Author: Bolt connections
When a connection is subject to a load reversal or to dynamic loading, a shear connection which acts by shear stress in the bolt and bearing stress in the plates, is not acceptable. By pretensioning of the bolts, however, a clamping pressure occurs between the connected parts which enables load to be transferred by frictional resistance.
Preloaded bolts exert a compressive stress on the connected plates. The compression gives rise to high frictional resistance, which enables load to be transferred between the connected parts. When the applied load F exceeds the frictional resistance which is developed between the plates, the plates will slip relative to each other allowing the bolt to act in bearing. Bolts which transfer load by friction are known as High Strength Friction Grip (HSFG) bolts. Controlled tightening of the bolts allows the frictional action to be quantified for design.
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The main advantages of HSFG bolted connections are their greater stiffness and their ability to withstand alternating forces. Their behaviour under fatigue loading is also better than that of bearing bolted connections. Against these advantages are the costs of HSFG bolted connections. The preparation of the friction grip surfaces and the controlled tightening require additional care (training of people). The costs are greater than for bearing connections. As a result, HSFG bolted connections are usually used only where the stiffness of the connection is important, where alternating loading would cause alternating slip, or where fatigue loading is present.
When an external tension force is applied to the connection, the force in the bolt will increase. At the same time the elongation of the bolt increases, and the shortening of the plate assembly decreases by the same amount. As a result, the force in the plate assembly decreases. The percentage of the force which is transmitted by an increase in the force in the bolts and the other part which reduces the clamping forces between the plates depends on the elastic behaviour of the connection (see Figure 3). Since the elastic behaviour is the same, the location of the contact pressure produced by tightening of the bolts is of crucial importance with regard to bolt fatigue. Typically HSFG bolts are used in bridges, cranes and crane girders.
In order to make practical use of the friction effect, high tensile steel bolts (usually grade 10.9) are used so that an adequate clamping force can be obtained with reasonably sized bolts. The stress induced in the bolts by the pretensioning is at, or near, the proof stress.
Three methods of tightening are available:
- Torque method: For this method of tightening a calibrated torque wrench is required which may be hand operated or, for bolts of larger diameters, power operated.
- Turn-of-nut method: This method is based on a predetermined rotation of the nut.
- Combined method: In this method the torque method and the turn-of-nut method are combined.
Failure of threaded fasteners due to over-tightening can occur by bolt shank fracture or by stripping of the threads of the nut and/or bolt. Shank fracture is sudden and therefore easily noticed. Stripping is gradual and therefore difficult to detect. It introduces the danger of partly failed fasteners being left in assemblies.
HSFG bolts in shear connections transmit the force by friction between the contact faces. The resistance of these connections depends on the preload, the slip factor and the number of friction faces.
Parts to be joined with preloaded high strength bolts must be protected against corrosion by suitable measures designed to prevent moisture penetrating to the contact surfaces and the bolt holes. Such a protection (e.g. sealing) may also be necessary as a temporary measure when joint faces are left wholly or partially exposed during erection. All necessary precautions shall be taken, during fabrication and erection, to ensure that the slip factors assumed for the calculation are achieved and maintained.
Prestressed bolt connections form the majority of bolt and threaded connections. In most bolted connection, the bolts are subjected to shear. The bolt assembly may be pretensioned or it may only resist shear. There are many options involved with a bolted connection. Single-bolt connection capacity cannot be directly extrapolated to predict. Design and strength check of a prestressed bolt connection. The condition of the high-tensile bolt connections is of great importance. highly sophisticated bolt connections.
Load-bearing performance of a retaining ring bolt connection. The calculation is designed for a geometrical design and strength check of a prestressed bolt connection, loaded by static or cyclic loading. The connections specify a bolt torque to achieve bolt pretension and full seating of the bolt head. The distribution of strength in a multiple-bolt connection is known for up to four. Connections Using Galvanized Bolts.
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