Bearing Capacity Adjustment Coefficient of Tension Angle Members Connected with One Bolt in Single Shear

The end distance L_d and edge distance L_z of bolt holes affect the bearing capacity of single angle steel members connected by a single bolt and the size of gusset plates at both ends in the transmission tower. Reducing L_d can reduce the geometric size of the connection area, reduce or cancel the gusset plate, and increasing L_d can improve the tensile bearing capacity of members. At present, the bolt L_d and L_z in DL/T 5442-2020 are fixed values. In order to study the tensile bearing capacity of such members, the material property test and tensile test of single bolt connection of angle steel of ∟56, ∟80, ∟90 and ∟110 specifications were carried out. The angle steel is made of Q235 B, and the diameters of high-strength bolt are 16 mm and 24 mm. Displacement meters were set to measure the member deformation and hole elongation. After the specimen was loaded, the bolt hole had plastic deformation and longitudinal elongation. When the ultimate tension was reached, both sides of the pressure hole wall and the middle of the end surface of the corresponding angle steel were torn. The finite element model of connecting plate, angle steel and bolt was established according to the test piece parameters, and the grid size near the contact surface of each component was less than 2 mm.
The calculation resuls show that the distribution of high stress areas are identical for specimens and FEA models, and the average error of ultimate tension foce between the two is 5.8%, and the load-hole elongation curves are consistent with the test results. Four groups of one bolt connected angle models(L_d=(0.8~3.7)d₀) are built for further study with yield strength f_y=235 MPa and elastic modulus E=2.06×10⁵ MPa:∟40×3, 1 M12, ∟40×3, 1 M16, ∟50×4, 1 M16, ∟63×5, 1 M20.
The results show that the diameter of the bolt hole is larger than that of the bolt, resulting in the stress concentration on the compression hole wall during the single shear connection. When the total elongation of the specimen is small, plastic deformation has occurred in some areas, and then the specimen loses its initial stiffness, and the bolt hole continues to elongate. When the reference end distance is adopted, the bearing capacity N_rt of the specimen is about 95% of the DL/T 5486-2020 bearing capacity N_code, and the tensile bearing capacity N_t of the four groups of specimens is about 75% of the ultimate tensile force N_u. There is a certain margin in DL/T 5486-2020. The length of the linear section of the load-hole elongation curve, the position of stiffness change and the bearing capacity of the specimen are jointly affected by L_d and L_z. Taking L_d/L_z=1.5 as the boundary, there are mainly two failure modes:when it is less than the critical value, the specimen has end shear and tear failure; after exceeding the critical value, it gradually turns into the net section failure of steel angle, and the continuous increase of L_d has little effect on the bearing capacity. At present, M16 bolts are used for ∟40 angle steel in China. When M12 bolts are used, L_d/d₀ can be reduced to obtain a higher upper limit of bearing capacity. EC 3, ASCE and other specifications consider the influence of L_d and L_z on tensile bearing capacity. The values of material strength and other parameters are mainly for local steel, and the formula form is complex. Combined with the actual situation of steel in China and the calculation framework of transmission tower, the calculation method including bearing capacity adjustment coefficient is given according to the test and model results. Considering the different effects of L_d and L_z, the same calculation formula is used for members with different bolt diameters and steel angle leg widths, which is suitable for the calculation of tensile bearing capacity of hot-rolled steel angle members connected by single bolt and single shear within the range of L_d=(1.0~3.0)d₀.