A rapid strengthening approach for existing A-type RC frames in substations considering low disturbance: a finite-element-driven design study

The power system is an important lifeline engineering system, and substations play a crucial role in transferring the power supply. To ensure the reliability of the power supply, the safety of the existing A-type reinforced concrete frame cannot be ignored. To avoid long-term power outages and high overall costs, this study considered and analyzed the use of a low-interference rapid reinforcement method. All components used in this method are produced in the factory, and no grouting or welding operations are required on site. Additionally, ABAQUS numerical simulation was used to study the mechanical properties of the strengthened A-type reinforced concrete frame and analyze the effectiveness of the proposed reinforcement method. The results show that strengthening the A-type reinforced concrete frame by this method has increased its ultimate bearing capacity, initial stiffness, and energy dissipation capacity by 184%, 28%, and 52%, respectively. The proposed method improves the overall performance. This study provides a reference for reinforcing similar reinforced concrete structures in substations.

1 Introduction

With rapid urbanization and industrialization in China, the demand for electricity has increased significantly. The power system is an important lifeline engineering system, where substations play a crucial role in transferring the electricity supply. The substations are widely used. A substation failure causes a power outage and affects the production and life of local residents. Scholars worldwide have conducted relevant experiments and theoretical research on the structures that serve in substations. Yin et al. (2023) conducted an experimental study on the seismic performance of indoor RC substations with a combined base isolation technique under far-field earthquakes and near-fault earthquakes. Chen et al. (2025) proposed a partial isolation system (PIS), in which isolation bearings are installed between the equipment and the floor slab. This approach provides cost-effective seismic protection for indoor substations facing risks of equipment damage during earthquakes due to excessive acceleration. Gong et al. (2024) numerically investigated the seismic responses, bearing capacities, and frailties of long-span truss structures in ultra-high voltage substations subjected to near-fault pulse-like and far-field earthquakes. Gong et al. (2022) numerically investigated the seismic fragility of a 1,000 kV outgoing line frame, which has complex and multiple interactions with transmission towers and lines, subjected to earthquakes with multiple angles of seismic incidence. Gong and Zhi (2020) investigated the seismic responses, earthquake failure modes, and collapse fragility of a 1,000 kV outgoing line frame by considering the interactions in the tower line system. Zhang et al. (2023) developed a finite element (FE) model of a typical lightning rod structure with an intersecting joint weld and verified it using experimental results. However, only a few studies (Li et al., 2016; Gao, 2019; Wang et al., 2004) about existing A-type RC frames in substations are found in existing literature. To ensure a reliable power supply, the safety of existing A-type RC frames cannot be neglected.

For existing RC frames in coastal regions, the exposure to harsh environments leads to the degradation of their material properties due to physical or chemical effects (A et al.). These cause deterioration of the performance of RC frames and increase their risk probability. Common environmental factors, such as chloride ions, carbonation, and their combined effects, lead to performance deterioration and subsequent safety risk. What is worse is that many substations built during the 1980s and 1990s are still in use. This indicates that the bearing capacity of the investigated reinforced concrete frames may not meet the requirements of the current Chinese code (Ministry of Housing and Urban-Rural Development of China, 2013). These deteriorated existing RC frames need to be strengthened to ensure the ongoing substation operation. Therefore, there is an urgent need to strengthen the RC frame to ensure the operation and maintenance of substations.

With the development of the electric power system, the actual service life of some substations is close to the design service life specified in the building code. More and more substations are facing needs such as extending service life, upgrading, and renovation. Both space and clear height are limited in a substation. What is more, the devices must be kept in non-interruptible operation, and they are either difficult to move or cannot be moved at all. The task must be completed within the given time limit. However, common reinforcement methods for concrete structures have limitations for RC frames in substations (Norris et al., 1997; Abdalla et al., 2016). Traditional reinforcement techniques, such as carbon fiber and basalt fiber reinforcement, are used to improve structural durability. Their effectiveness in increasing structural load capacity is limited. The construction process, including bonding, section enlargement, steel pipe wrapping, and steel–concrete composites, is cumbersome. Other reinforcement methods can increase the load-bearing capacity of A-type RC frames, but these methods require power outages, which is unacceptable to the power grid managers (Ibrahim et al., 2025; Cheng et al., 2025; Ma et al., 2025; Li et al., 2022; Zhang and Liu, 2025).

Based on assembly technology, this study conducted a detailed investigation on a new method that can effectively and rapidly enhance the existing A-type reinforced concrete frame while minimizing disturbance. First, survey and load analysis were conducted, and common strengthening methods were compared to consider low disturbance (Peng et al., 2022; Bencardino et al., 2002; Bai et al., 2023). Next, ABAQUS numerical simulations were utilized to analyze the influence of the proposed strengthening method on the mechanical performance of the existing A-type RC frame. Then, the effectiveness of the new rapid strengthening approach in enhancing the safety of the existing A-type RC frame was verified. Finally, based on the analysis of design loads and the reinforcement methods, relevant suggestions were proposed to ensure the safety and sustainability of the substation structure (Sezen, 2012; Bai et al., 2024; Granata and Parvin, 2001).

In conclusion, in view of the prominent limitations of traditional reinforcement methods in power station applications, such as long power outage time and cumbersome foundation treatment, this article proposes a prefabricated low-interference reinforcement technology. Different from the existing processes, this method is based on the dry steel-bonding concept and achieves rapid reinforcement through standardized components and modular assembly, without requiring power outages. It significantly reduces interference to power grid operation and provides a new technical path for the reinforcement of existing A-type RC frames (Malek, 1997; Naser et al., 2019; Mitropoulou et al., 2022; Meier, 1995). The research approach is illustrated in Figure 1.

2 Strengthening analysis of existing A-type RC frames

2.1 Survey of existing A-type RC frames in substations

A survey was conducted on six typical existing substations. These substations are located in Maoming City, Guangdong Province. The voltage levels of the investigated existing substations range from 35 kV to 500 kV. The construction time of these existing substations spans from 1980 to 2020. Among these substations, four were constructed during the 1980s and 1990s. They have suffered severe damage, and the observed damage patterns showed certain regularity, as shown in Figure 2. These existing frames are precast RC structures. The frequency of various types of damage (cracks, peeling, and rusting) observed during the investigation is detailed in Table 1.

1. There are obvious, wide cracks on the surface of the existing A-type reinforced concrete frame.

2. The spalling on the concrete protective layer of the A-type reinforced concrete frame is quite obvious.

3. The rusting of rebar and corrosion propagation cannot be ignored.

Figure 1

Figure 1. Flowchart of research methodology.

Figure 2

Figure 2. Photographs of typical damage of A-type RC frames in an existing substation. (a) Concrete spalling and (b) exposed reinforcing bars.

Table 1

Table 1. Frequency (%) of various forms of damage.

2.2 Load analysis of existing A-type RC frames in substations

As mentioned earlier, among the substations surveyed, four were constructed in the 1980s and 1990s. As such, the bearing capacity of these A-type reinforced concrete frames may not meet current Chinese standards^[11]. To analyze the bearing capacity of these frames, a calculation model for an A-type reinforced concrete frame without durability damage was developed using the design software PKPM, as shown in Figure 3.

Figure 3

Figure 3. Computational model of A-type RC frames developed using PKPM software.

The bending moment curve of the A-type reinforced concrete frame is shown in Figure 4. As shown in Figure 4, the maximum bending moment occurs at point A, which is located at the top of the A-type RC frame. At point A, the actual single-sided reinforcement area of the member is 760 mm². However, to satisfy the requirement of the current Chinese code, the single-sided reinforcement area of the member should be 1,420 mm². For this member, the ratio of resistance to the load is only 0.54. This indicates that the bearing capacity of the A-type reinforced concrete frame at point A exceeds the limit value stipulated by the current Chinese standards^[11].

Figure 4

Figure 4. Diagram of the bending moment envelope of A-type RC frames. M1, M2, and M3, respectively, represent the maximum bending moment values of different sections.

In this load analysis, the initial defect factors were not taken into account. If the initial defect factors were included in the consideration, the bearing capacity of the bearing capacity framework might decrease. In fact, the various forms of damage have seriously compromised the durability of the existing A-type reinforced concrete frame. Therefore, the safety of some existing A-type reinforced concrete frames has potential risks. The investigation results and load analysis both indicate that these damaged A-type reinforced concrete frames must be reinforced to ensure the normal operation of the substation.

2.3 Limitations of common strengthening methods for existing A-type RC frames in substations

In substations, space and clear height are both limited. Moreover, the equipment must operate continuously without interruption, and substations are difficult or impossible to move. Therefore, the reinforcement work must be completed within a limited time. The commonly used methods of reinforcing concrete structures have limitations when applied to A-type reinforced concrete frames in substations. The fiber-reinforced composite material (FRP) bonding reinforcement method, steel plate bonding reinforcement method, and embedded steel-FRP composite bar (SFCB) reinforcement method all require preparation of the concrete base, including thorough removal of loose concrete and rust from the reinforcing bars. This process is very complicated. The enlarged section reinforcement method requires a longer construction time. It will increase the cross-sectional area, which is not suitable in substations. The replacement of the steel frame method requires a large-scale power outage, which is unacceptable for the operation of the power grid. Therefore, the common reinforcement methods have limitations for the existing A-type reinforced concrete frames in substations.

3 Methodology

As mentioned before, the existing A-type reinforced concrete frame in the substation plays a crucial role in the operation of the power grid. It has problems such as concrete cracks and corrosion. This poses a threat to the safety of the existing A-type reinforced concrete frame in the substation. However, the common reinforcement methods require a longer construction period. This would result in prolonged power outages and high overall costs, which exceed the acceptable range for regional power grid operations. To address this issue, this study proposes a rapid reinforcement method that takes into account low interference.

A rapid reinforcement method considering low disturbance was designed for the A-type RC substation frame. The schematic diagram of the rapid reinforcement method considering low disturbance is shown in Figure 5. The rapid reinforcement method consists of four parts. The first part is the reinforcing bars installed on both sides of the existing frame. Angle steel and channel steel can be used. The second part is the plates and angle steel bars attached to the first part, as shown in Figure 5d. The third part is the high-strength bolts pre-welded on the first part, as shown in Figure 5e. The fourth part is the longitudinal connection plates used to connect the segmented longitudinal reinforcing bars.

Figure 5

Figure 5. Assembled low-disturbance reinforced structure. (a) Facade reinforcement structure diagram. (b) Section 1-1. (c) Detail 1. (d) Detail 2. (e) Detail 3. (f) Detail 4. (g) Section A-A.

For strengthening the existing A-type RC frames, the condition of the actual site and transportation must be considered to determine the number of segments of the reinforced steel required.

The dry connection method is adopted for the connection between reinforced steel and existing A-type RC frames. Anchor bolts are used for connection at the top and bottom of the columns, as shown in Figures 5c,f,g. Thus, post-grouting is not necessary in this method.

All parts used in this method are manufactured in factories. On-site strengthening construction can be completed by modular assembly and mechanical anchoring as a design requirement. Thus, on-site welding is not necessary in this method; the specific construction process is detailed in Figure 6.

Figure 6

Figure 6. Construction process.

4 Finite element analysis

In order to further analyze the reinforcement methods and evaluate their effectiveness, we used ABAQUS finite element analysis software to conduct a numerical simulation analysis of the pseudo-static test of the A-type reinforced concrete frame (Gao, 2019).

4.1 Model establishment

4.1.1 Frame dimensions

The cross-section dimensions of A-type RC frame members are based on a schematic diagram of the original A-type RC frame in a substation. The cross-sectional dimension of the beam is 120 mm × 450 mm. The cross-sectional dimensions of the inclined columns are 130 mm × 450 mm. The cross-section dimension of the top of an A-type RC frame is 450 mm × 450 mm. The dimensions of the upper grooves on both sides are 160 mm × 450 mm.

4.1.2 Materials

The materials used in the model include concrete and steel, with the following main material properties, are given as follows:

The strength grade of concrete is C25, with a density of 24 kN/m³. The axial compressive strength, f_c, of concrete is 11.9 MPa. The elastic modulus E of concrete is 2.8 × 10⁴ MPa. The Poisson’s ratio of concrete is 0.2.

The reinforcing bars adopt HRB335 steel, with a yield strength of 335 MPa. The elastic modulus of bars is 2.0 × 10⁵ MPa. The Poisson’s ratio of bars is 0.3.

The reinforcement steel is Q235 steel, with a density of 78.5 kN/m³ (7,850 kg/m³). The steel plate thickness is 12 mm, with a yield strength of 235 MPa. The elastic modulus of the steel plate is 2.0 × 10⁵ MPa. The Poisson’s ratio of the steel plate is 0.3.

4.1.3 Element types

Concrete is modeled using solid elements (C3D8). Steel mesh is modeled using truss elements (T3D2). Steel plates are modeled using shell elements (S4R4). A surface-to-surface rigid contact is adopted between the steel plate and the concrete.

4.1.4 Constraint setting

All degrees of freedom are constrained at the bottom of the A-type RC frame. The loading position on the top of the A-type RC frame is coupled with the center point of the top. This point is the loading point.

4.1.5 Loading setting

For the pseudo-static test, the loading was carried out using the displacement control method, with each level of loading increasing by 4 mm. Displacement was applied at the top of the A-shaped reinforced concrete frame. The loading settings are shown in Figure 7. As shown in Figure 7, the maximum displacement amplitude is 168 mm.

Figure 7

Figure 7. Loading setting.

4.1.6 Model meshing

The mesh size of the A-type RC frame is 100 mm.

To clearly present the key parameters and settings adopted in the modeling process, the core modeling information systems, such as material properties, simulation properties, and modeling properties, are summarized in Tables 2–4.

Table 2

Table 2. Material properties.

Table 3

Table 3. Simulation properties.

Table 4

Table 4. Modeling properties.

The finite element analysis (FEA) models were developed and are illustrated in Figure 8.

Figure 8

Figure 8. FEA model established using ABAQUS: (a) Concrete of original A-type RC frame. (b) Steel bars of the original A-type RC frame. (c) Assembled strengthening steel. (d) Strengthened A-type RC frame.

4.2 Optimization analysis of strengthening methods

To analyze the design parameters of the proposed strengthening method, optimization analysis was conducted on the thickness and yield strength of the steel plate. Four thicknesses (10 mm, 12 mm, 14 mm, and 16 mm) were chosen. Three yield strengths (160 MPa, 235 MPa, and 345 MPa) were chosen. The mechanical behaviors of the strengthened A-type RC frames with different thicknesses or yield strengths were compared. The analysis results are as follows.

4.2.1 Different yield strength

The performance of an A-shaped reinforced concrete frame reinforced with steel plates of three different yield strengths (160 MPa, 235 MPa, and 345 MPa) was compared, as shown in Figure 9. Figures 9a,b, respectively, present the hysteresis loop and the skeleton curve. Figure 9c gives the initial stiffness and the maximum load-bearing capacity.

Figure 9

Figure 9. Analysis results of A-type RC frame strengthened by steel plates with different yield strengths: (a) Hysteresis loops. (b) Skeleton curves. (c) Initial stiffness and maximum load capacity.

As seen in Figure 9c, when the yield strength of the steel plate increases from 160 MPa to 235 MPa, the stiffness increases by 4.3% and the maximum load capacity increases by 4.8%. When the yield strength of the steel plate increases from 235 MPa to 345 MPa, the stiffness increases by 52% and the maximum load capacity increases by 25.3%. This indicates that the 345 MPa has the best load-bearing capacity.

4.2.2 Different thicknesses

The performance of A-type RC frames strengthened by steel plates of four thicknesses (10 mm, 12 mm, 14 mm, and 16 mm) was compared, as shown in Figure 10. Hysteresis loops and skeleton curves are shown in Figures 10a,b, respectively. The initial stiffness and maximum load capacity are given in Figure 10c.

Figure 10

Figure 10. Analysis results of an A-type RC frame strengthened by steel plates with different thicknesses: (a) Hysteresis loops. (b) Skeleton curves. (c) Initial stiffness and maximum load capacity.

As shown in Figure 10c, when the thickness of the steel plate increased from 10 mm to 12 mm, the stiffness increased by 19.9% and the maximum load capacity increased by 3.2%. When the thickness of the steel plate increased from 12 mm to 14 mm, the stiffness increased by 55.4% and the maximum load capacity increased by 29%. However, when the thickness of the steel plate increased from 14 mm to 16 mm, the stiffness increased by 13.7% and the maximum load capacity increased by 7.4%. A significant improvement of the performance in the strengthened A-type RC frame is observed when the thickness increases from 12 mm to 14 mm.

However, this method requires an economically viable reinforcement solution, and its cost should be lower than that of traditional reinforcement methods. It is recommended to set the yield strength at 235 MPa and the thickness at 12 mm as the design parameters.

4.3 Evaluation of the proposed strengthening method

To evaluate the proposed strengthening method, a comparison was conducted on an A-type RC frame between strengthened and not strengthened. A numerical simulation was conducted using ABAQUS. This reinforcement method adds steel components on the outside of the frame, mainly providing additional bending resistance, thereby significantly increasing the ultimate bearing capacity. However, the initial stiffness of the structure is mainly determined by the original concrete section. The post-installed steel components work in synergy with the original concrete structure, but the stiffness of the steel components is relatively small, resulting in a relatively smaller contribution of the steel components to the stiffness than to the bearing capacity. The detailed analysis follows.

4.3.1 Load capacity

The performance of the strengthened or not strengthened A-type RC frames was compared, as shown in Figure 11. In Figure 11, BDZ denotes the unstrengthened A-type RC frames, and BDZ-JG denotes the strengthened A-type RC frames.

Figure 11

Figure 11. Analysis results of strengthened and not strengthened A-type RC frames: (a) Hysteresis curve. (b) Skeleton curve.

The hysteresis and skeleton curves are given in Figures 11a,b, respectively. As shown in Figure 11a, the hysteresis curves of BDZ-JG are wider than those of BDZ.

As shown in Figure 11b, the energy dissipation capacities of BDZ and BDZ-JG were 35,600 kN·mm and 54,000 kN·mm, respectively. It indicated that the energy dissipation capacity increased by 52% due to the energy dissipation capacity of the assembled strengthening steel, as shown in Figure 8c.

As shown in Figure 11b, the ultimate bearing capacity and initial stiffness of BDZ are 87.5 kN and 1.55 kN/mm, respectively. In contrast, the ultimate bearing capacity and initial stiffness of BDZ-JG are 248.9 kN and 1.98 kN/mm, respectively. This indicates that after reinforcement, the ultimate bearing capacity and initial stiffness of the A-type reinforced concrete frame have increased by 184% and 28%, respectively. This significant increase in bearing capacity can be attributed to the reinforcement method proposed in this study.

4.3.2 Equivalent plastic strain

The equivalent plastic strain situation of the A-type reinforced concrete frame without reinforcement is shown in Figure 12. Figures 12a–c illustrate the increase in failure points. This enables a more realistic consideration of the mechanical behavior of the proposed reinforcement method.

Figure 12

Figure 12. Equivalent plastic strain of strengthened or not strengthened A-type RC frames: (a) Concrete of the original A-type RC frame. (b) Steel bars of the original A-type RC frame. (c) Concrete of the strengthened A-type RC frame. (d) Steel bars of the strengthened A-type RC frame. (e) Assembled strengthening steel.

As shown in Figures 12a,b, regardless of whether the reinforcement is added, the failure point of the A-type reinforced concrete frame always occurs at the top. This is consistent with the load analysis in Section 2.2. Comparing Figure 12a with Figure 12c shows that the equivalent plastic strain of the concrete failure point increased from $6.716\times {10}^{-3}$ to $4.061\times {10}^{-3}$. By comparing Figures 12b, d, it can be seen that the equivalent plastic strain of the steel bar failure point increased from $9.108\times {10}^{-2}$ to $9.439\times {10}^{-2}$. Moreover, as shown in Figure 12e, the equivalent plastic strain of the assembled strengthening steel failure point is $1.346\times {10}^{-1}$. The equivalent plastic strain of the concrete in the strengthened A-type RC frame was effectively reduced, and subsequently, the properties of the materials can be utilized.

5 Conclusion and outlook

5.1 Conclusion

To ensure substation safety and durability, an inspection and analysis of existing A-type reinforced concrete frames was conducted. In this process, finite element simulation calculations were carried out using ABAQUS to analyze the reinforced A-type reinforced concrete frame. A rapid reinforcement method considering low disturbance was proposed. Through the simulation analysis of pseudo-static tests, the bearing capacity and equivalent plastic strain were studied. The following conclusions can be drawn.

1. In substations, conventional reinforcement methods have limitations. To avoid prolonged power outages and high overall costs, a fast reinforcement method considering low interference was proposed. All components used in this method are manufactured in a factory. The on-site reinforcement construction can be completed according to the design requirements through modular assembly and mechanical anchoring. No on-site grouting or welding operations are required. Therefore, this provides a fast and cost-effective engineering solution for reinforcing RC structures in substations.

2. The ultimate bearing capacity and initial stiffness of the A-type reinforced concrete frame that was strengthened using the proposed method were increased by 184% and 28%, respectively. Due to the reinforcement, the bearing capacity has significantly improved.

3. For the A-type RC frame strengthened by the method proposed, the hysteresis curves became wider, and the energy dissipation capacity increased by 52%. This indicated that the ductility characteristics and hysteresis performance improved significantly due to strengthening, which benefited the seismic resistance of RC structures in a substation.

4. This method, through factory prefabrication and assembly construction, can shorten the construction period by 60% and reduce power outage losses. The overall cost is approximately 20% lower than that of traditional reinforcement methods.

5.2 Outlook

1. The finite element analysis in this study is based on certain assumptions, and the range of parameter values must be further expanded.

2. The performance of the reinforcement scheme under actual complex loads must still be verified through long-term monitoring and full-scale experiments.

3. Future research will focus on optimizing the connection nodes and conducting corresponding experimental validations.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding author.

Author contributions

WC: Writing – original draft, Writing – review and editing. WZ: Writing – original draft, Writing – review and editing. YW: Writing – original draft, Writing – review and editing. JG: Writing – original draft, Writing – review and editing. XG: Writing – original draft, Writing – review and editing. LL: Writing – original draft, Writing – review and editing. GN: Writing – original draft, Writing – review and editing. JW: Writing – original draft, Writing – review and editing. SW: Writing – original draft, Writing – review and editing.

Funding

The authors declare that no financial support was received for the research and/or publication of this article.

Conflict of interest

Authors WC, WZ, YW, JG, XG, and LL were employed by Guangdong Power Grid Co., Ltd. Authors GN, JiW, and SW were employed by China Academy of Building Research Co., Ltd.

Generative AI statement

The authors declare that no Generative AI was used in the creation of this manuscript.

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