The difference in tensile strength between my country's high-strength bolts and DIN

There is a huge difference in the tensile bearing capacity of high-strength bolts in the DIN standard and the Chinese standard, for the following reasons:

1. In our country's standard, the pretension P is required to be applied when the conflicting high-strength bolt is installed, P=0.9*0.9*0.9*fu*Ae/1.2.

In the meantime, fu is the smaller tensile strength, and Ae is the useful area of the bolt.

The planned pretension of high-strength bolts is determined by the material strength and the useful cross-section of the bolt, and takes into account:

a. When the bolt is tightened, the shear stress generated by the torque will reduce the tensile capacity of the bolt, so the tensile strength of the material is divided by a factor of 1.2;

b. In order to compensate for the slack of the pretension during construction, the bolt must be over-tensioned by 5%~10%, so it is multiplied by the coefficient 0.9;

c. The influence of variation of material resistance, etc., multiplied by a coefficient of 0.9;

d. The tensile strength introduces an additional safety factor of 0.9.

The resistance type is to rely on the resistance force between the connected members to transmit resistance, and the shear force is equal to the resistance force as the limit of the bearing capacity. In order to avoid slack after unloading when the external force is greater than the bolt pretension, the planned value of tensile bearing capacity shall not be greater than 0.8P.

Taking the 10.9 grade bolt as an example, the smaller tensile strength is 1040N2, and the planned value of tensile bearing capacity is 1040*0.9*0.9*0.9*0.8/1.2=500N2.

2. The planned value of tensile bearing capacity of pressure-bearing high-strength bolts in our country is also judged according to 0.8P, but slippage of the contact surface is allowed. At this time, the force is the same as that of ordinary bolts, and the bearing capacity is the strength of the bolt itself. Pressure-bearing type means that when the shearing force exceeds the resisting force, the screw is sheared or the hole wall is damaged by pressure, which is the limit of bearing capacity. Although the shear deformation of pressure-bearing high-strength bolts is larger than that of the interference type, the bearing capacity is higher than that of the interference type.

This type of bolt cannot be used in structures subject to dynamic loads.

3. According to the DIN standard, conflicting high-strength bolts cannot be used in earthquake areas, so the plan is based on the strength of the bolt itself, and only the material sub-factor and safety factor are reduced, which is higher than our country's standard value.

Take the 10.9 grade M20 high-strength bolt as an example. The Chinese standard is controlled by the pre-tension force, and the tensile planning bearing capacity is 124kN; the DIN standard is controlled by the strength of the material itself, which is 178kN. The accounting process is as follows:

N<=314*900/1.1/1.1=234kN, 314 is the cross-sectional area, 900 is the yield strength, 1.1 and 1.1 are the additional safety factor and material sub-factor respectively;

N<=245*1000/1.25/1.1=178kN, during which 245 is the net cross-sectional area, 1000 is the smaller tensile strength, 1.25 and 1.1 are the additional safety factor and material sub-factor respectively;

Both take the small value, that is, 178kN.

Therefore, assuming that the planned value of our country’s code is selected, the overall structure is guaranteed to be less deformed under normal working conditions (non-earthquake); assuming that the planned value in the DIN code is selected, the structure is also safe (we will still impose pre-deformation during construction). Tightening force, to ensure a certain resistance of the contact surface and small deformation of the overall structure, only when the bolt exerts a greater effect during an earthquake, it is resisted by its own strength).