A325 vs A490 Structural Bolts: Which One to Choose

In steel structures, bridges, and heavy machinery, you'll encounter A325 and A490 high-strength bolts. They look similar but differ significantly in material and strength. The RCSC (Research Council on Structural Connections) specification defines their design and installation requirements.

Material and Strength Comparison

A325 Bolts: Heat-treated medium carbon steel, minimum tensile strength 830 MPa (120 ksi), yield strength 660 MPa (90 ksi). Sizes from 1/2" to 1-1/2". Roughly equivalent to metric 8.8 grade.

A490 Bolts: Quenched and tempered alloy steel, minimum tensile strength 1040 MPa (150 ksi), yield strength 940 MPa (130 ksi). Sizes from 1/2" to 1-1/2". Roughly equivalent to metric 10.9 or above.

Key difference: A490 is about 25% stronger but cannot be welded (alloy steel welding risks hydrogen embrittlement) and must not be used above 425°F (218°C).

Preload Requirements Are the Same Principle

RCSC requires both A325 and A490 to be preloaded to 70% of minimum tensile strength:

• 3/4" A325: ~185 kN preload
• 3/4" A490: ~250 kN preload

Bearing-Type vs Slip-Critical Joints

Bearing-Type: Allows minor slip between connected parts; bolts carry shear. Less precise preload control needed.

Slip-Critical: Load transferred by friction only. Requires precise preload, clean faying surfaces (blast or wire-brush), and verified friction coefficient. Used in bridges, wind-sensitive structures, and cyclic loading.

Installation Methods

RCSC accepts three preload methods: torque wrench (lowest accuracy), turn-of-nut method (medium), and direct tension indicators (DTI washers with protrusions that flatten at target preload — highest accuracy).

Toprecision supplies metric equivalent high-strength bolt solutions (10.9/12.9 alloy steel) with torque calculations for structural applications.