Steel frameworks, bridges and industrial machinery, everything depend on structural bolts. These structural fasteners transfer loads, resist vibration and protect steel joints from separating during wind, seismic activity or the constant pounding of heavy equipment. In the construction industry, high-strength bolts are used since they offer an appropriate load path but do not create bulk. This guide gathers data in the known engineering resources to describe the key structural bolt types, the standards governing these bolts and the appropriate selection of fastener to use in your project.
Types of Structural Bolts
Structural bolts are purpose‑built; the grade and head design reveal where they should be used. The most common types include:
- A325 bolt – These quenched and tempered carbon‑steel fasteners are among the workhorses of structural steel connections. They are applied in buildings, bridges and other constructions where massive loads have to be moved A325 bolts are offered in 1/2 -1/2 inch to 1 1/2 inch diameter and designed to provide both shear and tension. Tensile strength is approximately 105,000-120,000 psig and elongation minimum requirements protect the joint flexibility. In the SAE system they correspond to Grade 5 strength.
- A490 bolts – Similar in size to A325 bolts but made from alloy steel. These high‑strength bolts are used where the connection demands a higher mechanical performance; they have tensile strengths above those of A325 bolts and are often compared with SAE Grade 8 fasteners. Because of their elevated strength, A490 bolts are limited to smaller diameters to control brittleness.
- Tension Control (TC) bolts – TC bolts also known as twist-off bolts have a spline which shears away when the correct amount of torque is applied. This aspect eliminates the guess work in tightening and is desirable in heavy construction where stability in tightening of bolts is of great importance.
- Heavy hex bolts – These bolts have a bigger bearing surface and head than regular hex bolts and this disperses the clamping force over a greater area. Their six-sided head can easily be handled using wrenches, so they are used in high-temperature and high-tension applications. The heavy hex nuts are commonly used with heavy hex bolts.
Grade 8 and Grade 9 structural bolts – In the SAE grading system, Grade 8 bolts are the highest standard grade; they have a tensile strength of roughly 150 000 psi and are comparable to ASTM A490 bolts. Grade 9 bolts go beyond the SAE scale; they are exceptionally strong (around 180 000 psi) and are used in heavy machinery where shock or vibration is extreme. Grade 9 bolts often use yellow zinc chromate coatings to improve corrosion resistance.
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Get a QuoteBolt Dimensions, Bolt Sizes and Standards
Structural bolts are manufactured to meet standards set by organizations such as ASTM (American Society for Testing and Materials) and SAE (Society of Automotive Engineers). These standards specify the minimum mechanical properties, dimensions and thread pitches to achieve consistent performance. The bolt diameter and length are chosen based on the thickness of the materials being joined (the “grip”) and the type of load like shear or tension, that will act on the connection. Thread pitch influences how easily a nut can loosen under vibration, while markings on the bolt head indicate the grade and therefore the bolt’s yield and tensile strength.For quick reference on nominal diameters, thread pitches and body lengths, engineers use dimension charts. Jade Alloys includes a handy Bolt Dimensions Chart for ASTM F3125 structural bolts to check diameters and thread lengths for different grades:
| Bolt Diameter | Width Across Head Flats | Bolt Head Height |
|---|---|---|
| 1/2 | 7/8 | 5/16 |
| 5/8 | 1 1/16 | 25/64 |
| 3/4 | 1 1/4 | 15/32 |
| 7/8 | 1 9/16 | 35/64 |
| 1 | 1 5/8 | 39/64 |
| 1 1/8 | 1 13/16 | 11/16 |
| 1 1/4 | 2 | 25/32 |
| 1 3/8 | 2 3/16 | 27/32 |
| 1 1/2 | 2 3/8 | 15/16 |
Strength Criteria and Grading Systems
The performance of a structural bolt is defined by three key properties:
- Tensile strength – the maximum stress the bolt material can withstand while being pulled before breaking. This value is often used interchangeably with the maximum load rating.
- Yield strength – the stress at which the bolt begins to deform permanently. Bolts with high yield strength maintain clamping force even under heavy loads.
Proof load – the highest load a bolt can carry without causing permanent deformation.
The grading systems of ASTM and SAE grade bolts using these properties. ASTM alloys like A325 and A490 stipulate tensile and yield strength that is suitable in structural connection. SAE grading system is between Grade 2 to Grade 9, the better grade the better the minimum tensile strength. For example, Grade 5 bolts (comparable to A325) offer intermediate strength and are common in machinery, while Grade 8 bolts meet or exceed A490 strength requirements. Grade 9 bolts sit at the top of the SAE spectrum, delivering about 180 000 psi of tensile strength for extremely demanding applications.
A325 and A490: Flagship Structural Bolts
The A325 bolt is widely used in building and bridge construction. These bolts are made of quenched and tempered carbon steel, and have a diameter ranging between 1/2 inch and 1 1/2 inch. They are also applicable in tension and shear joints and can offer tensile strength of 105 000-120 000 psi. Minimum elongation (14% of short bolts and 12% of long ones) is required by the standard to provide sufficient ductility and eliminate brittle failures. Engineers often classify A325 bolts into three material types:
- Type 1- medium carbon, alloy or boron steel
- Type 2- low‑carbon martensitic steel, no longer in use
- Type 3- weathering steel
Each type offers a different balance of strength and corrosion resistance.
The A490 bolt is made of alloy steel and is treated to develop even higher strength. It is roughly equivalent to an SAE Grade 8 bolt. Because A490 bolts achieve higher tensile strength, they are recommended for connections where slip resistance and clamping force are critical. Hot-dip galvanizing is not compatible with them, as the temperatures required may cause changes in their mechanical properties; instead, they have a bare finish or may be supplied with corrosion-resistant coats.
Standard Bolts vs. High‑Strength Bolts
Not all bolts are designed to support structural loads. Ordinary commercial bolts are produced from mild steel and have lower tensile strengths; they are used where failure would not compromise safety. High‑strength structural bolts, on the other hand, must meet strict ASTM standards and are designed to work with heavy hex nuts.
Key differences include:
- Material and coatings – High‑strength bolts use quenched and tempered or alloy steels with controlled chemistries, whereas standard bolts may use low‑carbon steel. Some high‑strength bolts (such as Grade 9) receive yellow zinc chromate to resist corrosion.
- Head design – Structural bolts typically have heavy hex heads, while standard hardware bolts may use finished hex heads.
- Thread length and diameter range – Grade 5 bolts span 1/4–1 1/2 inches in diameter, whereas A325 bolts start at 1/2 inch. Standard thread lengths for Grade 5 fasteners are generally longer than those for A325 bolts.
Installation Methods for High‑Strength Fasteners
Even the highest quality bolt can fail if it is installed incorrectly. Two standard methods are widely recognised:
- Turn‑of‑nut method – Tighten the assembly by turning the nut (or bolt head) a prescribed number of turns or degrees after the joint surfaces are snug. The angle depends on the bolt’s length and diameter and is chosen so that a predictable amount of tension is induced. A backup wrench prevents the other element from rotating.
- Calibrated wrench method – Use a torque wrench calibrated against daily testing to apply a specific torque value. This value is derived from a representative bolt assembly under conditions similar to the job site. The method relies on regular calibration to account for variations in friction, lubrication and thread fit.
Applications of Structural Bolts
High-strength structural bolts are integral to diverse industries. They clamp together girders of skyscraper buildings and bridges, they clamp heavy machinery like bulldozers and clamp flanges in pipelines and petrochemical facilities. They can be found in shipbuilding, power plants, car assembly and aerospace gear, in any area where the safety of the product relies upon a strong mechanic bond. These bolts are loaded with a huge dynamic load and in most of the cases work under harsh environment conditions, it is essential to specify the appropriate grade and mode of installation.
Selecting the Right Structural Fastener
When choosing a structural fastener, consider the following factors:
- Application requirements – Determine whether the connection will experience primarily shear, tension or a combination of forces. Bridges and building frames often require slip‑critical connections that rely on friction, whereas machinery may need bolts designed to withstand dynamic loads.
- Material compatibility – Match the bolt material to the base materials to avoid galvanic corrosion or premature failure. For example, weathering steel bolts (Type 3 A325) are used with weathering steel components.
- Size and thread- Check that the diameter, length and thread pitch are as per specifications. The assembly should be finished with the appropriate heavy hex nuts and washers.
- Strength and grade – ASTM and SAE grade Compare the grades of bolts to choose one with the right tensile and yield strength. For very high loads, consider Grade 9 bolts; for general building work, A325 bolts or Grade 5 equivalents may suffice.
- Standards compliance – Determine whether the specifications of the project require ASTM F3125, A325, A490 or any other standards and make sure that all the issued fasteners are marked and identifiable.
In some cases, bespoke fasteners may be necessary. Experienced manufacturers like Jade Alloys can produce custom high‑strength bolts tailored to your project’s material, threading and shape requirements.
Conclusion
Jade Alloys supplies a broad range of structural bolts, high-strength bolts, nuts and washers that meet ASTM standards. The company stocks A325 bolts and A490 bolts in various diameters, along with tension control bolts and heavy hex fasteners. All products comply with current ASTM standards for bolts, and material test certificates are available upon request.
Contact the Jade Alloys team on product selection, stock level and torque. Their experienced personnel can help them in selecting materials, sizing and compliance forms to ensure their next project runs smoothly.
