret shown in Fig. 5 is a good exampleillustrating that the transverse reinforcement had 180◦ endhooks and all smooth longitudinal bars with 180◦ end hookswere cut at the same location where the failure occurred. At thiscross section, due to longitudinal bar end hooks, the amount

2256 H. Sezen et al. / Engineering Structures 30 (2008) 2253–2264

Fig. 5. Splicing of transverse reinforcement and longitudinal bars with 180◦ end hooks.

Fig. 6. Failure around mid-height of a minaret.

or effective area of concrete is reduced with both possiblepoor concrete confinement and potential unnecessary steelcongestion. The current Turkish Standards Code (TS 500, 2000)does not allow 180◦ end hooks at the end of longitudinal barsin RC components or structures. Similarly, the 180◦ hooks atthe end of the transverse reinforcement or hoops (e.g., Figs. 4cor 5) may open up under cyclic loads, and do not confineconcrete as effectively as spirals. In minarets which have a ringshaped cross section, the effective confinement of concrete is achallenge. The use of 180◦ hooks at the end of both longitudinaland transverse steel exacerbates the problem near the bottom ofthe minaret cylinder where the longitudinal rebar is usually lapspliced.

Structural failure or damage to the upper portion of thecylindrical body of the minarets was observed less frequently.If and when structural damage occurs, it is typically associatedwith some irregularities such as larger mass or stiffnessconcentrations around balconies.

Longitudinal rebar often may be lap spliced and notanchored well at those locations. Fig. 6 shows one such casewhere lap spliced longitudinal rebar with 180◦ end hooks exist.No sign of distress or damage at other locations, including thebottom of the cylinder, suggests that longitudinal reinforcementdiscontinuity created by the lap splices may have been theprimary cause of this specific failure.

No damage was reported to the spires that are RC andmonolithically connected to the minaret body. Metal sheet iscommonly used for spires because of its lightweight and easyinstallation. There were few instances of metal spire failureover the virtually undamaged minaret body. If the metal spire

is anchored to the top of the minaret body properly, no damageshould be expected.

In almost all cases, as shown in Figs. 2 through 6, the stifferminaret base or boot is not damaged. Also, the boot is usuallyattached to the mosque structure, making it relatively rigid.However, if the boot is not attached to the mosque or if nearbystructures of part of the mosque structure hit the boot, the rigidbody rotation of the boot and minaret failures may be observedas shown in Fig. 7.

The above discussion of minaret failures focused on how andwhy minarets may have failed during the recent earthquakesin Turkey. In many cases minarets fell on top of the mosquecreating a potential structural hazard and causing casualties(Fig. 3). It is also possible that the minaret may fall on nearbybuildings. As shown in Fig. 8, during the 17 August earthquakeone minaret fell on an otherwise virtually undamaged buildingcausing damage in the upper stories. It is recommended thatthere should be a safe distance between the minaret andsurrounding buildings. To the authors’ knowledge, no safedistance requirement exists in the current Turkish design andconstruction regulations. A practical safe distance from theminaret to the nearest structure could be the cylindrical bodylength between the top of the transition region and spire (Fig. 1).This assumes that the minaret is not going to fall on the mosquestructure and the spire is made up of light metal sheet material.

3. Dynamic analysis of a representative minaret

The architectural, geometrical, and material properties ofminarets vary widely. For example, the height of a typicalminaret can be between 10 m and 55 m. The minaret may

H. Sezen et al. / Engineering Structures 30 (2008) 2253–2264 2257

Fig. 7. Rotation and failure of boots (photos by: (a) Anatolianquake [2], and (b) Locke [10]).

Fig. 8. Collapse of a minaret onto a nearby building [9].

have one or more balconies. The representative RC minaretinvestigated in this study is assumed to be 30 m high, includinga 6 m boot or base, 2 m transition segment, 17 m cylindricalbody, and 5 m spire. The assumed outer diameter and thicknessof the cylinder are 1.76 m and 0.18 m, respectively. The planview at the balcony level and elevation of the representativeminaret are shown in Fig. 9.

The assumed material properties are as follows: specifiedconcrete compressive strength, f ′

c = 16 MPa (designcompressive strength is 11 MPa according to the Turkishcodes), modulus of elasticity of concrete, Ec = 27 000 MPa,concrete unit weight, γ = 25 kN/m3, limiting concretecompressive strain εcu = 0.003, smooth reinforcing barswith minimum specified yield strength, fy = 220 MPa,strain at hardening = 0.002, and fracture strain = 0.12.It is assumed that the minaret is located in a high seismicregion with a soft soil site (Subsoil class Z4, and Zone 1in Turkish Earthquake Code, [18]). According to the TEC,the structural behavior factor, R is 3, and importance factor,I is 1.2 for such structures. In this study, R = 3 is usedin the response spectrum analysis as required by the TECfor the design of such structures. On the other hand, elasticmaterial properties are used in all dynamic analyses presentedbelow. This is mainly because the vast majority of RC minaretsfailed to develop plastic hinges during recent earthquakes. Theyeither failed without any indication of ductile response (Fig. 3),or remained elastic with virtually no visible damage. Thedamage observed in Fig. 2b is an exception to more than 40RC minarets surveyed by Sezen et al. [14].

It is usually burdensome to include all components in astructural model and consider their effect on the total behavior.

In this study, four finite element models representing the sameminaret will be used to show how certain structural componentsaffect the dynamic response, and thus to decide whether theyshould be included or can be ignored in a simplified model.The first model (Model 1) includes all components of theminaret. The interior spiral stairs are ignored only in Model 2,the two balconies are ignored only in Model 3, balconies anddoor openings at the balcony levels are ignored in Model 4,and stairs, balconies, and door openings at the balcony levelsare all ignored in Model 5 (Fig. 10). The computer program,SAP2000 [12] was used to analyze the models shown in Fig. 10.

The response spectrum analysis of each minaret model iscarried out using the design spectrum specified in the TEC [18]as shown in Fig. 11. Two ground motions recorded during the12 November Duzce and 17 August 1999 Kocaeli earthquakes(Fig. 12) are also used in the dynamic modal time historyanalyses of the minaret models.

The minarets are essentially slender towers with almostuniformly distributed mass, including additional concentratedmasses if there are balconies. Thus, the dynamic behavior ofthese towers is strongly influenced by the higher mode affects.All modes with total modal mass participation of 90% or moreare included in the time history analysis. Periods, participationfactors and directions for the first seven modes estimated fromthe analyses are given in Table 1. As shown in the table,the dynamic response participation of the first few modes ofthe minarets are relatively small compared with those of atypical frame structure. The total participation factor for thefirst few modes is usually larger than 90% for typical framestructures, indicating that the effect of higher modes on thedynamic response of minarets is significant. Also shown in thetable are the periods for the first eight and their percentageparticipations in the total response. First two modal shapes inthe two directions, and the 7th mode shape for Model 1 areshown in Fig. 13.

The maximum lateral displacement distribution over theheight of the full minaret model (Model 1) is shown in Fig. 14.The maximum lateral displacements at the top of the transitionsegment or bottom of the cylindrical body are 0.007, 0.008,and 0.010 m, as calculated using the TEC design spectrum andKocaeli and Duzce ground motions, respectively. The overalllateral displacement distribution shows that most of the flexuraldeformations occur over the height of the relatively slendercylindrical minaret body.

2258 H. Sezen et al. / Engineering Structures 30 (2008) 2253–2264

Fig. 9. Geometrical and cross-sectional properties of the representative minaret.

Table 1The first few modes and their participation factors

Modes 1st 2nd 3rd 4th 5th 6th 7thDirection y x y x y x Torsion

Model 1 Period (s) 0.643 0.626 0.136 0.131 0.056 0.055 0.053Participation (%) 46.3 0.0 22.7 0.0 9.0 0.0 0.0

Model 2 Period (s) 0.627 0.611 0.127 0.122 0.052 0.051 0.052Participation (%) 48.5 0.0 21.4 0.0 8.9 0.0 0.0

Model 3 Period (s) 0.608 0.592 0.128 0.123 0.052 0.052 0.046Participation (%) 42.8 0.0 24.0 0.0 10.9 0.0 0.0

Model 4 Period (s) 0.594 0.594 0.122 0.122 0.051 0.051 0.044Participation (%) 42.1 0.2 10.2 13.1 5.6 5.6 0.0

Model 5 Period (s) 0.578 0.578 0.113 0.113 0.047 0.047 0.043Participation (%) 43.6 0.4 21.6 0.2 8.4 3.0 0.0

The lateral displacements at the top of the minaret, ascalculated from the modal time history analysis of Model 1using the 1999 Duzce and Kocaeli input motions, are shownin Fig. 15. The maximum calculated displacements were 0.14 mand 0.10 m for the Kocaeli and Duzce earthquakes, whereasthe maximum displacement calculated using the code designspectrum was 0.08 m.

Post-earthquake investigation of minaret failures revealedthat the majority of minarets failed near the bottom of thecylindrical body immediately above the transition segment

(Fig. 3). The variations of axial and shear stresses at theintersection of cylinder and the transition segment are shownin Figs. 16 and 17. The maximum calculated compressive axialstress is 14 MPa, which is lower than the specified concretecompressive strength of 16 MPa. The maximum calculatedtensile axial stress is about 16.5 MPa, indicating that theconcrete has already cracked in tension. Considering that thespecified tensile strength of steel rebar is 220 MPa, a tensionfailure should not occur as long as the bars are not cut or donot have short lap splices at that location. It is noted that the

H. Sezen et al. / Engineering Structures 30 (2008) 2253–2264 2259

(a) Model 1: Includesall components.

(b) Model2: Stairsareignored.

(c) Model 3:Balconies areignored.

(d) Model 4:Balconies anddoor openings areignored.

(e) Model 5: Stairs,balconies and openingsare ignored.

Fig. 10. Finite element models for the representative minaret with and without spiral stairs.

Fig. 11. Design spectrum recommended by TEC [18].

maximum displacements and maximum stresses occur at aboutthe same time, at 4.1 s and 6.1 s for the Duzce and Kocaeliearthquakes, respectively. The axial compressive and tensilestress contours for the basic minaret model (Model 1) are shownin Fig. 18 when the maximum stresses and displacements werecalculated for the two earthquake motions. Consistent with theobserved performance, the stress contours show that both thelargest compressive and tensile axial stresses are concentratedin the cylindrical body within a few meters above the transitionsegment. The stress distributions were very similar for bothearthquakes with larger stresses measured for the Duzceearthquake. It should be noted that in the minaret modelanalyzed, the effect of longitudinal rebar lap splices or barpullouts were not considered. The existence of short lap spliceswithin the highly stressed minaret body probably intensifiedthe problem and led to collapses during the earthquakes. Eventhough no RC minaret failures were observed by the authors inor around the balconies, the calculated stresses were high butstill much smaller than those below the lower balcony.

2260 H. Sezen et al. / Engineering Structures 30 (2008) 2253–2264

Table 2The maximum internal forces at the top of the transition segment (height = 8 m), and at the bottom of the minaret or ground level

Height Shear forces (kN) Bending moments (kN m) Axial forces (kN)DSa THA-Db THA-Kc DS THA-D THA-K DS THA-D THA-K

Model 1 8 m 204 370 272 2490 4695 3371 592 593 591ground 214 465 304 3902 7295 5414 960 966 962

Model 2 8 m 173 303 249 2210 4056 3076 541 541 541ground 187 349 285 3145 6184 4956 860 860 860

Model 3 8 m 172 296 241 2151 3657 2897 507 508 507ground 187 365 275 3351 5970 4742 873 878 876

Model 4 8 m 174 298 246 2179 3746 2954 514 515 514ground 189 371 281 3394 6018 4834 888 893 891

Model 5 8 m 145 251 231 1901 3724 2821 458 458 458ground 160 322 281 2913 5345 4628 785 785 785

a Analysis using design spectrum.b Modal time history analysis using Duzce record.c Modal time history analysis using Kocaeli record.

Fig. 12. Ground motions recorded (a) at DZC station in Duzce during the 12 November Duzce, and (b) at YPT station in Yarimca during the 17 August Kocaeliearthquakes.

Fig. 13. First two modal shapes in the two directions, and the 7th mode shapesfor Model 1.

Fig. 19 shows the distribution of maximum shear stressesalong the minaret’s height. The existence and location ofspiral stairs noticeably influence the magnitude and distributionof shear stresses. Fig. 20 shows the maximum shear stresscontours calculated using the code design response spectrum. Inthe same figure, the locations of spiral stairs inside the minaretare also shown. Evidently the stairs increase the shear strengthand reduce the stress significantly.

Fig. 14. Lateral displacement distribution over the height of minaret calculatedusing (a) TEC design response spectrum, and (b) modal analysis using Kocaeliand Duzce motions.

The maximum internal forces calculated from the dynamicanalysis of the representative minaret models using the Duzceand Kocaeli earthquake records and code design spectrum aresummarized in Table 2. The internal forces calculated from the

H. Sezen et al. / Engineering Structures 30 (2008) 2253–2264 2261

Fig. 15. Lateral displacement time histories at the top of the minaret subjected to (a) Duzce, and (b) Kocaeli earthquakes.

Fig. 16. Time history of normal stresses at the top of the transition segment for the (a) Duzce, and (b) Kocaeli earthquakes.

Fig. 17. Time history of shear stresses at the top of the transition segment for the (a) Duzce, and (b) Kocaeli earthquakes.

code design spectrum seem to be relatively small compared tothe forces from modal time history analyses. Although the twoground motions result in fairly close shear and axial forces,the moments, and hence the axial bending stresses, appear toincrease significantly at the bottom of the minaret compared tothe top of the transition segment located at 8 m above ground.

4. Demand versus capacity

Both the dynamic analysis results and observed minaretfailures indicate that the bottom of the cylindrical body or thetop of the transition segment is the most vulnerable section inRC minarets. As shown in Table 2, the maximum internal forcesat that level were calculated for Model 1, which includes allstructural components of the minaret. These maximum elasticmoment and shear force demands will be compared with thepredicted moment and shear strengths here.

During the post earthquake reconnaissance visits, theauthors observed that the most common steel rebar used inthe earthquake affected region was S220 with a yield strength

of 220 MPa. For easy workmanship, small size bars withdiameters of 12 or 14 mm were commonly used. In manycases, it was found that the longitudinal reinforcement ratiowas close to or smaller than the minimum code specified ratioof 0.01. Many studies carried out after the 1999 earthquakesfound that the concrete strength could be as low as 10 MPa.In this study, a concrete design strength of 11 MPa is used. Itshould be noted that according to the current Turkish buildingcode [19], the minimum concrete strength is 20 MPa. Althoughthe critical section includes two layers of longitudinal rebar(Fig. 8), a single layer of steel is assumed at the centerlineof the section (Fig. 21). The calculated axial load–momentinteraction diagram is plotted in Fig. 21 for a typical reinforcedconcrete ring section with a 0.18 m thickness and 1.76 m outerdiameter (do) and 1.40 m inner diameter with a correspondingcross sectional area, Ac of 0.9 m2. The diagram also showsthe elastic axial load–moment demands calculated from thedynamic time history analysis and code design spectrum. Theaxial load, N is assumed to be 592 kN (N/Ac fc = 0.06).

2262 H. Sezen et al. / Engineering Structures 30 (2008) 2253–2264

Fig. 18. Maximum tensile stress contours, Smax for (a) Duzce, and (b) Kocaeli records, and compressive stress contours, Smin for (c) Duzce, and (d) Kocaeliearthquakes.

Fig. 19. Maximum shear stress contours for (a) Duzce, and (b) Kocaeli records.

The normalized moment demands (M/Acdo fc) are 0.14, 0.27,and 0.19 for the code design spectrum and dynamic responsewith Duzce and Kocaeli motions, respectively. The comparisonof axial load–moment demands and the capacity curve suggeststhat the flexural capacity of the minaret model considered

Fig. 20. Effects of spiral stairs on shear stress distribution in Model 1.

here is not sufficient, and smaller than the inelastic demandscalculated using the code design spectrum and elastic demandsfrom the dynamic time history analysis. It should be notedthat the flexural demand calculated from the code designspectra is significantly lower than the demand imposed on theminaret during the two earthquakes. This, and the displacementresponse plotted in Fig. 14, imply that the strength anddisplacement capacity required by the code design spectra maybe much lower than the expected demand during large eventssuch as Kocaeli and Duzce earthquakes.

H. Sezen et al. / Engineering Structures 30 (2008) 2253–2264 2263

Fig. 21. Axial load–moment interaction diagram and predicted demands.

The shear strength of the cylindrical minaret body ispredicted from the column shear strength equation provided inthe Turkish Building Code, TS500 [19].

Vcap = 0.8(

0.65 fctd Ac

(1 + 0.007

N

Ac

))+

Asw fywdd

s(1)

where Ac is the gross cross-sectional area, fctd is design tensilestrength of concrete, N is the factored axial force calculatedunder simultaneous action of seismic lateral and axial loads,Asw is the transverse steel area, fywd is the design yieldstrength of transverse reinforcement, d is the effective depthof the section, and s is the transverse reinforcement spacing.Ignoring the contribution of shear reinforcement, the shearcapacity of a typical minaret cross section is estimated asVcap = 423 kN with Ac = 900 000 mm2, fctd = 0.9 N/mm2,N = 592 kN. The predicted shear strength, even without thestrength contribution from transverse steel, appears to be muchlarger than the calculated shear force demands listed in Table 2.

5. Conclusions

Observations from the minarets that collapsed duringrecent earthquakes and the analyses of a representativeminaret showed that the bottom of the cylindrical minaretbody immediately above the transition segment is the mostvulnerable section under seismic loading. The poor designpractices include the use of: (1) smooth steel rebar, (2) 180◦

end hooks at the ends of both the transverse and longitudinalreinforcements, (3) unstaggered and short longitudinal lapsplices, (4) inadequate transverse hoops instead of a spiralreinforcement, and (5) a short transition length between thesquare boot and cylindrical body. These practices exacerbatedthe problem of insufficient bending strength and deformation

capacity near the bottom of cylindrical part and increased thesusceptibility of this section to failure. Practicing engineers andcontractors can improve the design of minarets by providinga more gradual change from a square or polygonal section toa smaller circular section using a longer transition segment,and by eliminating lap splices near the critical section andusing instead staggered lap splices over the height of theminaret body. When either stairs or balconies are ignored inthe analysis, the maximum shear and flexural demands wereunderestimated by approximately 20%.

It was found that the shear strength of the minaret waslarger than the maximum shear demands calculated from thedynamic analysis, indicating that shear was not the likely causeof failure. In addition, the shear stress demands were reducedat locations where spiral stairs existed. The results of the elastictime history analyses have shown that the flexural capacity atthe critical section would be exceeded when the representativeminaret is subjected to ground motions recorded during recentearthquakes. This is partially because the flexural strength issmaller under relatively small axial loads. The strength anddisplacement capacities calculated using the inelastic codedesign spectra may be much lower than the elastic demandsimposed during large seismic events such as the Kocaeliand Duzce earthquakes. Almost all minarets surveyed afterthe recent earthquakes either behaved elastically or collapsed(Fig. 3). The existing design and construction practices shouldbe improved to provide sufficient ductility. Otherwise, it willbe misleading to use the inelastic response spectrum analysisprescribed by the current code.

References

[1] Acar R, Livaoglu R, Dogangun A, Sezen H. The effects of subsoilson the seismic response of reinforced concrete cylindrical minarets. In:

2264 H. Sezen et al. / Engineering Structures 30 (2008) 2253–2264

Fifth international conference on seismology and earthquake engineering.2007. SE-24.

[2] Anatolianquake. A curated repository of data on the Kocaeli-Golcuk andBolu-Duzce earthquakes of 1999. http://anatolianquake.org/; 1999.

[3] Dogangun A, Sezen H, Tuluk OI, Livaoglu R, Acar R. Traditional Turkishmonumental structures and their earthquake response. InternationalJournal of Architectural Heritage 2007;3(1):251–71.

[4] Dogangun A, Tuluk OI, Livaoglu R, Acar R. Traditional Turkish minaretson the basis of architectural and engineering concepts. In: Proceedingsof the first international conference on restoration of heritage masonrystructures. 2006. P34, p. 1–10.

[5] Dogangun A, Acar R, Livaoglu R, Tuluk OI. Performance of masonryminarets against earthquakes and winds in Turkey. In: Proceedings ofthe first international conference on restoration of heritage masonrystructures. 2006. P32, p. 1–10.

[6] Duzcedamla. Mosques expose the effects of earthquake. http://www.duzcedamla.com/text/index.dwx?TextID=2811; 2006 [in Turkish].

[7] El-Attar AG, Saleh AM, Osman A. Seismic response of a historicalMamluk style minaret. In: Earthquake resistant engineering structures III.WIT; 2001. p. 745–54.

[8] Firat YG. A study of the structural response of minarets in the1999 Anatolian earthquakes. M.S. thesis. West Lafayette (IN): PurdueUniversity; 2001. Also, http://cobweb.ecn.purdue.edu/∼anatolia/FIRAT/5adapazari/adapazari2.ppt#257,2,Slide2.

[9] Hurriyet. Newspaper earthquake photo archive. http://arsiv.hurriyetim.com.tr/fix98/deprem/depremgaleri.htm; 2006.

[10] Locke J. Damage photos for 1999 Earthquakes. http://www.marin.cc.ca.us/∼jim/photos/turkey/duz6.jpg; 2006.

[11] Menon A, Lai CG, Macchi G. Seismic hazard assessment of the historical

site of Jam in Afghanistan and stability analysis of the minaret. Journal ofEarthquake Engineering 2004;8(1):251–94.

[12] SAP2000. Integrated software for structural analysis & design. California:Computers and Structures Inc.; 2006.

[13] Scawthorn C. Preliminary Report Kocaeli (Izmit) Earthquake of 17August 1999. http://mceer.buffalo.edu/research/Reconnaissance/turkey8-79/ScawPrelim.pdf; 1999.

[14] Sezen H, Fırat GY, Sozen MA. Investigation of the performance ofmonumental structures during the 1999 Kocaeli and Duzce earthquakes.Paper no: AE-020. In: Fifth national conference on earthquakeengineering. 2003.

[15] Sykora D, Look D, Groci G, Karaesmen E, Karaesmen E. Reconnaissancereport of damage to historic monuments in Cairo. Egypt following theOctober 12, 1992 Dahshur earthquake. Technical report NCEER-93-0016.National Center for Earthquake Engineering Research, State University ofNew York at Buffalo; 1993.

[16] Syrmakezis CA, Asteris PG, Sophocleous AA. Earthquake resistantdesign of masonry tower structures. In: Structural studies, repairs andmaintenance of historical buildings. Advances in architectural series,vol. 3. Southampton (UK): Computaional Mechanics Publications; 1997.p. 377–86.

[17] Syrmakezis CA. Seismic protection of historical structures andmonuments. Structural Control and Health Monitoring 2006;13:958–79.

[18] TEC. Turkish code. Ministry of Public Works and Settlement. 1998.Specification for structures to be built in disaster areas; Part III-earthquakedisaster prevention, Government of Republic of Turkey; 1998.

[19] TS500. Requirements for design and construction of reinforced concretestructures. Ankara (Turkey): Turkish Standards Institute; 2000 [inTurkish].