Looking for the bolt tightening methods? Here I have discussed in detail the methods so that you can understand them very easily.
There are three basic types of bolt mounting: simple clamping, pre-polling force, and friction-effective (slip-controlled). The differences between these types of joints are mainly the fastening force obtained when tightening the bolts and the extent to which the joining parts can move under the effect of service loads. The contact surface between the combined parts is called the contact surface. In each project, the engineer must specify the joint type and contact surface for each joint.
Simple bolt tightening is the situation where the bolts are in direct leaning and the joint layers are in tight contact. This type of tightening can be achieved using all the strength of a worker with a spanner with an open end and about 40 cm long. The other end of this wrench is tapered so that an ironworker can align the holes of the assembled parts. The simple-crimp connection can be specified in most simple cutting joints and only in joints trying to pull. The use of simple-tightening connections with A490 bolts under tension and in connections under the effect of non-static loads is not permitted. (Tensile and Tensile Stress of Bolts)
A prestressed bolted connection has much greater bonding strength than a simple-crimp joint and therefore provides a higher level of slip-resistance for the joint. Preliminary tensile connections are used in connections subject to reversible or fatigue loads. It is also a must for A490 bolts in pulling effect. Some examples of using pre-pulled bolt connections:
- Column joints in buildings with high height-width ratio,
- Joints in the load transfer path of the horizontal load-bearing system,
- Combinations under impact or reversible loads such as cranes and machines.
It should be noted here that the design strength of a pre-pull connection is equal to the design strength of a simple-tight connection. Slipping is prevented until the friction force is exceeded in a pre-pull joint. When the frictional force is exceeded, the bolts about the sliding plates and the pre-pull or fastening force disappear (eg, equivalent to a simple-tightening case). In both simple-squeeze and pre-shrink joints, the contact surface is allowed to be uncoated, painted or galvanized, but must be free of dirt and unwanted materials.
Preliminary bolts should be tightened until a minimum bonding force is achieved between the parts that join when installed. The AISC specification stipulates that the minimum required bond strength is at least 70% of the characteristic tensile strength of the bolt, Rn. The table below contains the minimum pulling values for the different bolts.
Minimum Bolt Pre-tension (connection with pre-pull and friction effect)
|Lowest bolt pre-pull,||= 0.70Rn * (kN)|
|Bolt size,||A325 and A490||A490 and F2280|
|mm||Group A bolts||Group B bolts|
Source: AISC Table J3.1
Equation J3-1. R n = Fnt.Ab
Fnt = 620 MPa (A325 and F1852)
Fnt = 780 MPa (A490 and F2280)
A b = characteristic area of the bolt without grooves
Bolt Tightening Methods
To achieve this minimum pulling force, bolts should be placed in one of the following ways:
1. Nut Rotation
As a nut runs along the length of the bolt, each full turn corresponds to a certain pulling force at the bolt. Therefore, there is a known relationship between the number of rotations and the tractive force found. The starting point (eg, the point where the tensile force in the find is just above zero) is expressed as a simple-tightening case.
2. Tightening with Indicating Tightening Wrench
In this method, point wrenches are used to obtain a minimum torque corresponding to a certain pulling force. In any project. calibration should be done daily for each bolt size and class.
3. Tensile Controlled Bolt Tightening with Twist and Break
These bolts, which are manufactured with slotted ends, are tightened with a special wrench and when a certain torque is obtained, the slotted end is broken. These bolts comply with ASTM F1852 and are equivalent to ASTM A325 in strength and design.
4. Indicator Directly Indicating Tensile Force
Washers conforming to ASTM F959 have grooved lugs on the crushing surface that press in a controlled manner so that the pressure is proportional to the tension on the bolt.
The deformation of the protrusions is checked to see if proper shrinkage has been achieved.
A special case concerning placement is that bolted joints slip into crushing while the building is in operation. When this happens, the occupants may hear inherently disturbing gunshot sounds. However, this event is not a sign of structural power depletion. This is known as the “impact bolt”. To prevent this from happening, the bolts should be simply tightened, or, if possible, the steel installer should not tighten the bolts before the displacement studs are released and the finders are allowed to slip before they are tightened.
The last connection we will consider is the friction-effect connection. This type of connection is similar to the pre-pull connection except that power depletion is assumed to occur when the acting load is greater than the friction force and slip occurs between the contact surfaces. As with prestressed joints, friction-acting joints are used for joints that are subject to reversible loads or fatigue loads. In addition to these, it is used in connections with oval holes parallel to the direction of the load or on contact surfaces where both welding and bolts are used together on the same contact surface. The pre-pull or clamping force required for a friction-acting bolt is the same as for pre-pull joints. The design strength of a friction-effective connection is from the design strength of crush-type joints,
The main difference between prestressed and friction-acting joints is the type of contact surface between the mating parts. Two types of contact surfaces are defined in the AISC specification: Class A and Class B. Each type has a specific surface preparation and coating condition, corresponding to a minimum friction force. A Class A contact surface is either an unpainted clean mill scale surface or a surface containing a Class A coating to steel cleaned with compressed air. Class A also includes hot-dip galvanized roughened surfaces. The average shear coefficient of a Class A surface is μ = 0.35.
A Class B contact surface is either an unpainted compressed air purged steel surface or a surface containing a Class B coating applied to a compressed air purged steel surface. The average shear coefficient of a Class B surface is μ = 0.50.
The average shear coefficient for any contact surface can be obtained by testing for special coatings and steel surface conditions.
Another important factor in friction-affect joints is the likelihood of slippage occurring at service loads or increased loads. In some cases, slippage between contact surfaces can lead to usability issues without causing an issue in strength levels. In essence, there is slippage between the contact surfaces, but this does not cause the bolt to lean against the mating parts or to leak or break. In some cases, it may cause problems in the level of shear strength between contact surfaces (for example, in a long span roof truss attachment). Slip in the joint may cause extra displacement and increase the potential for unwanted buoyancy.