Investigation of Precast Reinforced Concrete Structures during the 6 February 2023 Türkiye Earthquakes

Abstract

On 6 February 2023, two major earthquakes, M_W7.7 and M_W7.6, occurred in the Kahramanmaraş city region of southeast Türkiye. As a result of the earthquakes that affected the region, around 60,000 people died and thousands of buildings suffered various levels of damage. In this study, the collapse and failure mechanisms of precast reinforced concrete structures, most of which are industrial buildings, and the damages occurred in these structures, were investgated through on-site observations. As a result of the investigations carried out in the field immediately after the earthquake, it was understood that most of the damage was caused by the connection system of the precast structures. The most common damage is the separation of the roof beam from the column due to the weak column–beam connection system. At the end of the paper, studies to strengthen and improve suggestions for this poor behavior will be mentioned. Also, by analyzing a sample structure, the performance levels were determined by comparing it with the damage limit values in TBEC-2018.

1. Introduction

The earthquakes that have occurred in Türkiye’s history show that Türkiye is located in a very high tectonic region, considering the seismic aspect. Two earthquakes on 6 February 2023, which affected 11 provinces and 14 million people, occurred at intervals of nine hours, causing a total of 60,000 deaths and injuries to more than 100,000 people in Türkiye and Syria. According to official data, it has taken its place in history as the earthquake with the highest loss of life in the history of Türkiye (the previous one saw approximately 33,000 people lose their lives with a magnitude of 7.8 Mw from the Erzincan earthquake that occurred in 1939) [1]. In addition, after these earthquakes, many houses, transportation structures, historical buildings, and industrial structures were destroyed and damaged at different levels [2,3,4,5].

Industrial buildings in Türkiye are generally built as precast reinforced concrete structures. These structures (columns, roof beams, gutter beams, purlins, etc.) are produced separately in the factory environment and assembled on-site. Wide spans can be passed with prefabricated structures and, since they are produced in a factory environment, they have high-quality materials and a high strength. However, due to the weakness of the joint area of the precast building members, different types of damage were seen in many buildings after the earthquake. In the 1988 Spitak (Armenia) earthquake, multi-story prefabricated frame-panel buildings performed poorly, mainly due to the low ductility of the connections [6]. After the 1999 Kocaeli (Türkiye) earthquake, severe damages related to connection deficiencies between prefabricated structural elements and insufficient bending reinforcement in prefabricated reinforced concrete columns were also documented [7]. Several different non-linear time history analyses of typical single-story prefabricated industrial buildings in Türkiye have been made and bending damage at the base of the column has been mentioned [8]. Sezen and Whittaker [9], after the 1999 Türkiye Earthquake, examined some facilities with different prefabricated structures in the region. Structural and non-structural damages at these facilities were summarized and reported. In the studies by Magliulo et al. [10], Bournas et al. [11], Minghini et al. [12], and Savoia et al. [13], data collected after the 2012 Emilia earthquakes are presented to explain the damage and seismic performance of the precast structure type. Ozden et al. [14] examined the performance of precast concrete structures in the 2011 Van (Türkiye) earthquake. The effects of poor prefabricated connection design and detailing during the building of prefabricated concrete structures are presented. In the study by Henry et al. [15], the structural damage observed in buildings during the 2016 Kaikoura earthquake in Wellington was explained by focusing on critical damage states used to categorize damage. A precast structure was modeled and analyzed by Pekgökgöz and Avcil [16] in order to investigate the reasons for the collapse of a prefabricated building in Şanlıurfa.

Because the ability to accommodate relative displacements between the members without losing the beam support and an adequate transfer of the lateral horizontal forces to the column would be the proper design, the connection between the column and the beam is referred to as connection failure. To address the disadvantages of the precast structure that were previously mentioned, extensive and methodical research has been undertaken [17,18,19,20,21]. There are also some studies on the behavior of precast structures under dynamic loads and retrofitting techniques which will be discussed in detail in the following sections [22,23,24,25].

In this paper, first, details about the 6 February 2023 Kahramanmaraş earthquakes and the area’s seismicity are provided. Then, initial findings obtained from field research data on the damages of prefabricated reinforced concrete industrial buildings hit by the 2023 Kahramanmaraş earthquakes are presented. Also, a sample precast model is analyzed with the SAP2000 program and compared with the damage limit values in TBEC-2018. The single-story reinforced concrete precast structures, that do not have moment-resisting frames and in which the column–beam joint is made with steel dowels, of which make up a large part of the industrial buildings in our country, are the most preferred building style in this region as well. The failures and damage patterns observed in these industrial structures examined in the disaster area are explained in detail in terms of structural engineering. The flow chart of the study is shown in Figure 1.

2. Kahramanmaraş Earthquakes

The East Anatolian Fault Zone (EAFZ), with a length of approximately 580 km, is located between Bingöl Karlıova and Hatay. Many major earthquakes have occurred throughout history in this fault zone, which is one of the most active fault zones in Türkiye [26,27,28,29,30,31]. Some historical earthquakes such as the 1822 Antakya earthquake, 1866 Karlıova earthquake, 1872 Amik earthquake, 1874 and 1875 Hazar Lake earthquakes, and 1893 Malatya earthquake have occurred over a magnitude of 7. There has not been an earthquake greater than 7 on the EAFZ in the last century. On 6 February 2023, at 4:17 local time and 7.7 magnitude, an earthquake with an epicenter in Pazarcık/Kahramanmaraş occurred. Nine h after this earthquake, which was the beginning of a series of earthquakes, the second largest earthquake with a magnitude of 7.6 with an epicenter of Elbistan/Kahramanmaraş occurred. These earthquakes, centered in Kahramanmaraş, were felt in a large part of the country and over 50,000 people lost their lives in 11 different provinces, shown in Figure 2.

In the first earthquake with a magnitude of 7.7, the maximum accelerations recorded by station 4614 (Pazarcık) of the Turkish National Earthquake Network were around 2380 cm/s², 2080 cm/s², and 1540 cm/s² for the E-W, N-S, and vertical (U) earthquake components, respectively, as shown in Figure 3.

Figure 4 shows the acceleration time graphs of the E-W, N-S, and vertical (U) earthquake components recorded at the 4612 Göksun (Kahramanmaraş) station, which is the closest to the epicenter of the second earthquake, with a magnitude of 7.6. The recorded maximum ground accelerations (PGA) were around 520 cm/s², 620 cm/s², and 430 cm/s², respectively.

3. Description of Precast Buildings in the Region

In this section, non-structural and structural damages in precast reinforced concrete structures during the Kahramanmaraş earthquakes are presented with photographs, with on-site examinations. Most of the prefabricated buildings in our country are single-story structures with pinned column–beam connections and spans of 15–30 m, as shown in Figure 5. These precast structures generally have a rectangular plan typology and double slope roof beams. Since precast structures are produced in a factory environment, each structural element can be produced with the desired strength. In the examinations made after the earthquake, it was understood that the majority of these structural elements had sufficient strength. However, weakness of the joint connection precast structures is the main cause of damage in prefabricated structures, which causes a loss of support of roof members (purlin, roof covering, etc.) from beams, beams from columns, and columns from foundations due to the low strength given by the friction mechanism.

3.1. Damages Observed in Precast Buildings

During the week following the earthquake, 11 provinces affected by the earthquake, including industrial zones, were visited by the author. Hundreds of buildings have been inspected with varying degrees of damage depending on permits and access possibilities: from simple exterior visual inspections to detailed inspections of the interior and exterior of the building.

Figure 6 shows an industrial structure in Adıyaman, one of the cities most affected by the earthquake. The precast reinforced concrete column and beam are connected with double steel bars. As a result of the insufficiency of the column beam connection under the effect of the earthquake, the roof beam was separated from the joint and a large part of the structure collapsed, together with the roof coverings and purlin beams. In addition, excessive rotation in the column bases due to the shear force caused the formation of plastic hinges, buckling, and yielding of the longitudinal reinforcements.

A part of a large industrial structure in Adıyaman is shown in Figure 7. The roof elements have partially collapsed due to failure caused by the connection between the roof beam and the purlin beam. While there was no deviation of the column axis, cracks formed along the column and most of the column–beam joints of the structure were severely damaged.

Loss of support of the roof beam from the column is one of the most common damages in precast reinforced concrete structures in the earthquake area. Although there was no damage to the column and gutter beams, due to the weak column–beam joint, the roof beam lost its support and collapsed completely together with the roof elements, as shown in Figure 8.

The failure of most of the precast buildings was caused by the roof beams being separated from the columns with consoles. When the sitting length of the roof beam was relatively short and the relative displacement between the column and beam exceeded the allowed width, as shown in Figure 9, the loss of support was mostly seen in the middle columns.

Occasionally, large rotations of precast columns occurred because of the settlements or failure of the precast footings. Figure 10 shows an industrial building under construction with unfinished side walls and roof coverings. The concrete-filled and double slope precast roof beams with a span of approximately 25 m lost their support as a result of excessive rotation of the columns and the entire structure collapsed with the purlin beams. It was made quite evident that anticipated framing activity and structural integrity were not achieved. Shear failures have occurred in the lower region of the column as a result of deviation of the column axis, and wall and gutter beam collapses are also observed in some parts of the buildings.

Figure 11 shows a rectangular two-span industrial structure in Pazarcık (Kahramanmaraş), the epicenter of the earthquake. It was observed that the two corner columns, which were thought to be caused by the failure of the foundation–column joint, collapsed together with the beams and roof members. Due to the damage to the column-beam joint with the dowel connection, half of the rest of the structure collapsed together with the roof beams, purlin beams, and roof coverings. In the other part of the structure, partial collapses occurred in the roof coverings and walls, and severe damage occurred in the column–beam joints.

The single-story, single-span precast industrial structure located at the epicenter of the first earthquake is shown in Figure 12. As a result of the ground-based failures and foundation settlement and, hence, separation of the roof beams from the column joints, it is seen that one part of the structure collapsed and the other part was heavily damaged. In addition, the precast columns were damaged from the various interaction parts with the masonry walls due to shear force. Additionally, it is observed that there are cracks in a large part of the building floor.

Plastic hinge formation at the bottom of the columns due to earthquakes is a type of failure encountered in precast reinforced concrete structures. There was no collapse in the structural members of this precast structure in the Kahramanmaraş industrial zone, shown in Figure 13. Concrete spalling occurred at the entire column base and buckling was observed in some longitudinal reinforcements.

In observations made after the earthquake, there were also precast buildings with damage but no collapse. Local bearing failures occurred, as in the factory example in Kahramanmaraş shown in Figure 14. Because no elastomeric bearings were employed in the column–beam connections at the connection interface, there was cracking and crushing at the corners of the structural members in the joint zones.

Due to the earthquake affecting the structure, various damages can be seen in the buildings. These damages are seen together in some precast structures, such as the industrial structure in Hatay shown in Figure 15. The column and trough beams of this building, which consist of two different sections, were built with reinforced concrete. The roof beam, which was built as reinforced concrete precast, was completely destroyed, and the roof beam, which was built as a steel truss system, was severely damaged. Plastic hinges were formed at the column base of this structure, which was exposed to large deformations, separations occurred in the column–beam junctions, and shear failures were observed in some columns.

Many buildings were destroyed or severely damaged in the provinces of Kahramanmaraş, Adıyaman, and Hatay, which suffered great destruction during the earthquakes. However, there are also a few slightly damaged or undamaged precast structures in the region. Examples of four different precast reinforced concrete structures surviving in these most affected provinces are shown in Figure 16.

The schematic representation of the damages frequently seen in precast structures during the surveys carried out in the earthquake region is shown in Figure 17.

In the examinations made in the disaster area, it is understood that Adıyaman, Kahramanmaraş, and Hatay are the provinces with the most structural damage [32,33]. According to the data obtained, there are 57 collapsed or heavily damaged buildings, 119 moderately damaged, and 116 slightly damaged buildings out of 394 examined precast buildings, as shown in Figure 18 [34].

3.2. Analysis of Sample Precast Model

In order to compare the results of the design approaches according to the Deformation-based Design Approach specified in the Turkish Building Earthquake Code (TBEC-2018) [35], the sample model was analyzed with the SAP2000 program [36]. The precast industrial structure sample is single-span and 25 m long in the transverse direction, consists of 10 spans in the longitudinal direction, and each bay width is 8 m long. The building columns have a square cross-section shape with dimensions of 0.36 × 0.36 m and a height of 7.8 m. A fully fixed column footing was produced by setting the column’s boundary conditions. In the numerical analysis, columns and beams were modeled as frames, and slabs and walls were modeled as shell finite elements. The modeling within the scope of the study was designed according to the unfavorable combination specified according to TBEC-2018. In addition, this structure consists of roof beams, gutter beams, purlin beams, walls, and roof coverings with various dimensions and cross-sectional areas, as shown in Figure 19.

For Adıyaman city center, design spectrum graphs drawn according to DD-2 ground motion level (for precast industrial buildings, the calculation is made for an earthquake with a probability of 10% to be exceeded in 50 years and an approximate return period of 475 years) and spectral accelerations measured in the E-W and N-S directions were compared, as shown in Figure 20.

Considering the conditions specified in TBEC-2018 in the design of all structural members, materials with 30 MPa and 420 MPa strengths were selected for concrete and rebar, respectively. First, the modal analysis was performed and the frequency and period values for the first 10 modes were found as shown in

Table 1. Modal analysis results of the precast structure model.

Mode	Frequency (Hz)	Period (s)
1	0.810373	1.234
2	3.174603	0.315
3	3.225806	0.310
4	3.311258	0.302
5	3.424658	0.292
6	3.571429	0.280
7	3.717472	0.269
8	3.846154	0.260
9	3.968254	0.252
10	4.065041	0.246

. The dominant period calculated for the short-span direction by modal analysis was found to be 1.234 s, as shown in Figure 21. Then, the non-linear time history analyses were performed using the spectral accelerations of the Pazarcık (Kahramanmaraş) earthquake that were recorded in the station code 0201. As a result of the analysis, the maximum displacements of the roof and rotations were obtained and the performance levels were determined by comparing it with the damage limit values in TBEC-2018. When the periods values are compared with previous precast structure analysis studies, the results are seen to be compatible [37,38].

Based on the definition of Building Performance Targets, Building Performance Levels for building bearing systems under earthquake effects are defined as the Damage Limitation (DL), the Controlled Damage (CD), and Prevention of Collapse (PC), as shown in Figure 22.

The non-linear behavior of column and beam elements is represented by the plastic hinge defined at the element ends according to TBEC-2018. $f_{\mathrm{c}\mathrm{e}}$ and $f_{\mathrm{c}\mathrm{k}}$ show the average and characteristic compressive strengths of concrete, and $f_{\mathrm{y}\mathrm{e}}$ and $f_{\mathrm{y}\mathrm{k}}$ indicate the average and characteristic yield strengths of rebar. Here, $M_{\mathrm{y}}$ and ${\theta }_{\mathrm{y}}$ are the averages of the effective yield moments and yield rotations of the plastic hinges at the ends of the frame member. L_s is the shear span (the ratio of moment/shear force in the section). The plastic hinge yield rotation ${\theta }_{\mathrm{y}}$ in Equation (3) will be calculated by Equation (4). Here, ${\varphi }_{\mathrm{y}}$ denotes the effective yield curvature in the plastic hinge cross-section. For beams and columns, η = 1 will be taken, and h is the section height. $d_{\mathrm{b}}$ in the last term expressing the reinforcement slip rotation for yielding indicates the average diameter of the rebars clamped in the support. To be used in the performance evaluation for the PC Performance Level, the allowable limit for the calculated plastic rotations is calculated by Equation (5). A performance evaluation for CD Performance Level is defined in Equation (6), depending on the values defined for PC Performance Level.

$$f_{\mathrm{c}\mathrm{e}}=1.3f_{\mathrm{c}\mathrm{k}}$$

(1)

$$f_{\mathrm{y}\mathrm{e}}=1.2f_{\mathrm{y}\mathrm{k}}$$

(2)

$$(EI{)}_{\mathrm{e}}=\frac{M_{\mathrm{y}}}{{\theta }_{\mathrm{y}}}\frac{L_{\mathrm{s}}}{3}$$

(3)

$${\theta }_{\mathrm{y}}=\frac{{\varphi }_{\mathrm{y}}{L}_{\mathrm{s}}}{3}+0.0015\eta \left(1+1.5\frac{h}{L_{\mathrm{s}}}\right)+\frac{{\varphi }_{\mathrm{y}}{d}_{\mathrm{b}}{f}_{\mathrm{y}\mathrm{e}}}{8\sqrt{f_{\mathrm{c}\mathrm{e}}}}$$

(4)

$${\theta }_{\mathrm{p}}^{\left(\mathrm{P}\mathrm{C}\right)}=\frac{2}{3}\left[\left({\varphi }_{\mathrm{u}}-{\varphi }_{\mathrm{y}}\right)L_{\mathrm{p}}\left(1-0.5\frac{L_{\mathrm{p}}}{L_{\mathrm{s}}}\right)+4.5{\varphi }_{\mathrm{u}}{d}_{\mathrm{b}}\right]$$

(5)

$${\theta }_{\mathrm{p}}^{\left(\mathrm{C}\mathrm{D}\right)}=0.75{\theta }_{\mathrm{p}}^{\left(\mathrm{P}\mathrm{C}\right)}$$

(6)

According to TBEC-2018, the Prevention of Collapse (PC) and plastic rotation limits for the Controlled Damage (CD) performance level and maximum rotation, made by non-linear time-history analysis using the SAP2000 program, are shown in

Table 2. Plastic rotation limits and analysis result.

Performance Levels (%)		Analysis Result (%)
${\mathsf{\theta }}_{\mathit{y}}^{\mathbf{\left(}\mathit{C}\mathit{D}\mathbf{\right)}}$	${\mathsf{\theta }}_{\mathit{y}}^{\mathbf{\left(}\mathit{P}\mathit{C}\mathbf{\right)}}$
3.78	5.04	3.56

. Since the plastic rotation demands calculated at the beam ends are close to the controlled damage limit value but remain below this value, the structure is at the Controlled Damage performance level. This performance level relates to the degree of damage that is not too severe and is typically repairable in the building’s structural components in order to ensure life safety.

3.3. Strengthening and Improvement Recommendations

It is clear that these investigations into precast reinforced concrete structures, especially in light of the significant damage these earthquakes inflicted, will result in an improvement in how these structures respond to earthquakes in the future. In this part of the paper, besides making suggestions for the existing building stock, in particular, studies on the development of the column–beam junction area, which is the most common failure situation, will be discussed. Additionally, it is crucial to consider whether to demolish or strengthen these buildings by conducting the necessary inspections and structural analyses on the stock of existing structures that have not suffered heavy damage as a result of earthquakes. The first step of this application will be to identify the buildings that require a detailed inspection and to prioritize the risks by using rapid assessment techniques [39,40,41,42].

Most of the precast structures made in Türkiye are used as industrial structures and the majority of them are single-story frame-type systems. Because these precast elements are produced in the plant, they have a high strength and high-quality materials. However, as it is known, after earthquake damage, the weakness of precast concrete element connections poses quite a big problem under seismic loads [43]. In Türkiye, one of the most common precast beam–column connections is a dowel connection and there are various cases of damage included in this connection, as described above. This type of precast structure mostly consists of two embedded steel dowels in the precast column and put in a beam hole, filled up with mortar (Figure 23). The connection behavior is quite complex and should be improved.

Pinned connections are used in structures where the moments generated by the forces acting on the precast elements are not transferred to other elements. Metelli and Riva [44] present a new type of precast concrete structure connection between beams-to-columns. Experimental results have been investigated in order to increase the shear resistance of the beam–column connection by placing a Z-shaped steel plate and high-strength steel bar fittings in the beam–column interface, as shown in Figure 24. The experimental test results present fine achievements of the connections in terms of ductility, energy dissipation capacity, and shear resistance under cyclic loading.

The behavior of the beam–column connection of the precast structures under seismic loading was investigated with four different combinations, which are semi-rigid, rigid, pinned, and a new connection by Farsangi [45]. The new proposed connection has a 10 mm plate and 22 mm bolt. The precast beam column was modeled by LUSAS and SAP2000 programs. The result shows that the new connection demonstrates higher ductility, strength, and stiffness. A total of five semi-scale beam–column assemblies, with the inclusion of four prefabricated monolithic specimens, have been experimentally investigated under seismic loading by Choi et al. [46]. The new connection of the precast beam–column consists of steel tubes and steel plates which are bolted and placed inside the precast beam and column, as shown in Figure 25. This study shows the seismic performance of the beam–column connection based on energy dissipation, stiffness, drift capacity, and connection strength, which is generally better than similar conventional cast-in-place connections.

In the study conducted by Al-Salloum et al. [47], the effectiveness of bolted steel plates on the behavior of prefabricated column–beam connections under sudden column loss scenarios was investigated. Experiments were conducted involving a half-scale precast reinforced concrete beam–column assembly, representing the most common types of precast beam–column combinations available in Saudi Arabia. The study by Shannag and Higazey [48] evaluated the condition of an aging multi-story precast concrete structure built 40 years ago and recommended appropriate repair techniques to maximize its service life at a reasonable cost. For a precast concrete column–beam joint, Huang et al. studied the seismic behavior of a replaceable artificial controllable plastic hinge [49]. In the study conducted by Arslan et al. [43], in order to solve the problem of deformed internal axle columns of damaged multi-span structures, the columns in these axes are strengthened by connecting them with steel beams below the column crowns.

To find out the impact of the short cantilever beam formation under beam members in precast post-tensioned connections on the structural capacity of the system, this study is being conducted by Alver [50]. As a result, it is suggested as an effective solution for increased ductility demands that the short cantilever beam’s contribution to the structural capacity of the system be clearly observed.

In addition to many damaged buildings, there are numerous prefabricated industrial buildings that withstood the earthquake with no signs of damage. This turn of events made clear that seismic hazards can cause significant damage to in situ cast reinforced concrete structures as well as precast ones if their design and construction are not carefully planned and adhere to the computations and connection detailing criteria outlined in the TBEC-2018. Although it is clearly expressed in the relevant regulations how to find out the earthquake performance of existing precast buildings, the repair and strengthening processes are not very clear. Detailing the column–beam joint area, which is one of the main causes of damage, is an issue that should be taken into consideration. For the purpose of preventing progressive collapse, efficient rehabilitation approaches to upgrade column–beam joints in precast reinforced concrete structures are required.

4. Conclusions

The earthquakes that struck Türkiye on 6 February 2023, along the Eastern Anatolian Fault, resulted in a considerable number of fatalities and extensive property damage across 11 different cities. However, this study is limited to prefabricated building failures and damages in Adıyaman, Kahramanmaraş, and Hatay, the three provinces where the most damage and deaths occurred. The failure mechanisms of prefabricated reinforced concrete structures, most of which are industrial buildings, and the damages occurring in these structures were analyzed through on-site observations. The extent of structural damage was significantly impacted by the fact that both earthquakes were significant in magnitude, happened quite near the surface, and, most crucially, occurred at intervals of nine hours.

Prefabricated structures are being used more and more frequently in both commercial and residential constructions. The high-strength prefabricated building components are installed in their intended locations after being manufactured at a factory. Examining the disaster region following the earthquake, it was found that specific prefabricated constructions had suffered varied degrees of damage and destruction. The structural elements’ inability to support the earthquake load due to connection errors is the primary cause of the examined structures’ failures. A small number of buildings with unique design requirements and conditions necessary for prefabricated structures were seen, particularly in TBEC-2018.

Direct examination of industrial areas reveals that many industrial prefabricated structures showed severe damage, and numerous people lost their lives, were injured, and lost property. Most structures were not being worked on when the first major shock occurred at 4:17 local time. The number of fatalities would have been significantly higher if it had happened during working hours.

It has been established that this type of building, which differs from conventional reinforced concrete buildings in that it requires more specialized design principles, requires expertise in the design and manufacture of this building, and that the number of structures that receive this service in practice is relatively small. Typically, beam–column connections lack mechanical connectors, and columns are extremely thin (particularly in the direction perpendicular to the main axis). The cross-sectional depth of the roof beams may exceed 2 m at mid-span. Examining the precast buildings that were damaged after the earthquake reveals that they still have a low resistance to horizontal earthquake forces because of the joints’ vulnerabilities. Heavy damage has been done to precast buildings, which feature the most typical steel–dowel connections (pinned connections) between beams and columns in industrial buildings. The wide-span and heavy roof beams employed in prefabricated structures have been particularly vulnerable to damage as a result of the beam’s poor connection to the column.

The tie beams that link the socket tops have separated, according to field observations. In this instance, it led to the foundation system moving independently, which, in turn, caused the columns to move apart from one another. This is one of the causes of removing the roof beams from the short cantilever connection.