Structural Steel Fasteners: Bolts, Rivets, and Weld Types Explained

Connecting structural steel elements safely requires choosing the correct fastening method. Modern steel framing relies on high-strength bolts and field welding, while historic structures often incorporate hot-driven rivets.

1. High-Strength Structural Bolts

High-strength bolts are the primary mechanical fasteners used in modern field erection:

• ASTM F3125 Grade A325: High-strength medium carbon steel bolts used for general structural connections, offer high tensile capacity, and are suitable for most steel-to-steel framing joints.
• ASTM F3125 Grade A490: Alloy steel bolts engineered for higher tensile strength requirements. They require strict handling procedures because they are susceptible to stress-corrosion cracking if galvanized incorrectly.
• Tension Control (TC) Bolts: Feature a splined end that shears off at the required clamping force when installed using a specialized electric shear wrench.

2. Field Welds and Structural Connections

Field welds create continuous, rigid connections between structural members without adding extra plate material weight:

• Fillet Welds: Triangular welds applied at corner or lap joints. They handle shear stress well and require minimal joint edge preparation.
• Groove Welds: Complete Joint Penetration (CJP) welds join two plates or flanges edge-to-edge, fusing the metal completely across the thickness of the joint. They are standard in heavy moment frame connections.
• Plug and Slot Welds: Fill circular or elongated holes in overlapping plates to transmit shear load where edge space is limited.

3. Historic Structural Rivets

Hot-driven rivets were the standard connection method for steel construction prior to the 1960s. Heated to a red-hot state, inserted into aligned bolt holes, and hammered flat with pneumatic tools, rivets clamped connections tightly as they cooled. Today, ironworkers replace damaged rivets with A325 or A490 bolts during historic restoration projects.

Summary

Selecting and installing the right structural fasteners—whether torquing A325 bolts or executing CJP field welds—ensures joints perform safely under full engineering design loads.