Structural bolts are essential fasteners used to join steel-to-steel components in heavy-duty applications such as buildings, bridges, towers, and industrial infrastructure. These bolts are engineered for strength, reliability, and long-term performance, making them critical to structural integrity and safety.

What Are Structural Bolts Used For?

Structural bolts are primarily used in applications where high-strength connections are required to support loads, resist shear forces, and maintain joint stability over time. Common use cases include:

• Steel buildings: Connecting columns, beams, and girders in commercial and industrial structures.
• Bridges: Securing structural components in highway and railway bridges.
• Towers and cranes: Assembling high-load structures like radio towers, wind turbines, and lifting equipment.
• Heavy equipment: Fastening machine components subjected to intense stress and vibration.

These bolts are often used in pre-engineered structures where connection performance is as important as material strength.

Common Grades of Structural Bolts

Structural bolts are produced to strict ASTM standards to ensure consistent quality and performance. Two of the most commonly used grades are:

ASTM A325 (Now ASTM F3125 Grade A325)

• Material: Medium-carbon steel, quenched and tempered.
• Minimum tensile strength: 120,000 psi (for bolts up to 1" in diameter).
• Applications: General structural connections, such as in steel buildings and pedestrian bridges.

ASTM A490 (Now ASTM F3125 Grade A490)

• Material: Alloy steel, quenched and tempered.
• Minimum tensile strength: 150,000 psi.
• Applications: High-strength requirements, such as long-span bridges or heavy-load support beams.
• Note: A490 bolts cannot be hot-dip galvanized, as the process may lead to hydrogen embrittlement.

Both grades are now covered under the consolidated ASTM F3125 standard, which streamlines specifications while maintaining performance criteria.

Key Features of Structural Bolts

• High-strength steel for load-bearing capacity.
• Heavy hex heads and nuts for better torque distribution.
• Typically used with hardened washers to prevent embedment and ensure proper preload.
• Installed using tensioning methods to ensure consistent clamping force.

Methods of Installation

Proper installation of structural bolts is critical to achieving the required tension and maintaining the safety of the structure. The three most common methods are:

Turn-of-Nut Method

This method involves tightening the bolt to a snug condition, then turning the nut a specified additional amount (such as one-third or one-half turn) to achieve the necessary bolt tension.

Tension-Control Bolts (TC Bolts)

Also known as twist-off bolts, TC bolts feature a spline at the end that shears off when the correct tension is reached. This system provides quick, consistent results and is popular in large-scale projects like high-rises and bridges.

Calibrated Wrench Method

A torque wrench is used to apply a specified torque that corresponds to the desired tension. This method requires calibrated tools and is often used when precise preload values are necessary.

The Importance of Preload

In many structural connections—especially slip-critical joints—the clamping force provided by bolt tension is what resists movement between the connected parts. If the bolt is not properly tensioned, the joint may fail under load, leading to serious structural issues.

Conclusion

Structural bolts are more than just heavy-duty fasteners—they are a cornerstone of modern construction and engineering. With their high-strength properties, standardization through ASTM specifications, and various installation methods, structural bolts ensure that buildings, bridges, and other critical infrastructure remain safe and secure for decades. Understanding the types, applications, and proper use of structural bolts is essential for engineers, contractors, and anyone involved in the assembly of load-bearing steel structures.