What Is a High-Strength Bolt?
High-strength bolts are robust fasteners primarily used in steel-frame construction. Their tensile strength, typically at least 800 N/mm2, far exceeds that of common steel materials like SS400, which has a strength of 400 N/mm2. This makes them ideal for construction applications rather than general mechanical uses.
High-strength bolts are particularly prevalent in friction joints, a common element in steel construction, where they are used to securely fasten steel materials together.
Uses of High-Strength Bolts
These bolts are predominantly employed in the construction industry, especially for steel frame structures, a method often favored in Japan over concrete structures. In steel construction, welding steel frames together are labor-intensive and less stable in quality when performed on-site, leading to the preference for friction joining using high-strength bolts, which require high torque to generate the necessary frictional force.
Principle of High-Strength Bolts
Traditionally made from chrome steel or chrome molybdenum steel, high-strength bolts for steel-frame joints are now increasingly crafted from low-carbon boron-added steel. This change addresses the issue of delayed fracture common in chromium and chrome molybdenum steels.
The Torsia-type high-strength bolt, a variant, is designed to ensure fastening completion and axial force, and cannot be intentionally loosened once fastened.
Types of High-Strength Bolts
High-strength bolts are categorized into two main types based on their shape and application: hexagonal bolts for friction joints and Torsia-type bolts.
1. High-Strength Hexagonal Bolt for Friction Joints
These bolts are used for friction joints in construction, conforming to JIS B 1186 standards. They are classified as F10T or F8T based on strength. Although hot-dip galvanized high-strength bolts are sometimes classified as F8T, they often do not fully conform to JIS standards and are usually certified by the Minister of Land, Infrastructure, Transport, and Tourism.
2. Torsia-Type High-Strength Bolt
Torsia-type bolts require special tools for fastening due to their unique shape. The tightening section of these bolts breaks at the specified torque, simplifying torque control. The head is circular, resembling a rivet, and fastening is achieved by turning a grooved screw tip that breaks upon completion. This design ensures a consistent axial force and prevents intentional loosening post-fastening.
Other Information on High-Strength Bolts
Difference Between High-Strength Bolt and High-Tension Bolt
High-strength bolts and high-tension bolts are essentially the same, made from high-tensile (high-tension) steel. Variations in high-tensile steel include types with higher nickel content for increased salt resistance near coastal areas and weather-resistant types with added Ni and Cu for enhanced atmospheric corrosion resistance.