Mechanical Performance and Failure of the Braced Frame Joint in Tower Cranes

The standard section of the tower crane connects with the building by PESC, the most commonly used BFJ is shown in Figure 2A. The vertical load of PESC and concrete in the joints include the dead weight of the tower crane, lifting weight, and the weight of concrete counterbalance. The horizontal load includes wind load, anti-overturning moment, and anti-sliding load. The seismic load, the non-uniform motion of cable and trolley, and the rotation of the crane arm in the horizontal plane will lead to dynamic actions at the joints. In engineering, the force of the tower crane joints is complex, and its direction is also uncertain. The simplified BFJ and test device are shown in Figure 2.

Three steel beams are used in the test device. Steel beam A (SBA) aims to provide drawing force. The loading actuator protrudes from the hole in the middle of SBA. During the loading process, the hydraulic jack transmits the thrust to SBA. Steel beam B (SBB) and steel beam C (SBC) are used to the fixed specimen. The steel beam is provided with stiffeners to improve the rigidity and reduce the deformation of SBA, SBB, and SBC. During the loading process, the end of SBA also follows the upward displacement of the hydraulic jack, meaning SBA is slightly inclined. At the same time, the bottom of the SBA separates from the top of the hydraulic jack. To ensure contact between SBA and the hydraulic jack, a movable hinge is designed to ensure contact. During installation, the laser level is used to correct the position of components, which ensures that the center of the jack, the centerline of PESC, and the midpoint of the rotating hinge are in the same vertical plane. According to the Saint-venant principle, the local load effect only influences the stress field distribution within a certain range. Hence, the dimension of BFJ could be increased in number threefold with PESC. The dimensions of tested BFJ specimens are shown in Figure 3.

The mechanical performance of BFJ is mainly controlled by the PESC parameters. PESC is composed of five parts, including ear plate, stiffening plate, faceplate, anchor, and endplate, as shown in Figure 3. The PESC is an axisymmetric structure, therefore displacement and extreme stress in key locations can reflect mechanical performance. Two strain gages (strain gage U and strain gage D) are placed on anchor one and anchor four respectively.

2 Experimental Setup

2.1 Testing Device
The experiment was completed in the Structural Laboratory of Chongqing University. The quantitative value of load added on the BFJ is measured with Digital Strain Gauge, and the quantitative value of crack is measured with Crack Width Observer, as shown in Figure 4. The detailed parameters and types of main test instruments are shown in Table 1.

3.2 Sensor Placement

To study the mechanical properties and failure mechanism of the BFJ, the data measured include tension, displacement of PESC, and the anchor plate strain of PESC. The strain gauges are arranged on the end of the anchor, the measurement points are shown in Figure 5.

The pressure sensor is placed between nut and SBA, and hence the measured load value is the exact quantitative value applied on the PESC. It could directly measure the load quantitative value of the hydraulic jack, avoiding the amplified error. Considering the deformation of SBA, SBB, and SBC, measured points are directly installed on the upper side and lower side welded rebar of the PESC and named measured point I and measured point II.

3.3.3 Steel Plate Performance Testing
The PESC is made of Q345C, and the nominal yield strength is 345 MPa. The standard metal plates are made of the same steel, the tensile test is carried out on the INSTRON 1342 static test-machine, and the yield strength, ultimate strength, and elastic modulus are measured and shown in Table 4.

3.4 Loading Scheme and Test Result
A stepwise monotonic static loading project is taken in the test. Preloading is carried out before formal loading and all the test instruments are determined to work normally and the device is reliable. The preloaded data is used to judge the relationship between load and deformation, and the test piece and the support are in contact. After preloading, unloading to 0, and then starting the formal loading, the cracking point, yield point, and limit point are captured during the loading process, the differential is adjusted downward around the point locations, and the load of each level lasts 1.0 min.

The mechanic behavior of BFJ is closely associated with crack propagation. The destruction mechanism and failure mode can be reflected by crack propagation. The critical points and corresponding crack states are recorded and shown in Figure 8. The full load–strain relationships of BFJ are shown in Figure 9.

The cracking load is 141.0kN, with a 2.6 mm tensile crack at the bottom of PESC. When the load is 184.0kN, two tensile cracks appeared with 2.0 and 1.6 mm in width, as shown in Figure 8A. When the load is 282.0kN, shear cracks appear at the bottom center of PESC. The concrete at the bottom of PESC is displacement damaged with lateral uplift, as shown in Figure 8B. The upper corner of the PESC produced an oblique crack with 2.4 mm in width when the load is 361.0 kN, as shown in Figure 8C. When the BFJ is finally destroyed, there are two main tensile cracks with 10.8 and 11.2 mm, and the largest main shear crack is 10.6 mm. Figure 8D indicates that there is no plastic deformation in PESC, and hence the failure of BFJ is determined by concrete. Cracks of specimens concentrate mainly on the central and bottom of PESC. Moreover, tensile shear crack concentrates mainly on a 45° angle, as shown in Figure 8E. The extreme values in strain gage U and D are 1,524 and 711 respectively, as shown in Figure 9. The PESC deformation in the anchor end has an obvious difference from the tension. Both of the strains dropped after the final destruction of BFJ, and hence PESC deformation is elastic deformation without destruction.

4 Numerical Analysis

4.1 Numerical Simulation
Numerical models with different working conditions are established to study the mechanical performance of BFJ. The detailed parameters of numerical models are given in Tables 5, 6.

4.2 Comparison Between Model and Experiment
The cross-comparison between test and simulation was taken to verify the correctness and reliability of the numerical simulation, as shown in Figure 11.

Figure 12 shows that the load-displacement relationships could generally be divided into three stages, including the elastic stage, crack stage, and failure stage. During the initial loading period, the relationships show a linear relationship with a small displacement. It is speculated that concrete and the PESCs only produced elastic deformations, and hence the initial stiffness of each specimen is relatively similar. With the increase of displacements, track transition relationships occur during the crack stage. Combining the results of the tests, it can be speculated that the concrete is cracked and the stiffness is affected. The load-displacement relationships have a significant decline phase when the specimen is pulled out. The slope of the relationships becomes steeper with increasing CSG in the failure stage, and the brittle failure phenomenon becomes more obvious. The reason for this is the constitutive relation of the concrete material and the slip on the contact surface. The ultimate bearing capacity of C20–450, C30–450, C40–450, C50–450 and C60-450 were 341.1, 350.6, 355.1, 365.8, and 388.1 MPa respectively. The ultimate bearing capacity increase with increasing CSG. In the crack stage, each finite element model has an extreme value, combing with the experiment phenomenon, which has the characteristic of major cracks appearing. The characteristic crack values of C20–450, C30–450, C40–450, C50-450, and C60-450 are 252.4, 276.0, 291.2, 305.9, and 318.9 MPa respectively, and the characteristic crack values also increase with the increase of CSG.

To explore the destruction change in PESC and concrete components with ultimate bearing capacity, the stress and damage distribution of the specimens with different CSGs are shown in Figures, 13, 14.

Based on a comparison of the PESCs in Figures 13A–C, the stress distribution of different concrete grades of PESCs are similar, especially in the location of stress concentration. All three locations of stress concentrations are located on the interface of anchor and faceplate. The peak value is 555.9 MPa, 562.4.9 MPa, and 594.0 MPa. The stress in-ear plate and anchor are larger than the faceplate and endplate. With the action of transverse shear force, the anchor and concrete are mutually squeezed, thus stress on one side of the embed is concentrated. Other parts of the PESC are in the elastic stage, and the safety reservation is sufficient.

As shown in Figure 14, concrete crushing parts are always concentrated around the PESC. This is the result of the transverse shear action between PESC and concrete. The damage concentration area reduces with the increase of CSG. The failure modes of the specimens have similarities in Figure 14. A large number of cracks are generated on the concrete surface in a failure state, and concretes are cracked and damaged with severe tension. It is speculated that failure mode is controlled by concrete tensile strength. Ultimate bearing capacity increases with the increase of concrete.

Figure 15 indicates that the load-displacement relationships could generally be divided into three stages, including the elastic stage, crack stage, and failure stage. The initial stiffness of the specimens is close to different working conditions in the elastic stage. With increasing load, the specimens begin to yield. The specimens are destroyed and ultimate bearing capacity can be obtained in the failure stage. The ultimate bearing capacity of C30–900, C30–675, C30–450, C30–225 and C30-000 were 273.1, 301.6, 350.6, 363.1, and 375.7 MPa respectively. The ultimate bearing capacity has a great difference with changes of tension direction. In the crack stage, the characteristic crack values of C30–900, C30–675, C30–450, C30-225, and C30-000 are 223.7, 264.5, 276.0, 285.3, and 293.2 MPa respectively. Therefore, the tension direction has an obvious effect on the ultimate bearing capacity and characteristic crack value. When the tension direction changes to 0.0°, the concrete is subjected to tangential shear force, and compressive strength plays an important role. When the tension direction changes to 90.0°, the concrete is subjected to normal tension, and tensile strength plays an important role.

As can be seen from the numerical stress distribution of Figure 16, PESC with 0.0° tension direction has the largest deformation. There are two main locations of stress concentration in C30-000 PESC, at the top of the anchor and around the draw hole. Stress concentration is located at the top of the anchor in C30-450 PESC. Nevertheless, PESC with 90.0° tension direction is still in the elastic state when the specimen is destroyed. Stress on the anchor is larger than the ear plate and faceplate.

The numerical stress distribution outlined in Figure 17 indicates that the damaged area is largest in the C30-450 concrete component. Damage in the C30-450 concrete component is the second largest and the C30-000 concrete component is the smallest. The specimen shears mainly tangentially and the compression damage is mainly concentrated on the top of the anchor plate. The transition region from damage to undamaged has an obvious difference. The C30-450 concrete component has the largest transition region, and C30-000 concrete component has the smallest transition region. The concrete around the upper side of PESC is crushed by the shear force, which is the reason for the failure of the specimen with 0.0° tension direction. When the tension direction changes to 90.0°, the crushed area is gradually reduced, and the tensile area gradually increases. The tension direction can affect the final failure mode. The change of tension direction has a more obvious effect than CSG.

Conclusion

The results prove that braced frame joint cracks seriously with ultimate bearing capacity, simultaneously, the pre-embedded steel component (PESC) is pulled out. The failure mode is tensile and bending failure of BFJ in the test. The ultimate bearing capacity of failure is controlled by concrete strength grades. The tensile strength increases with the CSG, and the ultimate capacity of BFJ improves equally. The brittle fracture characteristic is more obvious in the ultimate state. The failure mode changes from crushing failure to tensile and bending failure with increasing tension angle.