Problems in the conventional calculation method At present, the calculation formula for solving the maximum load external tension of the bolts under the bending moment (pull and bend) of the high-strength bolt group in China is obtained according to the traditional design method, that is, one axis is selected as the neutralization. The shaft is then assumed to be linearly distributed along the direction of the bending moment. Two of these calculations are critical: 1) the determination of the position of the neutral axis; 2) the assumption of the distribution of the tensile force values ​​outside the load.
The end plates connected by the high-strength bolts undergo a certain degree of compression, bending and warping under the pre-tension of the bolts. The warping deformation determines that the end plates in a limited area around the bolts are close to the force. Then, under the action of bending moment, as the bending moment value increases, the close contact area of ​​the end plate around each bolt located in the bolt tension zone gradually decreases, and the close contact area of ​​the pressure receiving zone gradually increases, around the bolt The change of the area of ​​the close-fitting area is equivalent to the change of the compression stiffness of the end plate. Therefore, in order to maintain the force balance of the combined section of the bolt and the end plate during the stress process, the neutral axis will move away from the compression zone of the bolt group. The center of the bolt group, so the neutralization shaft is the center of the bolt group only assuming that the end plate around the bolt is close to the area of ​​the pressing area. On the other hand, according to the current specification, the tension of the high-strength bolt is controlled, and the neutral shaft is not moved to the side bolt. Therefore, the neutral axis of the high-strength bolt group under the bending moment is located somewhere between the center of the bolt group and the side row bolts.
In addition, assuming that the deformation of the bolt shaft linearly changes from the direction of the center end of the axial end plate, the axial force of the bolt also exhibits a linear change. The area of ​​the area surrounding the bolt is varied, that is, the compression stiffness of the end plate is changed, and the increase and decrease of the pressing force of the end plate exhibits a nonlinear change in the distance along the direction of the bending moment. Then, the bolt pull force formed by the shaft force of the bolt and the pressure reduction force of the end plate exhibits a nonlinear change with respect to the distance.
The relationship between the shaft augmentation force and the reduction force is the compression deformation ((a)), bending deformation ((b)) and warpage deformation ((c)) of the end plate around the bolt under the action of the high-strength bolt preload. The deformation of the actual middle end plate is a combined deformation of compression deformation, bending deformation and warping deformation ((d)). The warping deformation component in the end plate causes the end plates in a limited range around the bolt to closely adhere to each other, and It is not the entire section of the end plate that is close to the force, as shown by the annular area A in the middle. The area A of the end portion of the high-strength bolt that is pressed against the end is called the area affected by the bolt force of the end plate, which is simply referred to as the affected area, corresponding to the compression stiffness K of the steel plate in the affected area (K=AE/t, t is the thickness of the end plate, and E is the elastic modulus of the end plate), which is called the influence stiffness.
The first relational model is as shown in the figure, the initial compression deformation of the end plate under the action of the high-strength bolt pre-tension P0, the corresponding initial influence area is A0, and the initial influence stiffness of the end plate is K0 (K0=A0E/t), the influence The maximum compression deformation of the end plate in the area is zero. Under the external tension of the load, with the increase of the tensile force, when the compression deformation of the end plate completely disappears, when the tensile force T of the load changes from 0 to Tp, the influence area A changes from A0 to 0, and the influence stiffness of the end plate K changes from K0 to 0, and the maximum compressive deformation of the end plate in the affected area changes from 0 to 0. The axial force N of the bolt changes from 0 to Np, and the axial force N of the corresponding bolt increases from P0 to P0+Np. When the bolt is completely pulled apart, the external tension Tp of the bolt is equal to the axial force P0+Np of the bolt rod, and the pressure reduction force Nc in the area affected by the end plate is increased from 0 to P0. According to the literature <7>, when the average value of the end plate thickness and the column flange thickness is equal to the nominal diameter of the bolt, K08.65 Kb (Kb is the axial rigidity of the bolt).
Conclusion 1) The external tension of the bolt is nonlinearly distributed along the direction of the bending moment, which is consistent with the model established in this paper. 2) Under the action of external load (bending moment), the neutral axis deviates from the center of the bolt group, and the larger the external load (bending moment), the more deviation. 3) The maximum tensile force of the bolt is less than the result of the conventional calculation method. This is the result of considering the movement of the neutral shaft and the area affected by the end plate and affecting the stiffness change, which is equivalent to a kind of adjustment of the bolt tension.
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