What is Considered a Structural Bolt?

ASTM F3125/F3125M is a comprehensive specification covering both inch and metric grades of structural bolts. This specification includes four inch-series grades: A325, F1852, A490, and F2280. This article will address these grades. At first glance, the mechanical properties of grade A325 and SAE J429 Grade 5 bolts appear identical, as do the properties of grade A490 and SAE J429 Grade 8 bolts. However, substituting an SAE J429 Grade 5 for an ASTM F3125 grade A325 is not permissible, and the same applies to substituting a Grade 8 for a grade A490.

Firstly, A325 and A490 bolts are produced with a heavy hex head configuration, which provides a wider bearing surface to distribute the load more effectively. In contrast, Grade 5 and 8 bolts are manufactured with a standard hex cap screw configuration, which does not provide adequate load distribution for structural applications.

Secondly, the shank length (the non-threaded portion of the bolt) on A325 and A490 bolts is designed to be greater, resulting in shorter thread lengths compared to standard hex bolts or hex cap screws. Since the threaded region is typically the weakest part of standard carbon steel fasteners, having a longer shank enhances the tensile strength of the connection.

Thirdly, differences in thread length can also impact shear strength. For example, if both bolts are 3/4-10 x 4, Bolt X has the shear plane acting through the body of the bolt, with a minimum body diameter of 0.729 inches and a cross-sectional area of 0.417 square inches. With an ultimate shear capacity of 72,000 PSI, Bolt X can handle nearly 30,000 pounds. Conversely, Bolt N, with its shear plane acting through the threaded portion and a root area of 0.302 square inches, can handle approximately 21,740 pounds. Ensuring the shear load is applied to the body of the bolt rather than the threads can improve shear strength by 8,260 pounds. If a Grade 5 bolt (with longer thread length) is substituted, the threads would be in the shear plane, resulting in an 8,260-pound reduction in shear strength.

Structural bolts are specifically designed for use with heavy hex nuts in the connection of structural members, as opposed to tapped holes. The nuts for structural connections must conform to ASTM A563 or ASTM A194 standards. The washers used for structural connections must meet ASTM F436 specifications, covering both flat circular and beveled washers.

Mechanical Properties and Nut Compatibility

Grade A325 bolts are available in diameters from 1/2 to 1-1/2 inches. For diameters greater than 1-1/2 inches, ASTM A449 specifications should be considered. A325 bolts have a minimum tensile strength of 120,000 PSI and are available in two types. Type 1 is a medium carbon steel that can be galvanized, while Type 3 is a weathering steel offering atmospheric corrosion resistance similar to ASTM A242 or A588 steels. Type 3 grade A325 bolts must be marked “A325” to distinguish them from standard grade A325s.

Grade A490 bolts are available in diameters from 1/2 to 1-1/2 inches with a minimum tensile strength of 150,000 PSI for all diameters. A490 bolts are also available in two types: Type 1 is alloy steel, and Type 3 is weathering steel with corrosion resistance similar to ASTM A242 or A588 steels. Type 3 grade A490 bolts must be marked “A490”. ASTM A490 bolts should not be galvanized or electroplated because the hot dip galvanizing process can cause the fasteners to anneal, and there is a risk of hydrogen embrittlement during the pickling process before plating or galvanizing.

When a protective coating is required or specified, the only coatings allowed by the ASTM F3125/3125M standard on grade A490 bolts are ASTM F1136 Grade 3 or ASTM F2833 Grade 1. These coatings have undergone extensive hydrogen embrittlement research according to IFI 144. This provides an opportunity for those interested in corrosion-resistant high-strength structural bolts. For more information on the F1136 and F2833 coatings, contact the Fastenal Engineering & Design Support Group at engineer@fastenal.com.

Installation Methods

According to the Research Council on Structural Connections and the American Institute of Steel Construction (AISC), there are generally three types of connections in which a bolt is used: snug-tightened, pre-tensioned, and slip-critical connections. Bolts used in pre-tensioned or slip-critical connections must be installed to within 70% of the bolt’s minimum tensile strength.

Due to the uncertainties involved with torque, it is not reliable to use published values based on a torque-tension relationship from a formula to achieve the accuracy required for structural connections requiring a minimum clamping force of 70% of the bolt’s minimum tensile strength. Four methods of installation procedures recognized by the AISC to achieve the required tension for fully tensioned bearing or slip-critical connections are:

  1. Turn-of-Nut Method: This method involves initially tightening the fastener to a low “snug tight” condition and then applying a prescribed amount of turn to develop the required preload. The preload depends on how far the nut is turned and the initial snug tightness.
  • Snug the joint to bring the assembly into firm contact.
  • Apply a few impacts with an impact wrench until a solid sound is heard or apply full effort with a spud wrench.
  • Inspect the joint to verify “snug tight.”
  • Match mark the bearing face of the nut and the end of the bolt with a single straight line.
  • Apply the required turns systematically according to the appropriate bolting pattern.
  1. Alternative Design Bolts (Tension Control Bolts): Tension control bolts use design features that indirectly indicate tension. The most common type is the twist-off bolt. An assembly tool holds the bolt from the nut end while an inner spindle on the tool grips a spline section at the end of the bolt. An outer spindle turns the nut and tightens the fastener. When the designated torque is reached, the spline snaps off, indicating proper tension. This system requires bolts to be used as received and strict adherence to storage, cleanliness, and verification standards.
  2. Direct Tension Indicating Method (DTI): DTI washers have hollow bumps that flatten as the fastener is tightened. A feeler gauge is used to measure the gap created by the bumps. When the appropriate tension is reached, the feeler gauge will no longer fit in the gap. Some newer types fill the void under the bumps with colored silicone, which squirts out when compressed, indicating proper tension. DTI washers indicate minimum required tension but cannot show over-tensioning or bolt relaxation.
  3. Calibrated Wrench Method: This method involves daily calibration of installation procedures by tightening at least three representative fastener assemblies in a device that measures actual bolt tension. The calibrated wrench method ensures accuracy when published torque-tension values are unreliable.

Summary

Structural bolts are essential for securely connecting structural members. A325 and A490 bolts, with their specific head configurations and shank lengths, offer superior load distribution and tensile strength compared to standard hex cap screws. Proper installation methods, including turn-of-nut, tension control, DTI washers, and calibrated wrenches, ensure the necessary preload and tension for structural applications. Following ASTM specifications and guidelines ensures the reliability and safety of structural connections.

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