Historical structures are the values that are of great importance to that country, showing the roots of a country, and must be passed on from generation to generation. This study attempts to make a contribution to this goal. Seismic damage pattern estimation in a historical brick masonry minaret under different ground motion levels is investigated by using updated finite element models based on ambient vibration data in this study. Imaret Mosque which was built in 1481 AD is selected for an application. Surveying measurement and material tests were conducted to obtain a 3D solid model and mechanical properties of the components of the minaret. Firstly, the initial 3D finite element model of the minaret was analyzed and numerical dynamic characteristics of the minaret were obtained. Then, ambient vibration tests as well as operational modal analysis were implemented in order to obtain the experimental dynamic characteristics of the minaret. The initial finite element model of the minaret was updated by using the experimental dynamic results. Lastly, linear and nonlinear timehistory analyses of the updated finite element model of the minaret were carried out using the acceleration records of two different level earthquakes that occurred in Turkey, in AfyonDinar (1995) and ÇaySultandağı (2002). A concrete damage plasticity model is considered in the nonlinear analyses. The conducted analyses indicate that the compressive and tension stress results of the linear analyses are not as realistic as the nonlinear analysis results. According to the nonlinear analysis, the ÇaySultandağı earthquake would inflict limited damage on the minaret, whereas the Dinar earthquake would damage some parts of the elements in the transition segment of the minaret.
Architectural and structural characteristics of minarets are influenced by national and local building materials. Historical minarets are generally masonry structures in which stone or clay brick and mortar are used together. The behavior of the majority of historical structures differs from one another during an earthquake as local materials are used in their construction. There is usually a spiral staircase system made of wood or stone inside the minarets. The most important feature of historical masonry structures is that they reflect the architectural characteristics of the time at and the region in which they were built [
Many towertype masonry structures were completely or partially destroyed due to major earthquakes, strong winds, or suddenly without any indication. For example, the Yüregil Minaret in Afyonkarahisar and the Çavdır Minaret in Antalya, Turkey, are some of the structures that were damaged in earthquakes, whereas the Civic Tower in Pavia in Italy collapsed without any indication [
In general, model updating techniques are based on the use of appropriate coefficients (sensitivity coefficients) that iteratively update selected physical properties (properties of materials, stiffness of a link, etc.) in such a way that the correlation between the simulated response and the target value could improve if compared to an initial value [
Bolvadin is one of the oldest settlements in Anatolia. It dates back to 10,000 years ago according to available documentation (Bayar, 1996). Especially during the reign of the Seljuks, it was decorated with mosques, fountains, inns, baths, aqueducts, and bridges. The Imaret Mosque, built in 1481 AD, is one of these works that has survived to the present day. The bearing system of the minaret consists of clay brick except pulpit which is stone masonry. The height of the minaret is 24.5 m, the pulpit has 2.66 × 2.66 m octagonal section, and the cylindrical body has a diameter of 2.0 m. Parts of the minaret and vertical and horizontal cross sections of the minaret are presented in Figure
(a) Parts of the minaret. (b) Vertical and horizontal cross sections of the minaret.
The material properties of the minaret are taken from the similar study of Nohutcu et al. and are listed in Table
Stone and brick material parameters from another similar study [
Compressive strength (MPa)  Tensile strength (MPa)  Modulus of elasticity (MPa)  Ultrasonic pulse velocity (m/s)  Surface hardness (R)  Density (kg/m^{3})  

Andesite stone  30  2.08  12240  1813  54  2200 
Full brick  8.2  1.86  2985  1051  31  2100 
Khorasan mortar  6.25  1.43  1100  —  —  1340 
The minaret consists of stone/brick and mortar materials. Elastic material parameters of the masonry wall using materials test results were calculated by equations recognized in the literature. The compressive strength of masonry is determined by equation (
The modulus of elasticity of masonry is determined using equation (
Table
Initial material parameters of masonry walls.
Materials  Stone masonry  Brick masonry 

Compressive strength (MPa)  7.210  4.210 
Tensile strength (MPa)  0.721  0.421 
Modulus of elasticity (MPa)  4400  1300 
Bulk density (kg/m^{3})  2200  1750 
Poisson’s ratio  0.17  0.17 
According to the drawings obtained from in situ surveying measurements, the threedimensional solid model and FEM model of the minaret were prepared using the Abaqus [
Threedimensional solid model and finite element model of the minaret.
Mesh size convergence.
Mesh size (m)  The first frequency (Hz)  Number of elements 

0.55  0.896  8710 
0.50  0.869  9546 
0.45  0.863  11170 
0.35  0.854  17404 
0.25  0.849  28175 
0.15  0.846  105985 
Frequency and mesh size convergence graphic.
According to the convergence analysis values in Table
The first four mode shapes profile and plan views and frequency values from the initial finite element model. (a) Mode 1: freq. = 0.849 (cycles/time); (b) Mode 2: freq. = 0.851 (cycles/time); (c) Mode 3: freq. = 4.339 (cycles/time); (d) Mode 4: freq. = 4.364 (cycles/time).
The material values are the local properties of the wall material. When we apply these values to the whole structure, it is difficult to achieve realistic values due to the regional differentiations. Therefore, operational modal analysis is performed with the data obtained from the accelerometers, which takes the environmental vibration data on the structure. Thus, experimental modal behavior and damping rates of the structure can be obtained. By modifying the modulus of elasticity, a modal analysis is carried out by the finite element method until it reaches the modes we have found experimentally. When we reach the first mode value obtained from OMA, we obtain the modulus of elasticity and the damping ratio of the structure.
Four Testbox2010 data acquisition devices and sixteen uniaxial Sensebox7021 accelerometers were used for the OMA method. The accelerometers used in the experiment are sensitive to signals in the range 0–200 Hz. The signals from the accelerometers are combined in the fourchannel Testbox2010 data acquisition unit and transferred to the TestlabNetwork [
OMA is an outputonly method based on ambient vibration data. In the experimental study, the connection points of the accelerometers were identified using the initial finite element (FE) mode shapes of the minaret. A total of 12 uniaxial accelerometers were placed at these points as shown in Figure
Locations and directions of the accelerometers on the minaret.
Acceleration data obtained from the OMA test were transferred to the ARTeMIS Modal Pro [
Stabilization diagram of estimated state space models obtained from the SSI technique.
The experimental dynamic characteristics.
Mod  1  2  3  4 

Frequencies (Hz)  1.425  1.516  6.530  7.108 
It can be seen from Table
The dynamic characteristics obtained before and after the FEM calibration.
Mode  Numerical frequencies FEM (Hz)  Experimental frequencies OMA  Difference (%)  

Before update  After update  SSI  Before update  After update  
1  0.849  1.425  1.425  −57  0.00 
2  0.851  1.531  1.516  −66  0.15 
3  4.339  6.548  6.530  22  0.18 
4  4.362  6.692  7.108  27  −4.16 
The first four calibrated and experimental mode shapes. (a) Updated FEM mode shapes profile and plan views. (b) Experimental mode shapes profile and plan views.
Seismic damage propagation in the minaret was determined by means of linear and nonlinear finite element models in Abaqus. The nonlinear analyses were performed using the CDP model. The CDP model depends on the integration of damage mechanics, and plasticity is improved to analyze the failure of concrete and unreinforced brittle masonry structures. CDP describes the important characteristics of the failure process of concrete or masonry under multiaxial stress. Material parameters for masonry in the CDP model are summarized in Table
Material parameters for masonry in the CDP model.
Dilation angle  Eccentricity 



100  0.1  1.16  0.666 
The failure of masonry can be modelled under uniaxial compression and tension and by the plasticity characteristics. CDP model response under uniaxial compression and tension is linear until the initial value of yield stress. When the failure stress is reached, microcrack formation in masonry is activated. In the plastic regime, the response is typically characterized by stress hardening followed by strain softening. Beyond the failure stress, the formation of microcracks is represented macroscopically with a softening stressstrain response as shown in Figure
Damage plasticity stressstrain diagrams: (a) uniaxial tension and (b) uniaxial compression.
Material properties of the masonry used in the minaret were adopted from the study carried out by Kaushik et al. [
Stressstrain relationship for brick masonry. (a) Compression. (b) Tension.
The acceleration records of the earthquakes that took place in ÇaySultandağı (Mw 6.0) on Feb 3, 2002, and Dinar (Mw 6.0) on Oct 10, 1995, were used to determine the seismic damage patterns of Imaret Mosque Minaret. Time histories of two components of the earthquakes are depicted in Figure
(a) ÇaySultandağı and (b) Dinar earthquake acceleration records.
Linear timehistory analyses of the minaret are performed using the full Newton method in Abaqus [
Maximum displacement contour shapes under the earthquakes. (a) ÇaySultandağı earthquake. (b) Dinar earthquake.
Lateral displacements throughout the height for the linear analyses. (a)
Under the ÇaySultandağı earthquake, it was observed that the maximum (tension) principal stress (126 MPa) was concentrated around the door which is in the transition segment that lies between the pulpit and the cylindrical body, while the minimum (compression) principal stress (1.36 MPa) was concentrated on the opposite side of the door (Figure
Maximum and minimum principal stresses contour under the earthquakes. (a) ÇaySultandağı earthquake. (b) Dinar earthquake.
Timehistory and contour graphs of displacements in the minaret under the ÇaySultandağı earthquake.
According to the results of nonlinear timehistory analysis for the ÇaySultandağı earthquake, the maximum lateral displacement values are obtained as 4.8 cm and 3.6 cm in
Maximum and minimum principal stresses in the minaret under the ÇaySultandağı earthquake.
In CDP analysis, the damage of the minaret can be achieved. When this damage ratio exceeds 100 percent, it is accepted that the structure completely collapsed. When the plastic deformation obtained with the nonlinear solution is investigated, it was found that the safety boundary deformation values also exceeded. But the main criterion that determines the damage and destruction of the structure is the damage rate obtained by the CDP.
The damage effect compression stress on the minaret is calculated by CDP analysis, and the damage percentage contours at the end of time steps are presented in Figure
Compression damage distribution for the minaret at different time steps under the ÇaySultandağı earthquake.
The compression damage on masonry of the minaret begins at 11.73 s, and the plastic deformation value reaches 0.0057 at the end of the analysis (
Compressive plastic strain contour under the ÇaySultandağı earthquake.
The tension damage effects calculated by CDP analysis at the end of the analysis are presented in Figure
Tension damage distribution for the minaret at different time steps under the ÇaySultandağı earthquake.
Tension plastic strain contour under the ÇaySultandağı earthquake.
Nonlinear timehistory analysis of the minaret under the Dinar earthquake shows that the maximum lateral displacement values are 17.4 cm and 27.8 cm in
Timehistory and contour graphs of displacements in the minaret under the Dinar earthquake.
Critical maximum and minimum principle stress contours under the Dinar earthquake are presented in Figure
Maximum and minimum principal stresses in the minaret under the Dinar earthquake.
Compression damage distribution for the minaret at different time steps under the Dinar earthquake.
The compression damage on masonry of the minaret begins at 3.8 s, and the plastic deformation value reaches 0.17 at the end of the analysis (Figure
Compressive plastic strain contour under the Dinar earthquake.
The tension damage effects calculated by CDP analysis at different time steps are presented in Figure
Tension damage distribution for the minaret at different time steps under the Dinar earthquake.
Tension plastic strain contour under the Dinar earthquake.
The locations of compression and tension regions under linear and nonlinear timehistory analysis are coincided. Since the masonry cannot recover tensile stress, the observed tension regions are critical although these regions are limited. Under a high ground motion, the minaret may be considerably damaged from these regions, and a special precaution must be implemented to these regions.
Historical structures are of great importance to a country. The historical roots of a country have been exhibited by them. Therefore, they must be passed on from generation to generation. This study includes a valuable contribution to this goal. Seismic damage propagation estimations in the historical clay brick masonry minaret under different ground motion levels by using the updated FEM are implemented in this paper. Dinar (1995) and ÇaySultandağı (2002) earthquakes are considered in the linear and nonlinear timehistory analyses. It is seen that only the material information made by material tests is not sufficient. The structure must be fully investigated. After the OMA experiment, as a result of the finite element solution improvement, the modulus of elasticity increased from 1300 MPa to 5200 MPa. In the first mode, the natural frequency increased from 0.849 to 1.425. Mode shapes are in parallel with those in the experimental study. A threedimensional finite element model of the minaret is calibrated using the ambient vibration test results. The difference between the experimental and numerical frequencies, at the first 4 modes, is about 43% before the model calibration and 1.12% after the calibration. Due to their intensity, the results from the Dinar earthquake are bigger than those from the ÇaySultandağı earthquake. When the two earthquakes are compared, the end point of the minaret is 5 cm in the ÇaySultandağı earthquake and 38 cm in the Dinar earthquake. Approximately 8 times the displacement difference is observed. The minaret has a displacement difference of 8 times. Approximately 6 times a stress difference occurred. Under the ÇaySultandağı earthquake, damage parameters remain around 70% of the tensile and pressure. It is thought that the minaret will overcome this earthquake with minor damage. It can be generally said that the masonry minaret is safe under the ÇaySultandağı earthquake. Under the Dinar earthquake, damage parameters are around 85% of the tension and 90% of the pressure. It is thought that the minaret would face major damage and failure in this earthquake hazard.
The ÇaySultandağı earthquake would inflict limited damage on the minaret, whereas the Dinar earthquake would damage some parts of the elements in the transition segment of the minaret. It is observed that linear timehistory analysis results do not reflect actual response of the clay brick masonry minaret during the earthquake. The maximum and minimum principal stresses obtained from the nonlinear analyses exceeded the tensile and compressive strength values of the masonry, and they concentrated on the transition region. At the end of the study, the damage of the historical minaret against the two selected earthquakes was obtained, and it was determined that the minaret was at risk of collapse during a severe earthquake.
It can be seen from the past earthquakes that masonry minarets are damaged particularly at the transition segments. Besides, occurrence of stress concentration in the regions where section variations take place indicates that the calibrated finite element model represents the behavior of the minaret as close to reality as possible. Therefore, safety assessment of the masonry minarets under earthquakes should be evaluated considering calibrated finite element models and nonlinear timehistory analyses.
Readers can access the earthquake accelerations data. The earthquake accelerations data used to support the findings of this study have been deposited in the repository available at
The author declares that there are no conflicts of interest regarding the publication of this paper.
This research was conducted using the facilities of Manisa Celal Bayar University.