Study on reinforcement effect of H type assembled anchoring joint in portal frame

Abstract

With the development of society, portal frame has been increasingly used in industrial buildings because of light weight, high degree of industrialization, and high economic efficiency. Some existing portal frames need to be reinforced and improved due to the expansion of production and the functional change. As a safe, reliable, and convenient reinforcement method, the external prestressing reinforcement technology is often used to reinforce the portal frame. In this paper, a new type of assembled anchoring joint was designed to connect the steel strand to the steel beam of a portal frame without damaging the original structure. The static loading test was performed by adjusting length of the strut. And the changes of strain and mid-span deflection were investigated. The Midas Gen FEM simulation was used to investigate stress characteristics of the reinforced portal frame, including strain, bending moment, shear force, etc. The experimental results and simulation results were compared to verify effectiveness of the designed joint. The results indicated the new prestressed anchoring joints had the advantages of simple structure, effectiveness, and force transfer path of the joints was clear. Therefore, the new prestressed anchoring joint can be applied to the external prestressing reinforcement of the portal frame.

Keywords

achoring joint experiment external prestressing reinforcement FEM analysis portal frame

Introduction

In recent years, portal frames have been increasingly used in factories. Some factories have been rebuilt and extended due to the reasons such as the expansion of production. However, in many cases, after the expansion of production, the existing portal frame cannot meet the design requirements thus need to be reinforced. At present, two technical methods are mainly used to reinforce the steel structures. The first method aims to adjust the internal force of the existing structures. For example, through a prestressed technology, a component internal force opposite to the load effect can be obtained to achieve the structural reinforcement. The second method aims to improve the bearing capacity of the component by changing the beam section and column section or enhancing the joint strength (Shi, 2018). For example, steel beams and steel columns reinforced with CFRP (Bocciarelli et al., 2009; Liu et al., 2016) or welded steel plates (Bhowmick and Grondin, 2016; Liu and Gannon, 2009), and the joints reinforced with cover plate (Engelhardt and Sabol,1998; Whittaker et al., 2002), etc.

Among many reinforcement methods, the external prestressing reinforcement technology is convenient, cost saving, and thus has been increasingly applied to reinforce the steel structures (Abbas et al., 2018; Kambal and Jia, 2018; Yang et al., 2012), concrete structures (Herbrand and Classen, 2015; Marco, 2018; Su et al., 2017; Wang, 2009; Zhang et al., 2018), bridge constructions (Chen et al., 2017; Madaj and Mossor, 2019; Miyamoto et al., 2000; Park et al., 2004; Zhao et al., 2014) etc. For example, Kambal and Jia (2018) tested the strength of two steel box girders (prestressed girder and control girder), and verified that prestressing techniques can enhance steel girders flexural behavior. Wang (2009) used six external prestressed steel strands to reinforce a concrete continuous beam. Miyamoto et al. (2000) investigated the application of external prestressing technology in bridge reinforcement, which provided a reference for the bridge reinforcement.

When the external prestressing method is used to reinforce the structures, the anchoring method between the prestressed steel strand and the beam is the key to the reinforcement effect. Therefore, different anchoring methods have been proposed by researchers. For example, in the design of a grain warehouse with a 72 m span by Wang et al. (2008), the anchorage at the end of the strand was used to transfer the prestress to the steel beam and achieve the reinforcement purpose. In the reinforcement of the new steel structure in a hospital hall, Chen et al. (2009) used independent clamp anchorage to achieve the connection between the steel strand and the steel beam, on this basis, the bearing capacity of the anchorage can reach 300 kN. Shi et al. (2003) anchored the steel strand to the steel column using a porous clamp anchor. Lee et al. (2019) studied that the steel strand passed through the hole of the steel beam and was anchored on the steel beam with bolts. Shao et al. (2016) used external prestressing technology to anchor the strand with the anchor block to reinforce the box girder. And a reasonable structure of external prestressing anchor block was obtained. A Y-type lug plate was used to achieve the connection of steel strand at the anchoring joint and reinforce the swimming and diving hall in Henan sports center (Chen et al., 2016). However, the above anchorage methods would cause damage to the original structures.

In order to avoid damage to the original structure by using welding, drilling or other joint connection methods, a new type of anchoring joint was developed and applied in the external prestressing reinforcement technology in this paper. The developed joint can anchor the prestressed steel strand to the steel beam joint based on the assembled installation (removable) without causing any damage to the original structure.

Design of H type anchoring joint

External prestressing reinforcement technology

The external prestressing reinforcement system mainly consisted of two anchoring joints, two struts and a steel strand, as shown in Figure 1. The main working principle of external prestressing reinforcement system is described as follows: Firstly, the anchoring joints are connected to the ends of the steel beam. Next, the struts are installed. Then the steel strand is anchored to the anchoring joints and the ends of struts. After that, the prestress of the strand is applied to reduce the bending moment of the mid-span and the deflection of the beam thus achieving strengthening purposes.

Figure 1.

External prestressing reinforcement system.

Constructional details of the joint

In this paper, a new type of assembled anchoring joint was designed and applied to the external prestressing reinforcement system. Since the designed anchoring joint had a similar shape to the character “H”, it is called “H type anchoring joint”. The design is based on the specific reinforcement examples. In the general cases, the stress of the joints and the structure is similar to the discussed examples with corresponding adjustments in scale, size, angle, etc. The designed joint mainly consisted of eight parts: upper triangle friction component (1), lower triangle friction component (2), H type component (3), anchoring component (4), upper fixed rod (5), lower fixed rod (6), bolt rod (7), and adjusting rod (8). The details of each component and the installation diagram of the anchoring joint are shown in Figure 2. In Figure 2, component (9) is the steel beam. The entire H-type end anchoring device weighed about 30 kg and the cost of design and installation was only about $50.

Figure 2.

Details of each component and installation diagram of the anchoring joint (Unit: mm): (a) front view, (b) lateral view, (c) friction components of upper and lower triangle, (d) anchoring component, and (e) H type anchor joint structure.

Working principle of the joint

The H type anchoring joint was designed for the slope of the steel beam, which can be adjusted according to inclination of both the upper and lower flange of the steel beam. After assembly, the whole device can be self-locked by its weight. The joint was designed based on the lever principle, as shown in Figure 3, the H type component (3) was equivalent to the lever arm and the lower fixed rod (6) was equivalent to the fulcrum of the lever.

Figure 3.

Schematic diagram of lever principle.

After the prestress is applied, the mechanical schematic of the anchoring joint is shown in Figure 4, the upper fixed rod (5) is simplified into hinge A, the lower fixed rod (6) is simplified into hinge B, so the H type component is similar to a lever in which the B is the fulcrum. When end of the component (point C) is under tension of the steel strand, the force (F) will generate torque to the point B. Then the torque will be balanced by the torque which generated by reaction force (F₁) of upper flange to upper triangle friction component and frictional forces $(F_{1} μ, F_{2} μ)$ of the upper and lower flanges to upper and lower triangle friction components. Besides, if $F$ (tension of the steel strand) is decomposed along the upper flange (X direction) and perpendicular to upper flange (Y direction), $F_{y}$ will be balanced by the pressure difference between the upper and lower flanges to the triangle friction components ( $F_{y} = F_{2} - F_{1}$ ), $F_{X}$ will also be balanced by the sum of frictional forces of the upper and lower flanges to triangle friction components ( $F_{x} = F_{1} μ + F_{2} μ$ ). As the prestress became greater, the torque was greater, and the pressure and friction force of the steel beam became larger on both the upper and lower flanges. As long as the prestressed moment arm is designed properly, the whole device can be self-locked and stable under the prestressed condition. The adjusting rod prevented the upper and lower triangle friction components from tilting to the H type component.

Figure 4.

Mechanical schematic of the anchoring joint.

According to the mechanical schematic of the anchoring joint (Figure 4), and ignoring the influence of flange thickness, the pressures of the upper and lower flange to triangle friction components are as follows:

F_{1} = F \frac{L_{B} X - L_{A} μ b}{μ^{2} b^{2} - XY}

(1)

F_{2} = F \frac{L_{B} μ b - L_{A} Y}{μ^{2} b^{2} - XY}

(2)

And,

X = \cos γ L + μ b + μ \sin γ L

(3)

Y = μ b + μ \sin γ L - \cos γ L

(4)

The sum of frictional forces of the upper and lower flanges to triangle friction components is as follows:

F_{μ} = (F_{1} + F_{2}) μ = F (\frac{L_{B} μ b - L_{A} μ b + L_{B} X - L_{A} Y}{μ^{2} b^{2} - XY}) μ

(5)

Component of prestress along the upper flange is as follows:

F_{x} = F \cos β

(6)

In order to make the anchoring joint reach self-locking steady state, it should meet the following:

F_{μ} ⩾ F_{x}

(7)

Thus, the minimum friction coefficient between the triangle friction components and the upper and lower flanges should meet the following:

(\frac{L_{B} μ b - L_{A} μ b + L_{B} X - L_{A} Y}{μ^{2} b^{2} - XY}) μ ⩾ \cos β

(8)

According to the joint design data, the following parameters can be obtained: $L_{A} = 0.851 m$ , $L_{B} = 0.291 m$ , $L = 0.560 m$ , $b = 0.03 m,$ $γ = 73^{°},$ $β = 16^{°}$ . Taking them into the equations (3), (4) and (8), we can get $μ ⩾ 0.141$ . Thus, the joint can be self-locked.

In the reinforcement process, the device transferred the concentrated stress at the end joint to the upper and lower flanges of the steel beam through the upper and lower triangular friction components, thus preventing the concentration of stress.

Struts

In the reinforcement system, the strut 1, strut 2 were prestressed steel strand loading devices. The strut consisted of six parts: the pressure block (1), the fixed components (2), the upper round steel (3), the middle screw rod (4), the loading block (5) and the lower round steel (with a hinged hole at the lower end) (6). The upper end of the strut was hinged with the steel beam and the lower end was rigidly connected with the steel strand. The details of strut are shown in Figure 5.

Figure 5.

Details of strut: (a) full drawing, (b) local drawing, (c) block front view, (d) block cutaway view, (e) front view of fixed component, (f) side view of fixed component(1-1), and (g) top round steel top display.

In order to install struts, firstly, the fixed components were symmetrically placed on the lower flange (on both sides of the plate) of the beam. Next, the upper round steel and the screw rod were installed. Finally, the lower round steel was installed (the connection hole was located at the end of the lower round steel). After the strut was installed, the steel strand was passed through the connection hole. In the loading process, the welding blocks of strut 1 and strut 2 were simultaneously rotated to make the screw rod extend and load (the upper and lower round steels and the outer side of the screw rod were sheathed, the ends of the screw rod were opposite, and the screw rod was extended during the loading process). The screw rod (4) was subjected to pressure during the loading process. Meanwhile, the threaded ends of the screw rod were subjected to the shear force and the other components functioned as a fixed screw rod. In the test the loading mode was static loading.

The set of struts was located at the lower flange of the steel beam with the length of 1.05 m. Compared with the portal frame with the height of 5–7 m, the set of struts in this study was relatively short. The short length would not affect the production and processing space of the plant. Thus the clearance loss of the portal frame was small.

Example of reinforcement

Introduction of example

A propped portal frame was used as an example for the reinforcement. The reinforced portal frame is shown in Figure 6. The beam and column of the portal frame were made of rolled H-shaped steel (H section steel HN350 × 175 × 7 × 11). The variable section of the steel beam was welded T-shaped steel and the material was Q345B steel. The column spacing of the steel frame is 6 meters. Solar panels and other equipment were installed on the left span the portal frame, the roof dead load of left span increased 0.38 kN/m² (the roof dead load of original structure was 0.20 kN/m²), so the left span of the portal frame was strengthened using external prestressing reinforcement, and the anchoring joints used in the reinforcement were H type anchoring joint. Two upper-hinged lower-rigid struts were installed at the 1/3 and 2/3 spans of the steel beam, respectively. And their length is 1.05 m. The prestressed steel strand had the diameter of 15.2 mm and the standard of 1 × 7. In Figure 7, solar panels were installed within net-span of 12.6 m, which increased the roof dead load, so equivalent line load of every steel beam was 2.28 kN/m (q = 0.38 kN/m² × 6 m = 2.28 kN/m), and each strut provided support force 9.58 kN (F₁ = 2.28 kN/m × 4.2 m = 9.58 kN) for the steel beam. According to the calculation of mechanical balance, prestress 39.60 kN (F = 9.58 kN/sin40^o = 39.60 kN) was applied to the prestressed cable (the angle between the prestressing force direction and the steel beam axis is θ = 14°). Therefore, the prestress level of 40 kN was applied to the model.

Figure 6.

The reinforced portal frame (Unit: m).

Figure 7.

Mechanical diagram of steel beam strengthening (Unit: m).

According to the design standards of steel structure (GB50017-2017, 2017), the structural steel should be selected based on the comprehensive consideration on the characteristics including structure, load and working environment. The joints and struts were made of steel with the yield strength of 345 MPa. The high-strength bolt of 10.9 grade friction type was used. In addition, in order to increase the friction force the bottom of the friction component in contact with the steel beam was sandblasted (shot). And according to the code (GB50017-2003, 2003), the friction coefficient was set to be 0.3.

The additional load and the external prestressing reinforcement were only applied on the left span, and according to FEM analysis, reinforcement has little effect on the stress of the right span. To simplify the test, the sensors were placed on the left span only, and the reinforcement effect of the left span was compared and analyzed. In the test, the strain was measured at seven key positions of the left span of the portal frame using the DHDAS (Dong-Hua test real time Data measurement and Analysis software System). Electric resistance strain gauge was used in the test. The implementation of the reinforcement and the pasting position of strain gauge are shown in Figure 8.

Figure 8.

The implementation of the reinforcement and the pasting position of strain gauge.

Installation of the joint

Firstly, two upper triangle friction components were placed symmetrically on the lower side of the upper flange of the I-beam. The top plate of the upper triangle friction component needed to be close to the lower surface of the upper flange of I-beam. The lower triangle friction component was placed on the upper surface of the lower flange of I-beams. The lower triangle friction component was slightly backward relative to the upper triangular friction component, as shown in Figure 2. Then the H type components were fixed with the two fixed rods. Subsequently, the anchoring component was installed at the lower end of the H type components by the bolt rods. Finally, the distance between the H type components and the triangle friction components was adjusted to be in parallel through the adjusting rod. The steel strand passed through the anchoring component and then was anchored on the anchoring joint. The prestress was applied to make the joint self-locked. The installation of external prestressed reinforcement system and the H type anchoring joint is shown in Figure 9.

Figure 9.

Installation of external prestressing reinforcement system and the H type anchoring joint: (a) installation of external prestressing reinforcement system, (b) H type anchoring joint, (c) front view, (d) side view, and (e) local view.

Application of load

The prestress was applied through the struts. The photographs of the strut are shown in Figure 10. In the loading process, firstly the nuts on the end of the middle screw rod were loosened, and then the square loading blocks at the middle screw rod of both struts were rotated simultaneously to elongate the middle screw rod to apply the static loading. After the loading process was completed, the nuts at the middle screw rod were tightened to fix the prestress. And at the beginning of loading, the upper triangle friction component generated upward force to the upper flange, meanwhile, caused a slight slip to the left. And the lower triangle friction component generated downward force to the lower flange, and it also caused a slight slip to the right (as shown in Figure 9(c)). As the loading increases, the whole device can be self-locked and stable, and friction components will not slip.

Figure 10.

The photographs of the strut.

Data acquisition

The strain was measured at seven key locations as follows: the first position was located at north of upper flange, the second position was located at north of lower flange, the third position was located at south flange of end column, the fourth position was located at lower flange of middle span, the fifth position was located at south of upper flange, the sixth position was located at south of lower flange, the seventh position was located at north flange of middle column. The data were collected from the monitored locations every 5 min, as shown in Table 1 (Some abnormal data were discarded).

Table 1.

Strain comparison (Unit: 10^–6).

Loadingstep	The first position	The second position	The third position	The fourth position	The fifth position	The sixth position	The seventh position
1	−6.9	10.55	16.2	−5.9	−3.7	19.85	9.5
2	−9.95	31.2	32	−18.5	−16.6	29.5	21
3	−13.8	40.55	51	−28.8	−20.25	40.21	38
4	−20.6	48.1	62.2	−55.45	−29.71	49.61	60.5
5	−28.41	50.95	87.8	−76.05	−38.3	68.91	76.2

As shown in Table 2, the axial force of the strand was acquired with the increment step size of 8 kN. Between the recordings, the loading was stopped for 3 min. At the same time, the prestress loss was recorded. After 24 h, the stress of the steel strand was measured to be 38.6 kN by the stress ring at both ends of the strand (model: 2D001V, range: 0–220 kN). And the system achieved stable status and the prestress loss of the strand was approximately 4.00%.

Table 2.

Comparison of prestress loss.

Loading step	1	2	3	4	5
Initial value (Kn)	8	16	24	32	40
Stable value (kN)	7.6	14.2	22.8	31.9	38.7

Simulation analyses

Modeling

The Midas Gen FEM simulation software was used to establish a reinforcement model for the portal frame. A truss of the frame was selected for the calculation and analysis. The beam element was selected for steel beam, and then the 3D model was meshed. The column and the beam of the portal frame were rigid connected. The constant load and the dead weight were considered to apply to the upper flange of the steel beam in the form of uniformly distributed load. The FEM model of the frame is shown in Figure 11.

Figure 11.

The FEM model.

Simulation results

Comparison of strain

Using the finite element model described in 4.1, the loading process of the prestress in the loading range of 0.2 kN to 40 kN with the stepwise of 8 kN on the steel strand was simulated. The variation of strain at the key locations in the loading process was obtained from the simulation analysis. The experimental results and the simulation results of the strain in the beam near the anchoring joint were compared, as shown in Figure 12. From Figure 12, as the prestress gradually increased, the strain near the joint gradually increased. With the change of prestress, the strain from both the simulation and the experiment results had a consistent trend.

Figure 12.

Strain comparison: (a) strain comparison of the north of lower flange (the second position) and (b) strain comparison of the south of lower flange (the sixth position).

After the prestressed loading was completed, the experimental and simulation results on the strain were compared at each key location of the steel frame, as shown in Figure 13.

Figure 13.

Comparison of strain from experimental test and simulation.

From Figure 13, the deviation of the strain experimental and simulation results was within 25% at the first to the third positions and the fifth to the seventh position is within 25% while the deviation was 38.1% at the fourth position. The large deviation at the fourth position might be due to the reasons including inaccurate paste position of strain gauge, large resistance caused by long wire, slight vibration of steel beam during construction, etc. From Figure 13, the strain from the simulation analysis was mostly consistent with the experimental results, proving that the anchoring joint had high reinforcement effectiveness and FEM mode was feasible to simulate the structure.

Comparison of deflection

The mid-span deflection was measured by a dialgauge in the test. In Figure 6, a wire was first attached to the middle span of the beam, and then a dialgage with a heavy hammer installed on upper was connected at the bottom of the wire. The dialgage was mounted on a CZ-6A magnetic base which is fixed on the ground. When the beam deflection changed, the wire pulled the dialgage to obtain the measured deflection value.

When the loading was increased from zero to 40 kN, the deflection was changed by 14 mm. The calculation results from Midas Gen showed that the mid-span deflection increased from -7 mm to 3 mm during the entire loading process, with a total change of 10 mm. Compared to the value before reinforcement, the mid-span deflection decreased about 28%. The simulated results were in good agreement with the measured results, and the predicted reinforcement effect was achieved using the device. The results fully verified the effectiveness of the anchoring joint in the external prestressing reinforcement of the portal frame.

From the comparison of experimental and analysis results, it was shown that they were basically the same, but some results differ greatly. The reason is that there are some the bending components and knee-bracings in the original structure. But these components are not considered in the finite element simulation. And the paste position and angle of strain gauge are not very accurate and slight vibration of steel beam also has influence in the test.

Analysis of reinforcement effect

Using the FEM established in section 4.1, the reinforcement effect was analyzed under different combinations of load. The load conditions were as follows:

Load condition 1: 1.35 D (dead load) + 0.98 L (live load);

Load condition 2: 1.2 D (dead load) + 1.4 L (live load) + 0.84 W (wind load);

The standard value of the dead load was 0.20 kN/m², the standard value of the live load was 0.30 kN/m², and the standard value of the wind load was 0.30 kN/m². The roughness of ground was class B (GB50009-2012, 2012), and the value of prestress in the steel strand was consistent with that in the experiment.

In order to verify the effectiveness of external prestressing reinforcement of the portal frame using the designed joint, the bending moment and shear force before and after the reinforcement of the beam were analyzed.

Analysis of the bending moment

The distribution of the bending moment before and after reinforcement was shown in Figures 14 and 15, respectively. From the figures, the bending moment of the beam near the anchoring joint (labeled as 1 and 2 in the figure) exhibited a sudden change, which indicated that the additional bending moment by the prestress can effectively reduce the negative bending moment at the end of the beam.

Figure 14.

Comparison of reinforcement effect (the load condition 1): (a) before reinforcement and (b) after reinforcement.

Figure 15.

Comparison of reinforcement effect (the load condition 2): (a) before reinforcement and (b) after reinforcement.

The bending moments before and after reinforcement were compared at the four locations with large bending moments, including the top of end column, the middle of span, the south end of beam, and the top of middle column, as shown in Figures 16 and 17.

Figure 16.

Comparison of bending moments before and after reinforcement (the load condition 1).

Figure 17.

Comparison of bending moments before and after reinforcement (the load condition 2).

From the results under both load conditions, the bending moment under the load condition 1 was larger than that under the load condition 2. After the prestress was applied, the bending moments at the four locations, that is, the top of the end column, the middle span, the south of the beam end, and the top of the middle column were all decreased to the original structure. After the reinforcement, although the bending moments at some locations, such as the south end of the beam and the top of the middle column, were increased under the load condition 2, these results were smaller than the value of the original structure under the load condition 1.

In summary, under both load conditions, after the external prestressing was applied in the reinforcement of the steel beam, the bending moment at each tested location of the steel beam was decreased relatively to the original structure, thus the bearing capacity of the steel beam was further improved.

Analysis of shear force

The distribution of the shear force before and after reinforcement is shown in Figures 18 and 19, respectively. The shear force value of six locations, that is, top of end column, south end of beam, south beam ends near the anchoring joint, north beam ends near the anchoring joint, north end of beam and top of middle column, were shown in Figures 20 and 21.

Figure 18.

Comparison of reinforcement effect (the load condition 1): (a) before reinforcement and (b) after reinforcement.

Figure 19.

Comparison of reinforcement effect (the load condition 2): (a) before reinforcement and (b) after reinforcement.

Figure 20.

Comparison of shear force before and after reinforcement (the load condition 1).

Figure 21.

Comparison of shear force before and after reinforcement (the load condition 2).

Based on the comparison of the shear forces before and under the reinforcement, the maximum shear force of the original structure was obtained at the south end of the beam under the load condition 1 and the maximum shear force was decreased after the application of the prestress. At some locations, such as north end of beam, the shear forces had a slight increase, but still remained far less than the maximum shear of the structure. The results indicated that the external prestressing method was effective to reinforce the portal frame.

Analysis of the lateral displacement

The lateral displacement of portal frame under earthquake was analyzed. According to the code (GB50011-2010, 2010), the design seismic grouping is 2, the seismic fortification intensity is 7 degrees (0.10 g) and the site classification is II. The damping ratio of the structure is 0.02, and the loading conditions are respectively:

Load condition 3 (the original structure of no additional load): 1.2 (D₁ + 0.5 L) + 1.3 E; Load condition 4 (the structure of additional load): 1.2 (D₂ +0.5 L) + 1.3 E; Load condition 5 (the structure under the external prestressing reinforcement): 1.2 (D₂ + 0.5 L) +1.3 E + P. And D₁ = 0.2 kN/m², D₂ = 0.2 kN/m² + 0.38 kN/m² = 0.58 kN/m², E is earthquake load, P is the prestressing.

Comparison of the lateral displacement of the portal frame under different load conditions was shown in Table 3. It can be seen that the lateral displacement of the portal frame under the load condition 5 was greatly reduced compared with that under the load condition 4, but slightly larger than that under the load condition 3. Therefore, the application of the external prestressing can reduce the influence of additional load on the lateral displacement. The feasibility of external prestressing reinforcement technique is further demonstrated.

Table 3.

Lateral displacement of portal frame under different load conditions (Unit: mm).

Load condition	Top of end column	Top of middle column
Load condition 3	9.936	9.911
Load condition 4	19.138	19.067
Load condition 5	10.593	10.009

Conclusion

In this paper, an assembled H type anchoring joint was designed for the external prestressing reinforcement. The designed anchoring joint was applied to the prestressed loading test of a portal frame. Midas Gen FEM software was used to simulate and analyze the reinforcement effect of the anchoring joint, and the simulation results were compared with the experimental results. The main conclusions are as follows:

1) After the external prestressing reinforcement was applied with the H type anchoring joint, the bending moment decreases by about 40%, and the mid-span deflection decreased by about 28%. The results showed that using the proposed anchoring joint, the stiffness of the structure was increased and the bearing capacity was increased. Thus the desired reinforcement effect can be achieved.

2) No slippage phenomenon was observed on the tests of the designed H type anchoring joints. In external prestressing reinforcement, the H type anchoring joint can effectively transfer the prestress and work as anchorage.

3) The results of FEM analysis were mostly consistent with the experimental results. In addition, the stress, strain, and mid-span deflection at each key location were within the allowable range, which demonstrated the anchoring joint had high effectiveness in external prestressing reinforcement.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The project was supported by Yanshan University Dr. Fund (No. BL17027).

ORCID iD

Cheng Shuliang

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