# Seismic Behavior of a Novel Blind Bolted Flush End-Plate Connection to Strengthened Concrete-Filled Steel Tube Columns

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Experimental Investigation

#### 2.1. Test Specimen

_{p}is the end-plate thickness; for specimens ST-1 and ST-3, t

_{p}= 12 mm; for specimens ST-2 and ST-4, t

_{p}= 24 mm; specimens ST-4 and ST-5 have the same dimension, except ST-4 with nut and ST-5 without nut.

#### 2.2. Material Properties

_{s}), the yield stress (f

_{y}), the ultimate stress (f

_{u}), and the elongation at fracture (δ). A summary of the results of the material tests has been presented in Table 2. Moreover, for the grade 8.8 blind bolts, the nominal values of f

_{y}and f

_{u}are respectively 640 MPa and 800 MPa, and the elastic modulus is 2.1 × 10

^{5}MPa. Commercially available SCC was employed to fill all the test specimen columns, and standard compressive concrete cubes were cast simultaneously as test samples. The compressive strength and elasticity modulus of the concrete cubes were tested on the testing day. The concrete cube compressive strength was 39.92 N/mm

^{2}, and the elastic modulus was 3.27 × 10

^{4}N/mm

^{2}.

#### 2.3. Experimental Test Setup and Loading Procedure

_{u}, where N is the axial load applied to the column and N

_{u}is the nominal ultimate axial compressive load of the CFST column predicted by the specification [19]. The distance from the loading point on the beam to the column flange was 1.2 m. The cyclic load is controlled through the layer angular displacement, which is the ratio of the horizontal displacement of the beam end to the loading point distance, which was 1.2 m, as mentioned above. The loading procedure suggested by the SAC Joint Venture (1997) [20] for cyclic testing was adopted and is summarized in Figure 4.

#### 2.4. Instrumentation

_{c}). LVDT3 and LVDT4 were adopted for monitoring the rotation of the beam (θ

_{b}), which were 30 mm away from column flange in the specimens with a strong beam and 180 mm away from column flange in the specimen with a weak beam, considering local beam buckling. LVDT5 was adopted for monitoring the displacement of the loading point at the beam end. The relative rotation between the column flange and beam can be derived from Equations (1)–(3).

_{1}, u

_{2}, u

_{3}, and u

_{4}are the corresponding displacements measured by LVDT1-LVDT4; and H is 350 mm for specimens ST-1~ST-5 and 180 mm for specimen ST-6, which is the horizontal distance between LVDT3 and LVDT4.

## 3. Results

## 4. Discussion

#### 4.1. Moment–Rotation Hysteresis Curves

#### 4.2. Moment–Rotation Envelope Curves

_{y}) and yield rotation (θ

_{y}), respectively. The moment and rotation at the peak point are the measured flexural resistance (M

_{max}) and corresponding connection rotation (θ

_{max}), respectively. According to Chinese specification JGJ3-2010 [22], the moment equal to 0.85M

_{max}is defined as the moment (M

_{f}) at failure, and θ

_{f}is the corresponding connection rotation used to represent the rotation ability of a connection in this study. The main results and key parameters of the specimens are summarized in Table 4.

#### 4.3. Classification of the Tested Connections

_{b}I

_{b}/L

_{b}, where E

_{b}, I

_{b}, and L

_{b}are the elastic modulus, second moment of area, and span of the steel beam, respectively. A rigid connection has an initial stiffness greater than 25EI

_{b}/L

_{b}for a non-braced frame and 8E

_{b}I

_{b}/L

_{b}for a braced frame. A connection with an initial stiffness between these two thresholds is classified as semi-rigid. A connection can be classified as full strength, partial strength, and nominally pinned according to the connection strength. The maximum required connection resistance for nominally pinned connection design is 0.25M

_{bp}, and the full-strength connection resistance is greater than M

_{bp}, where M

_{bp}represents the design plastic flexural resistance of the steel beam. If the flexural resistance is located between the two thresholds, the connection is partial strength. The classification of tested connections is illustrated in Figure 13. Most blind bolted flush end-plate connections to CFST columns are classified as semi-rigid and partial strength; however, specimen ST-6, with a weak beam, is classified as semi-rigid and full-strength.

#### 4.4. Rotation Ability and Ductility Ratio

_{e}] of 0.004 rad and an elastic-plastic layer angular displacement [θ

_{p}] of 0.02 rad. As is shown in Table 4, for all tested joints, the yield connection rotation (θ

_{y}) is 1.55–3.13 times [θ

_{e}], and the failure connection rotation (θ

_{f}) is 1.62–5.13 times [θ

_{p}]. To satisfy the ductility requirement in seismic design, FEMA 350 [25] suggests a ductility limit of 0.03 rad. It is indicated by comparison that the anchored blind bolted connections show excellent rotation ability, satisfying the specified earthquake design requirements in both GB50011-2010 and FEMA 350.

_{θ}) is adopted to define the ductility of the joint as follows:

_{f}is the failure connection rotation and θ

_{y}is the yield connection rotation.

#### 4.5. Energy Dissipation

_{total}), the equivalent viscous damping coefficient (ξ

_{e}), and the dissipated energy capability (E

_{e}). The total dissipation energy (W

_{total}) is the cumulative dissipation energy (W) described as a function of the drift ratio, where W is the area of the P-Δ hysteresis curve at a certain cycle time. The dissipated energy capability (E

_{e}) and the equivalent viscous damping coefficient (ξ

_{e}) are calculated from Equations (5) and (6). Figure 14 describes the idealized load versus deflection relationship, where S

_{ABC}and S

_{CDA}refer to the upper half area and lower half area of the hysteresis loop, respectively, and S

_{OBE}and S

_{ODF}refer to the corresponding triangular areas.

_{e}), the equivalent viscous damping coefficient (ξ

_{e}), and the total dissipation energy (W

_{total}) in the limit state. Figure 15 and Figure 16 exhibit the equivalent viscous damping coefficients (ξ

_{e}) of each hysteresis loop versus the drift ratio and the total dissipation energy (W

_{total}) related to the drift ratio, respectively. These experimental results show:

- (1)
- The specimens that failed in mode I (ST-1, ST-2, and ST-5) have a better energy dissipation performance than the specimens that failed in mode II (ST-3, ST-4, and ST-6).
- (2)
- Among the specimens that failed in mode II, the energy dissipation capacity of specimen ST-3, with a thinner end-plate, is the best; a reduction of the steel beam section helps to increase the energy dissipation capacity of joints.
- (3)
- An increasing end-plate thickness results in a reduction in the equivalent viscous damping coefficient (ξ
_{e}) for those specimens with failure mode II. The end-plate thickness has little influence on the energy dissipation capacity for the connections with failure mode I. Those connections mainly dissipate energy through CFST columns. - (4)
- The influence of the steel tube thickness on the equivalent viscous damping coefficient (ξ
_{e}) is related to the end-plate thickness. For the joints with thinner end-plates, the thickness of the steel tube had little effect on the equivalent viscous damping coefficient (ξ_{e}), which can be attributed to the fact that the thinner end-plate played a major role in energy dissipation. For the joints with thicker end-plates, the effective viscous damping coefficient (ξ_{e}) increased with a decreasing steel tube thickness. This result suggests that the CFST columns dominated the energy dissipation.

## 5. The Finite Element Model

#### 5.1. Finite Element Modeling

#### 5.1.1. Material Models

#### 5.1.2. Finite Element Modeling

#### 5.2. Validation

## 6. Research on the Initial Stiffness of Connection Based on the Component Method

#### 6.1. A Mechanical Model of the Initial Stiffness

_{eff, 1}and k

_{eff, 2}are the effective stiffnesses of the first bolt row and second bolt row, respectively, and they consist of four springs in series, representing the following stiffnesses: the end-plate in bending (k

_{1}), the bolt in tension (k

_{2}), the concrete in compression (k

_{3}), and the steel tube wall in bending (k

_{4}), as shown in Figure 22b. Therefore, k

_{5}and k

_{6}are the stiffnesses of the column wall in compression and concrete in compression, respectively. The CFST column is adequately rigid, therefore k

_{5}and k

_{6}are assumed to be infinite and neglected. These components are discussed further in the next section.

#### 6.2. Stiffness Calculation of the End-Plate in Bending and the Bolt in Tension

_{s}is the elastic modulus of steel; l

_{eff}is the effective length of the T-stub; t

_{p}is the end-plate thickness; m is the distance between the centre of the bolt hole and the intersection of stem and end-plate; A

_{s}is the effective tensile area of the bolt; L

_{bo}is the effective bolt length, given by L

_{bo}= t

_{c}+ t

_{p}+ d

_{emb}+ (t

_{w}+ t

_{nut})/2, where t

_{c}is the steel tube wall thicknesses; and t

_{w}and t

_{nut}are the thicknesses of the blind bolt washer and anchor nut, respectively.

#### 6.3. Stiffness Calculation of the Concrete in Compression and the Steel Tube Wall in Bending

#### 6.3.1. Stiffness Calculation of the Concrete in Compression

_{emb}is the embedded depth of the blind bolt, E

_{c}is the elastic modulus of concrete, A

_{x}is the compressive area of the concrete as a function of x, d

_{nut}is the diameter of the anchor nut, d

_{b}is the effective diameter of the bolt, and θ

_{cc}is the angle between the inclined plane and the horizontal plane of the cone. The parameters are shown in Figure 25.

#### 6.3.2. Stiffness Calculation of the Steel Tube Wall in Bending

_{s}is the elastic modulus of steel, L

_{e}is the distance between the fixed axis of the two ends of the steel tube wall, t

_{c}is the steel tube thickness, α is the ratio of the diameter of the bolt head (c) to L

_{e}, β is the ratio of the distance between the outer diameter of the bolt head to L

_{e}, and µ is the ratio of L

_{e}to the steel tube wall thickness (t

_{c}). The proposed value of φ is 35-10 β (refer to Neves et al. [29]), and λ

_{1}and λ

_{2}are 1.5 and 1.6, respectively.

#### 6.4. The Calculation Results of Initial Stiffness

_{eff r}of bolt row r can be derived from

_{i,r}is the stiffness for the i-th component corresponding to bolt row r.

_{eq}replaces components related to the first bolt row and second bolt row. The following expression can be derived for k

_{eq}.

_{r}defines the location of bolt row r with respect to the assumed center of rotation.

_{eq}is derived from

_{i}) can be calculated from Equation (16):

## 7. Conclusions

- (1)
- The observed results demonstrated that the application of the modified anchored blind bolts (Hollo-Bolt) and a locally strengthened steel tube column in the panel zone can effectively avoid the premature anchorage failure and CFST column damage for the bolted connections to square CFST columns in the moment-resisting frames. No obvious damage was observed for the embedded extensions of the blind bolts and C-shaped channels, providing a stable and sustained effect on the joint performance during cyclic loading. The proposed connection demonstrated its ability to reach the ultimate strength of the blind bolts and significantly improved the performance of the connections in terms of the flexural resistance, stiffness and stiffness degradation.
- (2)
- Two representative failure modes were presented in the test for the anchored blind bolted connections to square CFST columns. The tube wall thickness and blind bolt anchorage method mainly determined the failure mode, and the end-plate thickness and steel beam section had an influence on joint deformation. Connections that failed in mode II exhibited higher flexural resistances and stiffnesses, while better rotation ability, ductility and energy dissipation capability were observed for connections that failed in mode I. For connections that failed in mode I, the good deformation capacity is mainly achieved by the deformation of the steel tube walls and the crushing of concrete in the columns. For connections that failed in mode II, using thin end-plates and a weak steel beam could produce a good deformation capacity, avoiding premature brittle fracture of the connections while maintaining a high flexural resistance.
- (3)
- Due to the change of failure mode, the use of locally strengthened steel tube can not only enhance the initial stiffness and strength of a connection but also reduce the counterpart rotation capacity and ductility. The use of thin end-plate and weak beam can effectively enhance the hysteretic behavior, ductility, and energy dissipation capacity of joints. The change in anchorage method has a great influence on the flexural capacity but has little effect on the stiffness.
- (4)
- The rotation capacity and the ductility of these anchored blind bolted joints can satisfy the requirements for structural earthquake-resistant design in most seismic areas. In accordance with EC3 Part 1-8, most blind bolted flush end-plate connections to CFST columns were classified as semi-rigid and partial strength, except specimen ST-6 with a weak beam, which shows semi-rigid and full-strength behavior.
- (5)
- An FE model of these anchored blind bolted joints was also developed in this study using ABAQUS/Standard module. It is shown that the model could simulate the connection behavior successfully, particularly predicting the stiffness of the anchored blind bolted joints obtained in the early stage with sufficient precision. According to the mechanical behavior of the connection components, the component model of such a joint is proposed, and the initial stiffness expression is established. The expression was verified by the experimental results and the results obtained from numerous parametric FE models, demonstrating the reliability of the initial stiffness expression.

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 1.**Details of joint specimens (unit: mm). (

**a**) Specimens ST-1 and ST-2; (

**b**) specimens ST-3 and ST-4; (

**c**) specimen ST-6; (

**d**) end-plate.

**Figure 6.**Failure mode I (specimens ST-1, ST-2, and ST-5). (

**a**) ST-1; (

**b**) ST-2; (

**c**) ST-5; (

**d**) tube wall in specimen ST-1; (

**e**) core concrete in specimen ST-2.

**Figure 7.**Failure mode II (specimens ST-3, ST-4, and ST-6). (

**a**) ST-3; (

**b**) ST-4; (

**c**) ST-6; (

**d**) steel beam in specimen ST-6; (

**e**) fractured blind bolts; (

**f**) tube wall in specimen ST-3; (

**g**) core concrete in specimen ST-3.

**Figure 11.**Moment (M)–rotation(θ) envelope curves. (

**a**,

**b**) Effect of steel tube thickness; (

**c**,

**d**) effect of end-plate thickness; (

**e)**effect of anchorage method; (

**f**) effect of beam section.

**Figure 19.**Comparison of test and finite element moment–rotation curves. (

**a**) ST-1; (

**b**) ST-2; (

**c**) ST-3; (

**d**) ST-4; (

**e**) ST-5; (

**f**) ST-6.

**Figure 20.**Observed and predicted failure modes. (

**a**) The flexural deformation of the thin end-plate in specimen ST-1. (

**b**) The outward deformation of the thick end-plate in specimen ST-4. (

**c**) The local buckling at the root of the steel beam in specimen ST-6. (

**d**) The fractured bolt shank in specimen ST-6 (bending and shearing fracture).

**Figure 22.**Mechanical model of initial stiffness of connection. (

**a**) Component model for the joint; (

**b**) component model for the effective stiffness.

Specimen | Column Section (mm) | Beam Section (mm) | End-Plate Thickness (mm) | C-Shaped Channel Reinforcement | Anchor Nut |
---|---|---|---|---|---|

ST-1 | 250 × 250 × 5 | HN350 × 175 × 7 × 11 | 12 | No | Yes |

ST-2 | 250 × 250 × 5 | HN350 × 175 × 7 × 11 | 24 | No | Yes |

ST-3 | 250 × 250 × 5 | HN350 × 175 × 7 × 11 | 12 | Yes | Yes |

ST-4 | 250 × 250 × 5 | HN350 × 175 × 7 × 11 | 24 | Yes | Yes |

ST-5 | 250 × 250 × 5 | HN350 × 175 × 7 × 11 | 24 | Yes | No |

ST-6 | 250 × 250 × 5 | HN300 × 150 × 6 × 9 | 24 | Yes | Yes |

Steel Coupons | Steel Wall Thickness(mm) | E_{s} (GPa) | f_{y} (MPa) | f_{u} (MPa) | δ (%) |
---|---|---|---|---|---|

End plate-1 | 12 | 197.29 | 399.2 | 562.8 | 25.7 |

End plate-2 | 24 | 198.26 | 408.5 | 534.5 | 28.7 |

C-shaped channel | 7 | 192.29 | 395.2 | 567.8 | 27.7 |

Steel tube | 5 | 208.75 | 477.1 | 609.2 | 21.3 |

Beam web-1 | 7 | 202.23 | 412.6 | 593.6 | 24.83 |

Beam web-2 | 6 | 192.05 | 254.1 | 397.0 | 30.0 |

Beam flange-1 | 11 | 202.02 | 412.4 | 591.5 | 18.6 |

Beam flange-2 | 9 | 197.41 | 269.7 | 405.7 | 26.6 |

Component | Failure Mode I (Specimen ST-1, ST-2, and ST-5) | Failure Mode II (Specimen ST-3, ST-4, and ST-6) |
---|---|---|

Anchored blind bolt | Partial pull-out | Bolt rupture |

Column wall | Bulging deformation | Slight bulging around the bolt holes |

Core concrete | Severely damaged | Local concrete crushing |

Steel beam | No obvious local beam buckling | No obvious local beam buckling except ST-6 |

Specimen | K_{i} (kN·m/mrad) | M_{y} (kN·m) | θ_{y} (mrad) | M_{max} (kN·m) | θ_{max} (mrad) | M_{f} (kN·m) | θ_{f} (mrad) | μ |
---|---|---|---|---|---|---|---|---|

ST-1 | 10.83 | 56.99 | 9.0 | 70.24 | 14.4 | 59.70 | 62.6 | 6.96 |

ST-2 | 15.42 | 63.97 | 6.2 | 78.04 | 69.3 | 66.33 | 102.6 | 16.55 |

ST-3 | 15.21 | 80.12 | 10.5 | 106.34 | 49.2 | 90.39 | 54.3 | 5.17 |

ST-4 | 19.18 | 118.57 | 10.4 | 143.84 | 29.3 | 138.05 | 32.4 | 3.12 |

ST-5 | 17.77 | 90.38 | 9.2 | 118.64 | 88.8 | 113.08 | 98.5 | 10.71 |

ST-6 | 17.57 | 95.19 | 12.5 | 116.02 | 37.3 | 111.17 | 47.5 | 3.80 |

Specimen | E_{e} | ξ_{e} | W_{total} (kN·m) |
---|---|---|---|

ST-1 | 1.263 | 0.201 | 23.74 |

ST-2 | 1.318 | 0.210 | 39.80 |

ST-3 | 1.042 | 0.166 | 15.57 |

ST-4 | 0.540 | 0.086 | 5.51 |

ST-5 | 0.830 | 0.132 | 38.27 |

ST-6 | 0.770 | 0.123 | 11.86 |

Specimen | M_{y} (kN·m) | θ_{y} (mrad) | K_{i} (kN·m/mrad) | K_{FE}/K_{test} | |
---|---|---|---|---|---|

ST-1 | Test | 57.00 | 9.0 | 10.83 | 1.10 |

Model | 65.87 | 10.5 | 11.96 | ||

ST-2 | Test | 63.97 | 6.2 | 15.42 | 1.08 |

Model | 71.26 | 7.9 | 16.70 | ||

ST-3 | Test | 80.12 | 10.5 | 15.21 | 0.83 |

Model | 82.53 | 9.9 | 12.56 | ||

ST-4 | Test | 118.57 | 10.4 | 19.18 | 1.07 |

Model | 119.89 | 9.8 | 20.60 | ||

ST-5 | Test | 90.38 | 9.2 | 17.77 | 1.10 |

Model | 99.89 | 10.7 | 19.62 | ||

ST-6 | Test | 95.19 | 12.5 | 17.57 | 1.09 |

Model | 96.02 | 10.8 | 19.20 |

**Table 7.**Comparison of the component model to the test results and corresponding FE analysis results.

Specimen | Component Model K_{theory} | Test K_{test} | Ratio K_{theory}/K_{test} | FE Model K_{FE} | Ratio K_{theory}/K_{FE} |
---|---|---|---|---|---|

ST-1 | 9.90 | 10.83 | 0.91 | 11.96 | 0.83 |

ST-2 | 14.73 | 15.42 | 0.95 | 16.70 | 0.88 |

ST-3 | 12.15 | 15.21 | 0.80 | 12.56 | 0.97 |

ST-4 | 20.33 | 19.18 | 1.06 | 20.60 | 0.98 |

ST-5 | 19.99 | 17.77 | 1.12 | 19.62 | 1.02 |

ST-6 | 20.32 | 17.57 | 1.16 | 19.20 | 1.06 |

**Table 8.**Comparison between the component method and FE analysis results for overall connection stiffness.

Specimen | Tube Thickness (mm) | Endplate Thickness (mm) | Vertical Bolt Distance (mm) | Bolt Diameter (mm) | K_{theory} | K_{FE} | Ratio K_{theory}/K_{FE} |
---|---|---|---|---|---|---|---|

M-1 | 5 | 12 | 100 | 16 | 9.90 | 11.96 | 0.83 |

M-2 | 8 | 12 | 100 | 16 | 12.14 | 13.75 | 0.88 |

M-3 | 12 | 12 | 100 | 16 | 12.15 | 12.56 | 0.97 |

M-4 | 5 | 25 | 100 | 16 | 14.73 | 16.70 | 0.88 |

M-5 | 8 | 25 | 100 | 16 | 20.28 | 20.59 | 0.99 |

M-6 | 12 | 25 | 100 | 16 | 20.32 | 20.60 | 0.98 |

M-7 | 12 | 8 | 100 | 16 | 5.66 | 6.37 | 0.89 |

M-8 | 12 | 16 | 100 | 16 | 16.69 | 16.29 | 1.02 |

M-9 | 12 | 20 | 100 | 16 | 19.07 | 18.78 | 1.02 |

M-10 | 12 | 12 | 80 | 16 | 10.30 | 10.60 | 0.97 |

M-11 | 12 | 12 | 120 | 16 | 14.20 | 16.09 | 0.88 |

M-12 | 12 | 25 | 80 | 16 | 17.24 | 17.99 | 0.96 |

M-13 | 12 | 25 | 120 | 16 | 23.75 | 24.72 | 0.96 |

M-14 | 12 | 12 | 100 | 18 | 12.87 | 13.79 | 0.93 |

M-15 | 12 | 12 | 100 | 20 | 13.64 | 16.09 | 0.85 |

M-16 | 12 | 25 | 100 | 18 | 22.64 | 21.89 | 1.03 |

M-17 | 12 | 25 | 100 | 20 | 25.44 | 23.03 | 1.10 |

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**MDPI and ACS Style**

Wang, Y.; Wang, Z.; Pan, J.; Wang, P.
Seismic Behavior of a Novel Blind Bolted Flush End-Plate Connection to Strengthened Concrete-Filled Steel Tube Columns. *Appl. Sci.* **2020**, *10*, 2517.
https://doi.org/10.3390/app10072517

**AMA Style**

Wang Y, Wang Z, Pan J, Wang P.
Seismic Behavior of a Novel Blind Bolted Flush End-Plate Connection to Strengthened Concrete-Filled Steel Tube Columns. *Applied Sciences*. 2020; 10(7):2517.
https://doi.org/10.3390/app10072517

**Chicago/Turabian Style**

Wang, Yihuan, Zhan Wang, Jianrong Pan, and Peng Wang.
2020. "Seismic Behavior of a Novel Blind Bolted Flush End-Plate Connection to Strengthened Concrete-Filled Steel Tube Columns" *Applied Sciences* 10, no. 7: 2517.
https://doi.org/10.3390/app10072517