Using High Strength Bolts for Structural Bolting

High strength bolts or structural bolts are made to be used with heavy hex nuts to connect structural members. To be considered a structural connection, it has to conform to specific ASTM standards; specifically, ASTM A563 or ASTM A194.

Structural bolts are also categorized by grades. The grading system takes into consideration the bolt’s material, its size range, its proof load, the minimum yield strength, and the minimum tensile strength.

What Is ASTM A325?

The American Society for Testing and Materials, or ASTM, is responsible for setting the standards for various tools and specific industries. Although these standards are voluntary, most fabricators conform to them because it instills more trust in their customers.

In addition to ASTM A563 and A194, the A325 standard applies to heavy hex structural bolts. This standard defines mechanical properties for bolts that range from .5 to 1.5 inches in diameter.

The equivalent metric standard is known as A325M and defines mechanical properties for bolts in sizes M12-36. In 2016, the ASTM replaced the A325 designation with ASTM F3125; however, the markings on bolt heads are the same to avoid confusion.

Let’s Talk About Bolts vs. High Strength Bolts

Bolts are generally produced using some type of steel. The bolts strength is directly related to the materials used to make them.

So, what are bolts made of? Depending on the intended use for the bolt, different grades of steel are used. The steel used in each type of bolt varies, with some having a higher tensile strength than others.

The higher the tensile strength, the tighter the bolt joint is, and the more torque the bolt can take. The SAE (Society of Automotive Engineers) has assigned bolts seven different grades, starting with two and ending with eight. In addition, there are other grading systems that account for higher-strength structural bolts.

Grade 9 Bolts

Grade 9 structural bolts, also known as grade 9 hex cap screws, are one the strongest structural bolts that can be used today. While the typical grade 8 bolt has a tensile strength of 150,000 PSI, a grade 9 bolt has a tensile strength of 180,000PSI.

Grade 9 bolts are ideal for heavy-duty applications and are also often used in heavy machinery such as bulldozers. Not only are these bolts considerably stronger than they’re grade 8 counterparts, but they are also considerably more resistant to corrosive elements as they are made with yellow zinc chromate.

While the SAE grading system only goes up to grade 8, grade 9 bolts are without a doubt a stronger structural bolt.

Grade 8 Bolts

A grade 8 bolt is the highest grade SAE bolt. Grade 8 bolts have a minimum tensile strength of 150,000 PSI. As a comparison, the ASTM A325 bolt is roughly equivalent to the SAE grade 5. A grade 8 bolt would be one that meets ASTM A490 standards.

The A325 bolt strength, or SAE grade 5 bolt strength, is usually used for structural bolting projects. There are three primary materials used in the making of A325 bolts, and they are:

1. Type 1 – medium carbon steel or alloy steel, or boron steel
2. Type 2 – low carbon martensitic steel (this designation was removed from the standard in 1991)
3. Type 3 – weathering steel

Differences Between ASTM A325 and SAE Grade 5 High Strength Bolts

Strength is everything when it comes to structural bolting projects. When comparing ASTM A325 and SAE grade 5 bolts, there are more similarities than there are differences.

The general properties and the physical and chemical strengths between the two designations are essentially the same. The differences are seen in their classifications and the types of jobs recommended for each bolt.

For instance, engineers usually recommend ASTM A325 bolts for structural steel connections in heavy construction projects. On the other hand, SAE grade 5 bolts are common in OEM-type applications (original equipment manufacturer).

In addition, here are a few more differences between the two:

• Grade 5 bolts are usually available in a finished hex bolt design; A325 bolts must have a heavy hex head.
• The nuts required for grade 5 bolts are a finished hex pattern, whereas A325 bolts need heavy hex nuts such as A192-2H or A563-DH.
• Grade 5 bolts specify bolts from .25 to 1.5 inches in diameter; the A325 bolts cover sizes from .5 to 1.5 inches in diameter, which means that ASTM A449 should be used for bolts over the 1.5-inch size.
• Finally, the thread lengths required are different for each type of bolt. For grade 5 fasteners, the standard thread length is twice the bolt’s diameter or .25 inches for any bolt less than or equal to 6 inches. For bolts longer than 6 inches in diameter, the standard length is twice the diameter of the bolt plus .5 inches. For A325 bolts, there are still specific lengths, but they are generally much shorter than most other grades of bolts.

Different Bolting Methods

In structural bolting, using the right high strength bolts is just part of the equation. Excellent workmanship means using the proper methods to make sure the bolt and nut are fastened together correctly. Two standard bolting methods are:

1. Turn of nut method
   This method involves turning the nut or bolt of a fastener assembly a specific angle that is based on the length and diameter of the fastener, while at the same time making sure the unturned element doesn’t rotate.
2. Calibrated wrench method
   With this method, you apply an average calculated torque value based on the daily testing of your representative fastener assembly samples. These are based on the exact configuration of both your power supplies and your tools under conditions as close together as possible, within one day or less.

Custom High Strength Bolts For Your Applications

There are some hard-and-fast rules to apply when searching for the right bolts to use for structural applications. It’s important to have a clear understanding of your tensile strength requirements, specific material requirements, and anything else that may be pertinent to the final application.

Typically, custom metal fabricators will work closely with you to source or produce the components that best suit your project’s needs.  This includes anything and everything from material selection, threading, bolt shapes, and more.

If you’re looking for an experienced, reliable contract manufacturing partner to produce high strength structural bolts or any other custom fasteners, we’d be happy to give you a no-obligation quote on your project. With more than 40 years of experience in metal fabrication services, The Federal Group USA has the capabilities to produce high strength bolts no matter the requirements. Contact us today to find out more about our processes and capabilities.