Differences in tensile strength between high-strength bolts and DIN in China

Differences in tensile strength between high-strength bolts

and DIN in China

There is a significant difference in the tensile bearing capacity of high-strength bolts between DIN standards and Chinese standards, due to the following reasons:

1. In China's regulations, it is required to apply a pre tension force P, P=0.9 * 0.9 * 0.9 * fu * Ae/1.2, when using a high-strength bolt device with resistance.

Among them, fu is the smaller tensile strength, and Ae is the useful area of the bolt.

The planned pre tension force of high-strength bolts is determined by the material strength and the useful cross-section of the bolts, taking into account:

a. When tightening the bolt, the shear stress caused by the torque will decrease the tensile strength of the bolt, so the tensile strength of the material is divided by a coefficient of 1.2;

b. To compensate for the slack pre tension during construction, the bolts need to be over tensioned by 5% to 10%, so it is multiplied by a coefficient of 0.9;

c. The variation of material resistance and other factors, multiplied by a coefficient of 0.9;

d. Introducing an additional safety factor of 0.9 to resist tensile strength.

The resistance type refers to the transmission of resistance based on the resistance force between the connected components, with shear force equal to the resistance force as the ultimate bearing capacity. In order to avoid slack phenomenon after unloading when the external force is greater than the bolt pre tension, the planned value of tensile bearing capacity should not exceed 0.8P.

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

2. The planned value of tensile bearing capacity for pressure bearing high-strength bolts in China's regulations is also determined based on 0.8P, but it allows for contact surface slip. At this time, the stress situation is the same as that of ordinary bolts, and the bearing capacity is the strength of the bolt itself. Pressure bearing type refers to the ultimate bearing capacity when the shear force exceeds the resistance force, and the screw is sheared and broken or the hole wall is damaged under pressure. Although the shear deformation of pressure bearing high-strength bolts is greater than that of the resistance type, their bearing capacity is higher than that of the resistance type.

This type of bolt cannot be used in structures that bear dynamic loads.

According to DIN regulations, high-strength bolts with resistance type cannot be used in earthquake zones. Therefore, they are planned based on the strength of the bolts themselves, and are only reduced based on material partial coefficients and safety factors, which are higher than the standard values in China.

Taking the 10.9 grade M20 high-strength bolt as an example, according to Chinese regulations, the pre tension control is used, and the planned tensile bearing capacity is 124kN; The DIN specification is controlled based on the strength of the material itself, which is 178kN. The accounting process is as follows:

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

N<=245 * 1000/1.25/1.1=178kN, where 245 represents the net cross-sectional area, 1000 represents the smaller tensile strength, 1.25 and 1.1 represent the additional safety factor and material partial factor, respectively;

Take the smaller of the two, which is 178kN.

Therefore, assuming that the planning values specified in Chinese regulations are selected, it ensures that the overall structure undergoes minimal deformation under normal working conditions (non earthquake); Assuming the planning values in the DIN specifications are selected, the structure is also safe (we will still apply pre tightening force during construction to ensure a certain resistance force on the contact surface and small deformation of the overall structure. Only during earthquakes do bolts have a greater effect, relying on their own strength to resist).