Difference of tensile resistance between high strength bolts and din in China

Difference of tensile resistance between high strength bolts and din in China

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

1. in China's specifications, the pre tension P is required to be applied for the high-strength bolt device with resistance type, p=0.9*0.9*fu*ae/1.2.

Meanwhile, 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 section of the bolt, and the following factors are considered:

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

b. In order to compensate for the slack of pretension during construction, the bolts should be overstretched by 5%~10%, so multiply by a factor of 0.9;

c. Variation of material resistance, multiplied by a factor of 0.9;

d. An additional safety factor of 0.9 was introduced for tensile strength.

The resistance type is based on the resistance between connected members, and the ultimate bearing capacity is the shear force equal to the resistance. 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 grade 10.9 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 China is also determined according to 0.8p, but the sliding of contact surface is allowed. At this time, the stress condition is the same as that of general bolts, and the bearing capacity is the strength of the bolt itself. The bearing type is the ultimate case of bearing capacity when the shear force exceeds the resistance force and the screw is sheared or the hole wall is damaged under pressure. Although the shear deformation of pressure bearing high-strength bolt is larger than that of resistance type, its bearing capacity is higher than that of resistance type.

Such bolts cannot be used in structures subjected to dynamic loads.

3. according to the din specification, the high-strength bolt of resistance type can not be used in the earthquake area, so it is planned according to the strength of the bolt itself, and is reduced only according to the partial coefficient and safety factor of the material, which is higher than the value of China's specification.

Taking the M20 high-strength bolt of grade 10.9 as an example, the tensile planning bearing capacity is 124kn according to the pre tension control in the Chinese specification; The din specification 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, in which 314 is the cross-sectional area, 900 is the yield strength, and 1.1 and 1.1 are the additional safety factor and material partial factor respectively;

N<=245*1000/1.25/1.1=178kn, in which 245 is the net sectional area, 1000 is the smaller tensile strength, and 1.25 and 1.1 are the additional safety factor and material partial factor respectively;

The smaller of the two is 178kn.

Therefore, if the planning value of China's code is selected, the small deformation of the overall structure under normal working conditions (non earthquake) can be ensured; Assuming that the planning value in DIN specification is selected, the structure is also safe (we will still apply pre tightening force during construction to ensure certain resistance force on the contact surface and small deformation of the overall structure. Only in the event of an earthquake, the bolt will play a greater role and rely on its own strength to resist).