A Proposal for Reinforcement of Historical Masonry Minarets: Gaziantep Kabasakal Mosque Minaret

Abstract

This study investigated the historical Kabasakal Mosque minaret’s seismic performance in the Şahinbey district of Gaziantep province. After the 6 February 2023 Kahramanmaraş earthquake, the cone and honeycomb sections of the minaret were damaged. The minaret is a typical masonry structure with a cylindrical body and open balconies belonging to the Ottoman period, and the damage that occurred primarily in the weak areas, such as the honeycomb and the cone, caused serious structural problems due to the earthquakes. In the study, a finite element model (FEM) of the minaret was created. Nonlinear time-history analyses were performed using earthquake records recorded in the district where the minaret is located in the finite element model. First, the original state of the minaret (OM model) was analyzed, and its collapse mechanisms were investigated. Then, a proposal was developed to strengthen the minaret’s honeycomb and cone sections with steel rings. This reinforcement was tested with numerical analyses, and the dynamic performances of both the original and reinforcement models were compared. The durability and seismic performance of local materials commonly used in regional structures, such as Urfa stone, were evaluated. It was observed that the proposed reinforcement method reduced the displacement demands in critical areas and increased the overall rigidity of the structure. The study emphasizes the importance of reinforcement methods in protecting historical structures and reveals the proposed solution’s applicability to similar structures.

1. Introduction

Turkey has frequently faced natural disasters throughout history due to its geological, topographic, and meteorological characteristics. Earthquakes, in particular, affect a wide area and cause a severe loss of life, significant damage to the building stock, and the disruption of services in the public and private sectors. In addition, these disasters damage thousands of years of cultural heritage [1,2,3]. Therefore, they disrupt the country’s economic, social, and environmental development.

Although Turkey constitutes only 0.5 percent of the world’s land area, it ranks fourth in significant earthquakes since 1900 [4]. Turkey encompasses the Eurasian, Anatolian, African, and Arabian plates and is exposed to significant seismic activity on many tectonically active fault lines. Turkey’s neotectonic regime is controlled by the collision of the Arabian and Anatolian plates in the east and the southward subduction of the Hellenic subduction zone in the west. When statistical analyses of earthquakes are examined, it can be seen that a significant earthquake that causes the widespread loss of life and property occurs in Turkey on average every five years [5]. In addition, in 2023, when the Kahramanmaraş earthquake occurred, 830 earthquakes with a magnitude of over 4.0 occurred in Turkey [6]. The loss of life and property and the disruption of services necessary for the continuity of life resulting from these earthquakes reveal the importance of seismic preparation for earthquakes.

On 6 February 2023, two earthquakes with a magnitude of 7.7 Mw and a magnitude of 7.6 Mw occurred in Kahramanmaraş at 04:17 Turkish time, with the epicenters in the Pazarcık and Elbistan districts at 13:24. On 20 February 2023, an earthquake with a magnitude of 6.4 Mw occurred at 20:04 Turkish time, with the epicenter in Hatay Yayladağı. Since the earthquakes, which affected 11 provinces, occurred, 530 earthquakes of magnitude 4 Mw and above occurred in the region in the month (Figure 1). While the 1939 Erzincan and 1999 Gölcük-centered Marmara earthquakes have been the largest earthquakes in terms of loss of life and damage to date, the Kahramanmaraş earthquake was recorded as the largest earthquake in Turkish history, directly affecting 16.4% of the country’s demographics and killing more than 48,000 people [7].

Anatolian lands, which have hosted different civilizations for approximately 8000 years, have an architectural heritage built during the Roman, Byzantine, Seljuk, and Ottoman Empires and have survived to the present day. Architectural heritage has significantly contributed to forming the image of cities by carrying the traces of its period. Especially during the Ottoman period, mosques located in city centers have been a determining factor in urban planning. Mosques, which have played a central role in planning cities throughout history, have become essential elements of urban memory regarding their functionality and social aspects. In 2023, 6426 of the 22,150 historical structures that still exist in Turkey are mosques [9]. In the regions affected by the February 6 earthquakes, there are 864 religious structures, most of which are mosques. Religious structures have suffered extensive destruction and damage among the architectural works within the scope of cultural heritage. In particular, a large part of the mosques has been demolished. Of the mosques whose main structures remain standing, most of their minarets, which are among their essential elements, have been destroyed or damaged due to their structural form [7].

Minarets are slender and long tower-type structures with cubic, cylindrical, or polygonal shapes built adjacent to, integrated with, or separately from the mosque structure [10]. The primary formal elements of the minaret are the end ornament, spire (cap), upper part, balcony, cylindrical or polygonal body (shaft), transition segment, and boot (pulpit) (Figure 2). Regarding construction techniques, minarets are classified into different systems such as masonry, reinforced concrete, and wood. Minarets, especially those built with a masonry system, are essential in the architectural heritage inventory, and are one structure subject to reinforcement suggestions, especially in historical restoration works.

Historical masonry minarets were built using local construction techniques and materials. Since they are generally built in thin and long forms, their structural resistance to earthquake forces is weak. The behavior of masonry minarets in Turkey against seismic effects has been analyzed in terms of the base shear, acceleration, displacement, and maximum tensile forces over different minaret applications. These analyses, and the results obtained from earthquakes in Turkey, have shown that the main damage areas and causes in minarets are the upper body parts, transition segments from the shoe to body or body to the upper part, damage to the ornament and pointed-tip, rigidity difference damage, and shear force damage [12,13,14]. As a result of the 6 February 2023, earthquakes, various types of damage and destruction occurred in the spire, upper part, balcony, body, transition segment, and boots of many historical masonry minarets in the region (Figure 3). The historical minarets affected by the earthquake were damaged in the areas noted in the literature.

There are many strengthening methods in the literature to prevent earthquake-related structural damage. In stone and brick minarets, methods such as repairing shallow cracks with injections, metal clamps, and steel bracing are standard. In severe damage, the minaret may need to be reconstructed [19]. Numerical analysis and traditional methods have developed seismic reinforcement proposals in recent years. Usta [20] examined five minarets in Antalya with finite element analysis and suggested tension rings to reduce the stress concentrations at the body transitions. Hoseynzadeh and Mortezaei [21] compared FRP, ferro-cement, and fiber-reinforced methods in the Tarikhaneh Mosque minaret and determined that ferro-cement was the most effective. Şentürk et al. [22] used CFRP and steel plate strengthening methods in the Alaeddin Bey Mosque minaret and evaluated the seismic performance with base isolation and viscous dampers. In other studies, Altunışık [23] suggested reinforcement of the Trabzon İskenderpaşa Mosque with FRP, while Gürbüz and Kocaman [11] suggested a reinforcement line for the Erzurum Murat Paşa Mosque minaret. Preciado et al. [24] and Orlando et al. [25] suggested reinforcement of the masonry towers with steel tension bars and seamless stitching techniques. However, despite using advanced materials and analytical techniques, many reinforcement methods proposed in the literature do not simultaneously address minimal intervention and esthetic preservation. Many of the suggested strengthening approaches are specific to the examined structure and pose challenges when adapted to different buildings. Therefore, the lack of reversible reinforcement strategies that can be broadly applied to historical masonry minarets while considering esthetic concerns remains a significant gap.

This study looked at the minaret of the historical masonry Kabasakal Mosque located in Şahinbey district of Gaziantep province. First, the masonry minarets located in Gaziantep province were analyzed according to the shapes of their elements, and a typology was developed. Thus, the structures that the minaret to be considered could represent were specified. The minaret was damaged in the upper part and spire sections after the 2023 Kahramanmaraş earthquakes. A finite element model of the minaret was created. Then, the damage observed in the field was compared with the finite element model using the ground motion recorded in the district center. A reinforcement configuration was proposed by revealing the damage mechanism of the minaret. The dynamic analysis was repeated by defining the proposed reinforcement to the finite element model. The results obtained from both dynamic analyses were compared, and the proposed reinforcement’s contribution to the minaret’s seismic performance discussed. In historical masonry minarets, the upper part, which significantly differs in stiffness from the main body, is one of the most damaged sections. It was shown that the damage in this section could be prevented with the proposed reinforcement method.

2. Gaziantep Historical Masonry Minarets

Located in the southeastern region of Turkey, Gaziantep province hosts the architectural heritage of different civilizations. Mosques have an essential place among these architectural heritages. Of the 42 mosques identified in Gaziantep, all mosques except Kale Mosque and Araban Kale Mosque have minarets. Figure 4 shows examples of minarets with different architectural features in Gaziantep.

Considering the mosques in Gaziantep, the minarets in six mosques are placed on the narthex’s eastern wall, and seven are on the western wall. The minarets of 34 mosques are built adjacent to or embedded in the main wall. The minarets are generally positioned on the eastern or western corners of the northern facade [26,27]. These minarets can be classified into two types: open and closed balconies. Closed-balcony minarets were implemented for functional purposes according to the region. The minarets generally rise on square bases, and there is a short transition area on the boots. The bodies of some minarets are cylindrical, while some are dodecagonal. The balconies are also dodecagonal or circular. Open-balcony minarets, cylindrical, dodecagonal, and hexadecagonal cross-sections are made in the upper part. The entrances of the minarets are from the boot and at ground level. It can be said that the boots of the minarets are made of carved stone, and the bodies are made of cut stone [27]. The stone decorations, generally found in relief on the minarets, are dense on the body and under the balcony. There are muqarnas under the balconies, mainly in the form of a triangular pendentive. There are several rows of relief decorations under the muqarnas. The spires of the minarets have three different shapes: umbrella, cubic, and conical. In Figure 5, the sections belonging to the Gaziantep mosques and the differences in these sections are detailed.

3. Gaziantep Kabasakal Mosque and Minaret

3.1. Historical and Architectural Features

The Kabasakal Mosque, located in the Şahinbey district of Gaziantep province, was initially built as a masjid. It was converted into a mosque by adding a pulpit in the early 18th century. The mosque was partially destroyed during the war in the city in 1920, and the remaining parts remained open for worship for a while. In 1955, the mosque was expanded to the south and rebuilt [28,29]. It is not known precisely when and by whom the minaret was built. The fact that the minaret workmanship of the nearby mosques built by the masters of the 19th century is similar to the Kabasakal Mosque minaret allows only for an estimate. The literature mentions a renewal of the minaret in 1956 [29]. However, no precise information has been obtained as to the extent of the renewal. It is thought that the original structure of the minaret is preserved today because the joint spaces of the minaret body are thin, and no particular interventions, such as cement joint intervention, have been observed. Current images of the mosque and the minaret are presented in Figure 6. The minaret is located northwest of the mosque, adjacent to the west wall. The minaret’s upper part and spire sections collapsed in the Kahramanmaraş earthquakes in 2023. There is a plan for the original stones of these parts to be collected and used in the reconstruction process. Images of these stones are also presented in Figure 6.

The minaret has a cylindrical boot on a square body and was built as an open, single-balcony structure. In constructing the minaret’s boot, carved and regular cut stones were used in its body. The material used was Urfa stone, which is used in many historical structures in the region. There is a 700 mm high transition element with rounded corners on the base. After the bracelet, located at the junction of the body and the balcony, a sliced design was applied up to the balcony. The lower part of the balcony has intense ornamentation (these ornamentations can be seen in Figure 6); here, starting from the belt on the body, a twelve-cornered plan was adopted. These ornamentations are considered important handicrafts that increase the cultural value of the minaret. In the upper parts of the ornamentations, baçinis are placed in round cavities. In the other rows, a row of round rosettes and a row of stylized flower motifs are processed alternately. The upper part of the minaret is cylindrical, and its conical cone is built of cut stone. The dimensions of the minaret are presented in Figure 7.

3.2. Finite Element Model

In recent years, the seismic resilience of historical structures such as churches, bridges, mosques, towers, and minarets has become a prominent research focus [30,31,32,33,34]. Ensuring their safe preservation requires accurately predicting their seismic behavior and potential damage. However, complex geometries, traditional construction techniques, and material uncertainties complicate seismic performance assessments. Factors like boundary conditions, inclined transitions, connection details, and prior structural modifications increase the complexity of the analysis, necessitating advanced numerical methods [35,36,37,38,39].

Micro and macro modeling approaches are commonly used in historical structure analysis. Micro modeling provides high accuracy by considering detailed material properties, making it ideal for local damage assessment. In contrast, macro modeling assumes a homogenized structure, enabling faster large-scale analyses. While seismic loads significantly impact the computational demand in micro modeling, homogenization methods reduce this burden, ensuring efficient but reliable assessments. Nonlinear dynamic analysis based on macro modeling is widely applied to evaluate the seismic performance of masonry structures [40,41,42].

A three-dimensional model of the minaret was created using SolidWorks 2024, excluding stone carvings with negligible structural impact. This model was transferred to ANSYS 2022 R1, APDL for finite element analysis (Figure 8). The minaret’s base was assumed to be fixed, and four-node SOLID65 tetrahedral elements were used for meshing, with element sizes ranging from 20 cm to 35 cm. The final model consisted of approximately 18,634 elements and 5204 nodes, comprehensively representing the structure.

The SOLID65 element in ANSYS is crucial for comprehensive structural analyses. This 3D solid element has eight nodes, each with three translational degrees of freedom. Its geometric configuration, nodal coordinates, and coordinate system are shown in Figure 9a. SOLID65 can simulate cracking under tensile stress, crushing under compressive stress, and plastic deformation. Initially, the material is defined as isotropic, allowing cracks to form in three perpendicular directions at each integration point.

The masonry material was modeled as a homogeneous continuum using the five-parameter Willam–Warnke model [43,44,45]. This model defines the fracture surface based on the tensile and compressive strengths of the masonry units. Crushing is assumed when the material fractures under uniaxial, biaxial, or triaxial compression, leading to a complete loss of structural integrity. In such cases, the material’s stiffness contribution at the affected integration points is neglected.

While the Willam–Warnke model is effective for unconfined concrete, geomaterials, and masonry, it does not fully capture the post-peak softening behavior. A more realistic representation of crushing could be achieved by integrating it with a plasticity model. However, since minaret failure is primarily governed by separation and cracking due to low tensile strength, the Willam–Warnke model alone was deemed sufficient [46]. The corresponding uniaxial stress–strain behavior is shown in Figure 9b.

The SOLID65 element and the William–Wranke material model were initially developed to model concrete behavior. However, with the correct definition of critical parameters, they are also used in modeling masonry elements. Among the basic parameters used to determine the damaged surface in the material model, the transfer coefficients for open (βt) and closed (βc) cracks, compressive (crushing) strength (fc), and tensile strength (ft) were pretty significant. This analysis used the values of βt and βc as 0.05 and 0.8, respectively [47].

3.3. Material Properties

Determining the material properties of historical masonry structures is challenging due to the complex interactions of different building materials and element shapes. These structures, built centuries ago, have been subjected to various seismic loads, human interventions, and environmental effects. Researchers collect data using destructive and non-destructive testing methods to overcome these uncertainties. However, the unique material properties of each structure may cause these tests to be insufficient, although guiding. Therefore, it is recommended that local studies be prioritized whenever possible.

Determining the material properties of historical structures requires using multiple testing methods to ensure the accuracy of the results. It is inappropriate to rely solely on destructive tests or make assumptions with limited non-destructive tests. Researchers need to integrate as many experimental methods as possible with literature reviews. This multidisciplinary approach is crucial for scenarios where finite element models determine the seismic behavior. In addition, factors such as local and international code recommendations, studies on structures constructed in similar periods, and the structure’s restoration history should be considered. This integrated approach, which covers multiple fields of study, helps to accurately determine the material properties of the examined structure.

Since natural stone materials exhibit different mechanical properties due to their structure, it is recommended that more than one sample is taken from the structure to determine the mechanical properties. This process does not only determine the compressive strength and elastic modulus; the restoration history of the samples, the geological structure of the stone, and other factors should also be considered. This situation requires an approach that integrates information from various disciplines. However, these processes are usually costly, time-consuming, and require expertise. Therefore, it is often preferred to use some assumptions. Certain assumptions should be made by a review of the literature as well as materials with similar mechanical properties and regulations.

The historical Gaziantep Kabasakal Mosque was built using Urfa stone (also known as Yonu stone or Nahit stone), used locally in many different historical structures. Urfa stone is a solid, durable building material that withstands all climatic conditions. Urfa stone, also an excellent insulation material, hardens and becomes more durable in regions where winters are very harsh and cold, protecting the structure from storms, rain, and snow while keeping hot air out in the summer and preventing the interior from overheating. It is preferred for use in exterior and interior spaces due to its ability to maintain its esthetic appearance for many years. Urfa stone is very suitable for decoration due to its easy processing and shaping [48]. The reason for the stone carvings that have maintained their integrity in various structures over the years is due to the natural feature of the stone.

Urfa stone, used as the primary material in many different structures, has been studied intensively by various researchers. Destructive tests have determined the mechanical properties of the stone; studies show that the compressive strength of the stone varies between 5 and 20 MPa, and the elasticity modulus varies between 13 and 25 GPa [49,50,51,52,53,54]. In addition, in the literature on historical masonry minarets, it can be seen that the compressive strength is accepted as 5–50 MPa [55,56,57]. Considering these values and complexes, the compressive strength was 15 MPa in the finite element models. In addition, based on the study of Tomaževič [58], the tensile strength is accepted as 10% of the compressive strength. Thus, the tensile strength was determined as 1.50 MPa.

Determination of the elastic modulus in historical masonry structures is a rather complicated process due to its significant impact on the structural behavior, especially under seismic loading conditions. The elastic modulus directly influences the structure’s rigidity, natural frequency, and displacement response during dynamic analysis. In particular, the value of the elastic modulus is critical because it affects the natural period of the structure, and even slight variations in this value can lead to considerable differences in the seismic demand and overall structural response. Therefore, determining the elastic modulus with high accuracy is vital for reliable seismic performance assessment.

Elastic modulus recommendations for historical masonry structures, especially for Ottoman-period minarets, range from 3 to 25 GPa [59,60,61]. However, selecting any value within this broad range without proper justification would likely yield inaccurate results, potentially misrepresenting the structure’s seismic behavior. Kocaman [46] highlights that even small differences in the elastic modulus can significantly alter demand displacements, which are crucial for evaluating the seismic performance of a structure. A higher modulus increases the stiffness of the structure, thereby reducing the displacement response under seismic forces, while a lower modulus results in increased flexibility and larger displacements. Therefore, choosing an appropriate value requires careful consideration of the specific structure’s behavior, modal analysis results, and the surrounding material conditions.

To determine the correct elastic modulus for the Gaziantep Kabasakal Mosque minaret, a combination of approaches was adopted. First, frequency-based formulas from the literature were used to estimate the first mode frequency of the minaret [62,63,64,65,66]. These formulas consider key dimensions of the minaret such as its height, core diameter, and base dimensions. Using this estimated frequency, numerical modal analysis was performed to refine the elastic modulus value. The modal analysis results provided a more structure-specific estimate, which was further validated against the mechanical properties of Urfa stone, the primary material of the minaret. Given the geological characteristics and the durability of Urfa stone, a value of 24 GPa for the elastic modulus was selected. This value is not only consistent with the material’s mechanical properties as outlined in the literature, but also ensures compatibility with the dynamic characteristics of the structure.

Furthermore, considering the complex interactions between materials and the structural behavior of historical buildings, it is crucial to align the selected elastic modulus with the observed damage mechanisms in the structure. In this study, the elastic modulus value was chosen to ensure that the damage mechanism obtained through dynamic analysis aligned well with the actual post-earthquake damage observed in the field. This approach not only ensured the accuracy of the finite element model, but also helped to validate the material properties used in the analysis, providing a reliable basis for understanding the seismic behavior of the structure.

In conclusion, determining the elastic modulus for historical masonry structures is a multifaceted process that requires the integration of experimental methods, numerical analyses, and material-specific considerations. The value of 24 GPa used in this study was based on a comprehensive approach that combines the literature data, structural analysis, and material properties of Urfa stone. By carefully selecting this value, the study ensured that the seismic performance of the historical structure was accurately represented, and provides a reliable foundation for further seismic assessments.

Different sources and approaches have been used to determine the parameters used as inputs in finite element models. The suitability of these accepted values will become more meaningful with the compatibility of the damage mechanism to be obtained in the dynamic analysis of the structure and the damage mechanism observed in the field after the earthquake.

4. Proposed Reinforcement Details

Damage to minarets during earthquakes is generally due to the thin and long designs of the structures. Severe damage and collapse risks mainly occur in the upper parts of minarets, spires, balconies, and transition sections. When the lack of rigidity and strength in these areas is combined with the high stresses in the transition area, minarets are prone to damage at these points. Field examinations conducted after earthquakes in Turkey have shown that earthquake-related damage to historical masonry minarets is generally concentrated in sections such as the transition area, balcony, upper part, and cone. While the damage occurring in the transition area can be attributed to the high stresses occurring between the body of the minaret and the thicker boot section, the damage observed in the upper parts (upper part, balcony, spire) occur due to high displacement demands created by material incompatibility and insufficient rigidity. Such damage seriously endangers the earthquake safety of minarets.

As a solution, material compatibility should be observed, internal rigidity elements should be added, and strengthening methods such as covering the cone area with a metal sheet should be used [47]. Interventions that will increase the rigidity in the upper parts of the minarets could preserve the structure’s integrity and make it less affected by future earthquakes.

Reinforcement work on historical minarets is generally carried out on structures that have suffered moderate and severe damage. Minarets damaged in the 2023 Kahramanmaraş earthquakes were also evaluated within this scope. However, reconstructing such minarets with masonry elements following the original does not eliminate their vulnerability to natural disasters such as earthquakes. For this reason, the earthquake behavior of these historical structures should be analyzed carefully, and the necessary precautions should be taken.

In addition, it is essential to keep the interventions in these historic structures to a minimum level and comply with the principle of reversibility. In the reinforcement works, while increasing the structures’ seismic resistance, preserving their cultural and esthetic values should also be a priority.

In this study, two finite element models of the minaret were developed. In the initial model (OM—original/unreinforced model), no intervention was made, and the minaret was left with its unchanged masonry units. In the second model (RM—reinforced model), the reinforcement configuration detailed in Figure 10 was applied. It is helpful to explain some of the constraints and requirements used in determining this configuration. First, the minaret’s connection with the mosque’s west wall should be considered. Then, the focus should be on the damage to the minaret after the 2023 earthquakes. The absence of a clear transition section in the minaret eliminates the potential damage to the transition section. This situation can be verified by examining the damage to the minarets after the earthquakes.

For this reason, the focus was entirely on the balcony, upper part, and spire section. The absence of stairs forming the internal carrier system of the minaret in these sections causes it to exhibit a less rigid behavior than the main body. This situation causes more displacement demand in these parts under earthquake loads. Therefore, the proposed configuration focused on increasing the rigidity of the upper part of the minaret and spire. Although the example seems like a case study, it is noteworthy that many minaret structures have conditions similar to those in this case study. Therefore, it is necessary to investigate in detail how the application of only this proposed strengthening configuration in minarets with different boundary conditions will affect the seismic performance of the minaret. The strengthening process to be applied to these unique structures should be checked with particular case studies.

The proposed reinforcement mainly focused on the upper part of the minaret and spire, which are damage-prone areas in minarets (like the minaret under consideration). Here, a 10 mm thick ring plate (steel ring) is wrapped around these regions, which are less rigid than the main body of the minaret. First, a 100 mm high ring is positioned where the upper part meets the balcony. After this ring, three more steel rings of 10 mm thickness and 100 mm height are added at 1650 mm intervals. These rings are considered wholly connected with the stone units in the finite element model. These steel plates can be securely fixed to the minaret using threaded rods and epoxy during production. These reinforcement elements were defined in the element model using the SOLID185 element.

5. Numerical Analyses

Finite element analyses were performed to increase the seismic performance of the historical Gaziantep Kabasakal Mosque minaret. The OM model, which represented the original state of the minaret and did not contain any reinforcement elements, and the RM model, which defined the reinforcement configuration detailed in the previous section, were obtained. The numerical analyses performed in this section were performed on both models. Thus, it was revealed how the seismic performance of the minaret changed with the reinforcement effect.

5.1. Modal Analyses

The modal analyses performed on both models obtained each model’s mass, modal frequency, and modal mass participation ratios. These values are presented in

Table 1. Total mass, modal frequencies, and mass participation ratios (MPR) for the OM and RM models.

Model	Total Mass (t)	1st Mode (X Direction)		2nd Mode (Z Direction)		3rd Mode (X Direction)
		Freq. (Hz)	MPR	Freq. (Hz)	MPR	Freq. (Hz)	MPR
OM	142.5	1.75	0.441	1.84	0.435	10.95	0.106
RM	142.8	1.74	0.440	1.83	0.435	10.96	0.103

for a more precise comparison. The structural masses were 142.5 tons and 142.8 tons for the OM and RM models, respectively. It can be seen that the steel reinforcements added to the RM model were approximately 0.3 tons. These elements caused a mass increase of approximately 0.211% in the structure. For the OM model representing the original state of the minaret, the frequency values of the first three modes were obtained as 1.75 Hz, 1.84 Hz, and 10.95 Hz, respectively. In the RM model, where the reinforcement elements were defined, the frequency values of the first three modes were obtained as 1.74 Hz, 1.83 Hz, and 10.96 Hz, respectively. The change in the first three modes of the OM and the RM models was obtained as 0.6%, 0.5%, and 0.1%, respectively. The modal shapes of the first three modes of both models are given in Figure 11. These mode shapes were pretty similar, and it can also be said that the mode deformation patterns obtained from both models were compatible.

5.2. Dynamic Analyses

A dynamic analysis of the minaret in the time domain was carried out to evaluate the effects of the 6 February 2023 Kahramanmaraş earthquake. In this analysis, the two horizontal components of the selected acceleration record were applied to the minaret in the X and Z directions. The north–south (N–S) component was used in the X direction, while the east–west (E–W) component was applied in the Z direction. The ground motions’ peak ground acceleration (PGA) values were recorded as 0.17 g for the E–W component and 0.16 g for the N–S component. The vertical component was ignored in this analysis.

Figure 12 presents the acceleration–time graphs and the response spectrum of the N–S and E–W components of the earthquake recorded in the Gaziantep Şahinbey district center. These ground motion data were obtained from the Turkey Disaster and Emergency Management Authority (AFAD) [67] database. The same ground motion was used in the dynamic analyses for both the unreinforced model (OM) and the reinforced model (RM). Thus, the effects of the ground motions on the behavior of the minaret were investigated in two different models. In the analyses, the damping ratio was considered 3%.

Figure 13 shows the displacement–time graphs obtained from the dynamic analyses of the OM and RM models. The displacement data were obtained from the top of the minaret. While the displacements continued up to 30 s in the RM model, these values could be presented up to approximately 8.50 s in the OM model. The displacement limits seen in the minaret in the RM model were in the order of ±35 mm, and it was seen that the minaret survived the given earthquake load without collapsing. In the OM model, the increasing displacements exceeded 50 mm in both the X and Z directions between 8.00 and 8.50 s, so it can be said that the minaret collapsed at this point. Deciding whether this collapse occurred is impossible by looking only at the displacement–time curves. The decision that the collapse occurred should be evaluated as a whole, along with other analytical outputs. For this reason, it is essential to look at the force–displacement data and damage distribution in the following sections.

In Figure 14, displacement data taken from different points along the height of the minaret are shown at different time intervals. Displacement data are presented for both X and Z directions. The minaret height–displacement curves for the OM model in Figure 14a indicate that the minaret had limited displacement up to 8.00 s. However, after this second, it was seen that the minaret was exposed to displacements exceeding 50 mm from the top of the balcony section (upper part and spire). This 50 s can be considered as a collapse limit. Then, when we looked at the 10.00-s step, it was clear that the upper part and spire section had considerably more displacement than the main body. Figure 14b shows the minaret height–displacement distributions obtained from the dynamic analysis results using the RM model. These graphs are given only up to 10.00 s. The graphs showed no significant collapse in the RM model and that the displacements in all sections of the minaret were consistent and limited. This graph contains essential information about the collapse mechanism of the RM model. Here, the displacements in the X and Z directions along the height remained at the level of ±25 mm in the boot, transition section, main body, and balcony sections of the minaret. At the same time, they were more prominent in the upper part and spire sections. The proposed reinforcement proposal helped the displacements in the upper part of the minaret and spire to remain within this limit, as in the other sections of the minaret.

Figure 15 presents the force–displacement graphs obtained from both analyses. In the graphs, the displacement data were obtained from the top of the minaret, while the force data were obtained from the base reactions. When these graphs were examined, a stronger argument was presented that the OM model collapsed. It can be seen that the RM model remained within the elastic limits.

Figure 16 shows the damage mechanisms obtained from the dynamic analyses of the OM and RM models. The images were obtained using first principle strain distributions. Each image was taken using the natural scale. The colors in the figures represent the strain values by the scale. The aim was to reveal the damage mechanism, especially in the OM model. For this reason, a ranking was carried out according to the progress of the analysis. Multiplying the strain value given in the scale in the figure by the average element length of 35 cm helped to obtain the crack in that part of the model. The dynamic analysis results using the RM model showed no noticeable damage in the model with the applied ground motion. However, the situation was different in the OM model. A crack density was seen at the junction of the upper part and the balcony of the OM model as there is a door with dimensions of 160 × 60 cm to pass through to the balcony. This gap in the stonework caused the damage to start and intensify here. In the later steps of the analysis, this damage intensified and caused the minaret to break away from this part. The OM model assumed that the upper part and spire of the minaret collapsed between approximately 8.00–9.00 s of the analysis. It has been emphasized in previous sections that this collapse can be decided with different data. Here, the strain distribution was a solid indicator to accept that the collapse had occurred.

The OM and RM models used the data recorded in the Şahinbey district center of Gaziantep province from the 2023 Kahramanmaraş earthquake. The main difference between these two models was the addition of the proposed reinforcement configuration to the RM model. Damage distributions plotted with time displacement, minaret height displacement, force displacement, and first principle strain were obtained from both analyses. This showed that the proposed reinforcement prevented damage to the upper part and spire of the minaret. Especially when looking at the height–displacement graphs of the minaret (Figure 14), it is clear that there was an increase in displacements after the balcony section in the OM model. This increase was seen at the time of collapse and before the collapse. However, in the OM model, the displacements formed with the height of the minaret throughout the analysis steps were pretty compatible, and no sections moved independently of each other. In this situation, the proposed reinforcement made the upper part of the minaret more rigid. Thus, the displacement demands in this section decreased, and no damage or collapse occurred in the minaret. At the same time, the damage distributions shown in Figure 16 also support the reduced displacement demands due to the effect of reinforcement. In particular, the absence of obvious crack mechanisms in the damage distributions of the RM model showed the positive contribution of the addition of reinforcements. Finally, the force–displacement curves in Figure 15 also support that no damage is expected in the minaret due to the reinforcement effect.

The most common damage mechanisms in historical masonry minarets are collapse from the transition area and damage to the upper part–spire section. Especially in historical masonry minarets belonging to Ottoman architecture, there is a transition area where a thick boot meets a thinner main body. This area is where the stresses are the maximum under earthquake loads. Damage will likely occur in this area depending on the materials’ properties and the construction technique. Field and analytical studies have reported that many minarets have collapsed in this area [68,69]. The fact that no damage occurred in the transition area of the minaret can be explained by two different reasons. First, when looking at the transition area of the minaret, there is no significant cross-sectional narrowing. It is thought that the feature of this transition area also helps the minaret not to receive damage from this section. The second reason is using Urfa stone, which has a relatively high compressive and tensile strength. It is thought that the elements that can withstand the stresses accumulated in this area help to prevent damage. The collapse of the upper part and spire, which are common types of damage, is due to the lower rigidity of these sections. Experimental and analytical studies have shown that these sections are subjected to higher displacements due to the absence of staircase and core elements and a door gap [70,71]. The reinforcement configuration proposed in this study helped to increase the rigidity of the minaret’s upper part and reduce the displacements that may occur in this section. Thus, the damage seen in the OM model was not seen in the RM model.

In the study, a reinforcement method involving steel elements was used to improve the seismic performance of the minaret. In the literature, various approaches for the reinforcement of historical structures, such as the addition of FRP, CFRP, and concrete elements, can be found. However, there are certain disadvantages to using FRP and CFRP, which are commonly used in the reinforcement of masonry structures. It is known that the adhesion of these elements to masonry surfaces weakens over time, and mechanical connections are required [72]. Furthermore, it has been shown that FRP reinforcement can exhibit delamination under high vertical loads, which can jeopardize the structural integrity [73]. These elements are also prone to rapid strength loss at high temperatures [74], and temperature variations can reduce the adhesive strength, limiting the long-term durability of the reinforcement [75]. Additionally, the major disadvantage of reinforcing with FRP and CFRP is that it is irreversible and may harm the original texture of historical structures [76]. On the other hand, there are also some negative aspects of reinforcement approaches using concrete elements or concrete spraying techniques. These negative factors include adding heavy loads to the structure [77], damaging architectural and cultural values [78], and negatively affecting seismic behavior [79].

The advantages of using steel elements in the reinforcement of historical masonry structures, as highlighted in the literature, are as follows. These elements, with their high load-bearing capacity, have been shown to significantly increase the load-bearing capacity of the structure and improve seismic performance when proper techniques are used [73]. Additionally, mechanical connections can be easily applied to steel elements, and they can be removed and reused without damaging the structure [80]. Considering these factors, the approach of reinforcing with steel elements is recommended in this study.

6. Results and Conclusions

This study demonstrated the effectiveness of the reinforcement methods applied to understand the seismic behavior of historical masonry minarets and increase their performance against earthquakes. The Gaziantep Kabasakal Mosque minaret was among the structures damaged during the 2023 Kahramanmaraş earthquakes, and structural problems were experienced due to the weak rigidity, especially in the upper part and spire of the minaret. In the analyses performed, significant deformations and collapse mechanisms were observed in the minaret’s original state (OM). High stresses occurred in the transition areas between the main body and the balcony, and severe displacement demands were detected in the upper parts of the minaret.

The proposed strengthening method was based on increasing the rigidity by using steel rings in the weak areas of the minaret. The dynamic analyses observed that the displacement demands in the strengthened model (RM) were significantly reduced, and the risk of collapse was minimized. In particular, the deformations in the upper part and spire of the minaret were controlled, and crack formation in these areas was prevented. The original model determined that the cracks and displacements observed in these areas were reduced or even completely eliminated after the reinforcement. The results obtained from this study can be listed as follows.

The thin and long structure of the minaret makes it vulnerable to earthquake forces. The differences in rigidity, especially in the upper part and spire, cause these areas to become critical damage points. Although using local materials such as Urfa stone provides high strength in some areas of the minaret, the heterogeneous structure of this material creates uncertainties in the structural behavior. Therefore, detailed analyses of the material properties of historical structures and adaptation of strengthening measures in line with these material properties are of great importance.

The reinforcement with steel rings proposed in the study effectively preserved the structural integrity by reducing the displacement demands in the upper part and spire of the minaret. Dynamic analyses showed that critical cracks occurred in the model without reinforcement (OM), collapse occurred in these regions, and displacements were significantly controlled in the reinforced model (RM). These results prove that the proposed reinforcement method effectively eliminates the lack of rigidity in masonry minarets. Reinforcement studies carried out on historical structures should aim not only to increase the structural strength, but also to preserve the esthetic and cultural values of the structure. In this context, it is essential that the proposed reinforcement method can be applied with minimum intervention and is reversible. The study has shown the applicability of this reinforcement method on similar historical structures, but further analysis is required under different boundary conditions.

This study presented a case analysis. However, the architectural features of the examined minaret are in line with traditional Ottoman architecture. Therefore, the approaches, assumptions, and the proposed strengthening configuration adopted in the study can be applied to similar structures. Nevertheless, considering factors such as the cultural significance of historical buildings, differences in material properties, and varying environmental conditions, it is important to provide building-specific solutions or adapt existing solutions to the structure in question.

For future study, although the behavior of the minaret during the 2023 Kahramanmaraş earthquakes was identified in this study, it is recommended that future research explore the minaret’s detailed seismic behavior and assess the proposed strengthening using different ground motions. In light of the number of ground motions suggested in various national regulations, increasing the number of dynamic analyses will be crucial in deciding on the strengthening method.

In conclusion, this study again emphasizes the importance of numerical analyses and reinforcement methods for protecting historical masonry structures against earthquakes. The proposed steel ring reinforcement method has yielded successful results in historical structures with weak rigidity, such as the Gaziantep Kabasakal Mosque minaret. Testing this method on other historic masonry structures and comparing it with different structural systems will be essential in preserving historic structures.