1�� Basic definition of high-strength bolts
High Strength Friction Grip Bolt (HSFG), also known as "High Strength Friction Pre Tightening Bolt", is a special bolt that uses static friction force between connecting plates to transmit loads by applying pre tightening force. 'High strength' does not mean that the absolute value of material strength is high, but emphasizes the high stiffness connection characteristics achieved through pre tensioning force.
2�� Core characteristics of high-strength bolts
1. Unique force logic: precise pre tightening force is applied to tightly squeeze the connecting steel plates, relying on static friction to withstand shear forces. During normal operation, slippage is not allowed, and the elastic-plastic deformation is minimal. The node stiffness is much stronger than that of ordinary bolts.
2. Design strength paradox: Under the same level (such as 8.8 level), the tensile and shear design values of high-strength bolts are actually lower than those of ordinary bolts, but their advantage lies in deformation control, making them suitable for places with high stiffness requirements such as main beams.
3. Fatigue failure dominant: More than 50% of failures are fatigue fractures, commonly located at the root of threads (65%), the transition between screws and rods (20%), and the transition fillet of bolt heads (15%). Optimizing design (such as increasing transition corners and cutting unloading slots) can alleviate stress concentration.
4. The manufacturing process is rigorous: it requires heat treatment before rolling the wire (generating residual compressive stress), followed by phosphating (reducing friction coefficient and stabilizing torque), and then passing a 20 �� low-temperature impact test (Ak ≥ 27J) to maintain toughness.
3�� Difference from ordinary bolt core
|Comparison dimension | High strength bolt | Ordinary bolt|
|Principle of force | Pre tightening force generates static frictional force to resist shear, and after sliding, it can still be carried by elastic-plastic deformation | Screw directly carries shear, and after sliding, the load is transmitted by screw shear and hole wall pressure transmission|
|Material grade | Only 8.8, 10.9 (British and American standards) | Covering 4.6, 5.6, 8.8, 10.9, 12.9 (wider range)|
|Construction acceptance | Precise control of pre tightening force with torque wrench is required, and additional impact energy indicators should be tested | Ordinary wrench tightening is sufficient, with lower mechanical performance requirements and no impact energy requirements|
|Application scenarios | High stiffness scenarios such as main beams and steel bridges that require "strong nodes" | Secondary structures or temporary connections that can be reused|
|Failure mode | Fatigue fracture is predominant (over 50%) | The screw undergoes plastic deformation before being sheared.
From these characteristics, it can be seen that the "strength" of high-strength bolts lies in node stiffness, anti slip ability, and safety reserve, rather than single bolt bearing capacity. They are key components in achieving the seismic design concept of "strong nodes, weak members" in steel structures.