When the fastener is turned down on the thread helix, the rotatory motion transforms into linear motion. This causes the bolt to stretch, while parts are compressed. However, only a small amount of the input torque serves to pre-tension the bolt. Most of it is used to overcome the resistive effect of friction, which opposes the rotation of the fastener.
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Friction originates at two contact interfaces; between the turned part and the clamped part (often under the bolt head or under the nut) and in the threads.
This means that the portion of torque that transforms into preload is very small. Torque/Tensile tests help to characterize the coefficients of friction taking place at both contact interfaces. The breakdown of torque usually shows that only 10% of the input torque is useful. The rest is used up by friction.
For example, underhead torque represents 50% of the input torque and thread torque is 40%. Hence, the torque tightening process is not very efficient. This portion can be increased by applying a low friction lubricant. Using lubricant reduces friction and decreases the necessary torque to reach the same preload.
Friction coefficients are also highly variable (+/-20% usually), which introduces scatter in the process. Also, even though tightening tools can be very accurate (+/-1% for calibrated hand torque wrenches for example), the deviation on applied torque varies from +/-10 to +/-50%. Many operators get confused when they realize how much influence the position of their body and how they handle the tool has on the applied torque.
It is commonly known that the scatter in achieved preload is +/- 30%. This means that the maximum possible preload can be twice as much as the minimum possible preload. Scatter can be even higher for rusty bolts or for stainless steel fasteners, which are prone to seizing.
Luckily, scatter can be decreased by applying lubricant or using bolts with a specified coefficient of friction, for example with a top coat or wax.
High tension bolt
Among the bolts used for joining steel structures, high-tension bolts are commonly used as bolts with yield strength of 700Mpa or more and tensile strength of 900Mpa or more, and bolts used in steel structures are recognized as high-tension bolts.
2. Kind of High strength bolt for steel structural
According to KS B <High-tension hexagonal bolts, nuts, and flat washers for friction bonding>, high-tension bolts are divided into four types, and three types of F11T are not used for safety reasons, so they can be divided into three types: F8T, F10T, and F13T, and the tensile strength of each bolt can be divided into 800MPa, 1,000MPa, and 1,300MPa.
Bolt materials are classified in KS as steel that satisfies tensile strength, and similar bolt materials used in addition to construction generally use low-carbon heat treated alloy steel Ex) SCM (Quenching+Temperting) steel.
3. Type of tightening of high tension bolts for Steel Structural Connections
The tightening of steel structure bolts can be divided into three types: friction, acupressure, and tensile and friction method is most frequently used for tightening in steel structures likes beam, bridge.
Brige bolt connection working in Korea
The properties of the junction are summarized as follows.
3.1 Friction grip
The theory is that friction grip bonding generates a bolt preload by torque, resulting in friction between the two steel structures, and the two members do not slip due to the generated friction force. Therefore, except for the most important preload in bolt tightening, the important factor in friction bonding method is the surface state of the Friction grip. Frictional force is generated by the state of this surface, and the generated frictional force increases the resistance capacity of the bolt slip.
Surface states are defined as follows.
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Based on European standards, the slip factor value was found to have the highest sliding coefficient for short-blasted steel, and the just untreated steel had a sliding coefficient of about 0.2.
Here, the sliding coefficient is as follows.
Friction coefficient of high force bolts divided by the tension introduced into the bolt by the force of the sliding surface of the assembly in the friction set
μ= P(slip load) /F(preload)×N(number of bolt)×A(slip section number)
Classifications that may be assumed for friction surfaces (EN -2: &#; Table 18), &#;&#;: Coating of faying surfaces - Hempel
In conclusion, the principle of friction bonding in steel structure design can be seen as a design to minimize the slip of steel structure due to the vertical load of bolts.
3.2 Acupressure and Tensile Connections
Acupressure bonding prevents bolt slip through acupressure, and tensile bonding is not used well in steel structures, but is used in flanges or other mechanical or mechanical equipment joints of plants or large facilities.
In the two bonding methods, the sliding coefficient is not an important factor on the member surface, so other surface treatment is not important on the member, and bolts and fastening surfaces are considered more important factors in fastening.
4. Inspection of tightening of high tension bolts in Steel Structural Connections
The most important part of fastening the bolt is the preload (tension) of the bolt. However, in reality, it is almost impossible to accurately calculate this preload, and a high-strength bolt Torque shear bolt is used to check the design tension most easily in the field, and the tightening of the steel structure joint is inspected through a gold drawing method after the first tightening.
4.1 Torque shear bolt
Torque shear bolt which have pin-tail in edge side of bolt
The torque shear bolt, called the T/S bolt, looks like the figure above, and if there is a pin-tail at the end of the bolt, the pin-tail is broken if it exceeds a certain torque. That is, the weakest pin-tail part of the bolt is cut by torque, thereby having a similar target preload for almost all bolts.
4.2 Pre-tightening(1st) torquing
KCS 25:
According to KCS 25:, the bolt is tightened with the first tightening torque before the main tightening. For the primary tightening, use a preset torque wrench, an electric impact wrench, etc. to rotate the nut to the torque specified in the table. However, the primary tightening of bolts at the bridge junction of "D" classified by quality control applies torque equivalent to 60% of the standard bolt tension.
In other words, the specification suggests bolt torque suitable for the grade and size of the bolt for torque management, and the tightening mechanism suggests using a tool that can manage torque.
4.3 tightening(2nd) torquing
After the first tightening, the specification specifies the gold drawing as shown in the figure above, and after this gold drawing, it is assumed that the following bolt tension can be obtained after fastening at 100% torque.
The KS standard defines that the axial force of the bolt varies in consideration of temperature conditions, etc., and the bolt tension type is separately named. However, these values are defined as very easy expressions and may be different from the actual preload.
The rotation angle can be confirmed by the secondary tightening torque of the gold drawing, and the rotation angle of the bolt of the steel structure should be within 10%, according to the KCS specification.
In this way, the characteristics and friction joints of bolts used in construction steel structural joints in Korea were examined.
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