Structural Bolts Guide – Standards, Types, Installation & Specifications

What Are Structural Bolts?

Structural bolts are high-strength fasteners specifically designed for use in steel construction and structural connections. They are engineered to withstand heavy loads while maintaining consistent clamping force between steel components.

Unlike standard bolts, structural bolts are manufactured to strict engineering standards that define:

• mechanical strength

• dimensional tolerances

• installation procedures

• coating and corrosion resistance

Structural bolts are commonly used in:

• steel buildings

• bridges

• mining infrastructure

• heavy equipment frames

• large industrial structures

These bolts play a critical role in maintaining the integrity and safety of structural steel assemblies.

Structural Bolt Standards

Structural bolts are manufactured according to recognised international standards.

The most common include:

In Australia, AS/NZS 1252 structural bolts are widely used for structural steel assemblies.

Types of Structural Bolts

Structural bolts are available in several types depending on the application and installation method.

High Strength Friction Grip (HSFG) Bolts

HSFG bolts are designed so that load is transferred through friction between steel plates, rather than direct shear on the bolt.

Key characteristics include:

• high tensile strength

• controlled tightening procedures

• reliable clamping force

These bolts are widely used in bridges and structural steel connections.

Bearing-Type Structural Bolts

Bearing bolts transfer load through direct shear between the bolt and connected plates.

They are commonly used in structural assemblies where friction grip connections are not required.

Structural Bolt Grades

Structural bolts are manufactured in high strength grades.

These high tensile values allow structural bolts to withstand heavy loads in construction applications.

Structural Bolt Installation Methods

Correct installation is essential to ensure structural bolts achieve the required preload tension.

Common installation methods include:

Turn-of-Nut Method

The bolt is tightened to a snug condition, then rotated by a specified angle to achieve the required tension.

Calibrated Wrench Method

A torque wrench is used to apply a predetermined torque value based on bolt diameter and grade.

Tension Control Bolts

These bolts have a splined end that breaks off when the correct tension is reached.

Structural Bolt Preloading

Preloading is the process of tightening a bolt so it develops a controlled clamping force between connected plates.

Proper preload:

• increases friction between plates

• reduces movement in the joint

• prevents fatigue failure

Structural bolts typically achieve preload values of 70–90% of the bolt's yield strength.

Structural Bolt Dimensions

Structural bolts are manufactured in a range of diameters depending on the structural requirements.

Common sizes include:

Bolt length varies depending on the thickness of connected steel plates and washers.

Coatings and Corrosion Protection

Structural bolts may be supplied with different coatings depending on environmental conditions.

Common finishes include:

Hot Dip Galvanised

Provides strong corrosion protection for outdoor structural applications.

Plain Black

Typically used for indoor steel structures.

Zinc Plated

Provides moderate corrosion resistance but is less common for heavy structural applications.

Structural Bolt Applications

Structural bolts are essential components in many heavy engineering projects.

Common applications include:

Steel Buildings

Structural bolts connect beams, columns and bracing members.

Bridges

HSFG bolts are widely used in bridge structures to ensure high strength friction connections.

Industrial Structures

Mining plants, conveyor systems and heavy equipment frames use structural bolt assemblies.

Infrastructure Projects

Structural bolts are used in large infrastructure such as towers, stadiums and transport facilities.

Structural Bolts vs High Tensile Bolts

Although the terms are sometimes used interchangeably, structural bolts are specifically designed for steel construction connections.

Structural bolts follow specific installation and inspection procedures required in construction standards.

Frequently Asked Questions

What is a structural bolt?

A structural bolt is a high strength fastener used to connect structural steel components in construction projects.

What is the difference between A325 and A490 bolts?

A490 bolts have higher tensile strength than A325 bolts and are typically used in more demanding structural applications.

What is an HSFG bolt?

HSFG (High Strength Friction Grip) bolts transfer load through friction between steel plates rather than direct shear on the bolt.

Structural Bolt Products

KonnectDirect supplies a range of high strength fasteners suitable for structural and industrial applications, including:

• high tensile bolts

• hex structural bolts

• washers and nuts

• heavy duty fastening systems

All products meet relevant engineering standards for industrial and construction use.

Structural Bolt Inspection Requirements

Structural steel connections must be inspected to ensure that bolts are installed and tensioned correctly. Inspection requirements are typically defined in structural standards such as AS/NZS 5131, ASTM F3125, and AISC steel construction specifications.

Inspection procedures usually include:

Visual Inspection

Inspectors confirm that:

• the correct bolt type is installed

• washers are present where required

• bolts are correctly oriented

• nuts are properly engaged

Tension Verification

Bolt tension may be verified using:

• calibrated torque wrench testing

• direct tension indicators (DTI washers)

• tension control bolt indicators

Connection Verification

Structural inspectors confirm that:

• bolts are tightened to the specified preload

• connection plates are properly aligned

• there is no visible thread damage or distortion

Proper inspection ensures the connection meets structural safety requirements.

Structural Bolt Preload and Tension Values

Structural bolts are tightened to achieve a specific preload force, which clamps structural steel plates together.

Typical minimum bolt tensions for structural bolts are shown below.

These values ensure that the bolt develops sufficient clamping force to resist slip and maintain structural integrity.

Actual tension requirements may vary depending on the structural standard and bolt grade used.

Download our Structural Bolt Tension Chart.

Structural Bolt Installation Procedure

Structural bolts must be installed using approved procedures to ensure the required preload is achieved.

Typical installation steps include:

Step 1 – Fit the Bolt Assembly

Insert the bolt through the connected steel plates with the required washers and nut.

Step 2 – Snug Tightening

Tighten the bolt to a snug condition so that all steel plates are firmly in contact.

Step 3 – Final Tightening

Apply final tension using one of the following methods:

• turn-of-nut method

• calibrated torque wrench

• tension control bolt system

Step 4 – Inspection

Verify that the connection meets the specified tension requirements and inspection criteria.

Structural Bolt Washer Requirements

Washers are often used with structural bolts to distribute load and prevent damage to the steel surface.

Common washer types include:

Hardened Washers

Used to prevent embedding of the bolt head or nut into the steel plate.

Direct Tension Indicator Washers

These washers compress under load to visually indicate when the correct bolt tension has been achieved.

Slip-Critical vs Bearing Connections

Structural bolts are commonly used in two types of structural connections.

Slip-Critical Connections

Load is transferred through friction between steel plates, generated by bolt tension.

These connections are typically used in:

• bridges

• seismic structures

• dynamic load environments

Bearing-Type Connections

Load is transferred through shear between the bolt and the plate.

These are commonly used in:

• general structural steel construction

• equipment supports

• industrial structures

Structural Bolt Hole Sizes

Bolt holes are typically slightly larger than the bolt diameter to allow installation.

Oversized holes may be used in some structural connections depending on design requirements.

Structural Bolt Torque Considerations

Although structural bolts are primarily tensioned rather than torqued, torque values are sometimes used during installation. 

Torque values vary depending on:

• bolt diameter

• lubrication

• coating type

• bolt grade

For accurate tightening values, refer to a Metric Bolt Torque Chart.