What is the difference between Grade 8.8 and 10.9 bolts?

Grade 8.8 is the most common high-tensile structural bolt in the UK, offering a tensile strength of 800 N/mm². Grade 10.9 bolts are even stronger, with a tensile strength of 1000 N/mm², and are typically used in more demanding structural connections or heavy industrial projects.

Why is bolt torque important in steel structures?

Torque is used to apply a specific 'preload' (clamping force) to the connection. In many structural joints, the resistance to movement comes from the friction between the steel plates, which is created by the clamping force of the tightened bolts. This is known as a High-Strength Friction Grip (HSFG) connection.

How can I identify the grade of a bolt?

Structural bolts are marked on the head. You will see the manufacturer's identification mark along with the numerical grade (e.g., '8.8' or '10.9'). If a bolt has no markings, it should never be used in a structural application.

Should I use washers on both sides of a bolt?

Standard UK practice for structural connections often requires a hardened washer under the element being turned (usually the nut). However, many engineers specify washers on both the head and nut sides to prevent the steel from being marred and to ensure even distribution of the clamping force.

What is the difference between Grade 8.8 and 10.9 bolts?

Grade 8.8 is the most common high-tensile structural bolt in the UK, offering a tensile strength of 800 N/mm². Grade 10.9 bolts are even stronger, with a tensile strength of 1000 N/mm², and are typically used in more demanding structural connections or heavy industrial projects.

Why is bolt torque important in steel structures?

Torque is used to apply a specific 'preload' (clamping force) to the connection. In many structural joints, the resistance to movement comes from the friction between the steel plates, which is created by the clamping force of the tightened bolts. This is known as a High-Strength Friction Grip (HSFG) connection.

How can I identify the grade of a bolt?

Structural bolts are marked on the head. You will see the manufacturer's identification mark along with the numerical grade (e.g., '8.8' or '10.9'). If a bolt has no markings, it should never be used in a structural application.

Should I use washers on both sides of a bolt?

Standard UK practice for structural connections often requires a hardened washer under the element being turned (usually the nut). However, many engineers specify washers on both the head and nut sides to prevent the steel from being marred and to ensure even distribution of the clamping force.

Structural Bolt Calculator

Torque settings and weight takeoff for steel connections.

Bolt Size (Metric)

Bolt Grade

Quantity Required

Fastener Spec

Tightening Tip: Always use a calibrated **Torque Wrench** for structural steel connections. Overtightening can cause the bolt to stretch and fail, while undertightening can allow the connection to slip under wind or vibration loads.

Fastening the Future: Structural Bolt Standards

Tension & Friction

In structural steelwork, bolts do more than just hold pieces together—they create the clamping force necessary for safe load transfer. Whether you are installing a single RSJ or an entire portal frame, using the correct bolt grade and torque is vital.

Our **structural bolt calculator** provides the recommended torque settings for standard UK high-tensile bolts, along with weight estimates for shipping and logistics planning.

Torque Guide (Nm)

M12 (Grade 8.8)85 Nm

M16 (Grade 8.8)210 Nm

M20 (Grade 8.8)410 Nm

Snug-Tight vs Fully Tightened

Most structural connections are first brought to a **'snug-tight'** condition. This is defined as the tightness achieved by a person using a standard spanner or a few impacts from an impact wrench.

Final tightening to the specified torque should only be done once the entire structure is aligned and plumb. This prevents the building from being 'locked' into an out-of-square position.

Thread Lubrication

Structural bolts are usually supplied with a light coating of oil. Do not clean this off, as it ensures the relationship between torque and tension remains consistent. If a bolt is rusty or dry, the friction in the threads will cause your torque wrench to 'click' before the correct clamping force is reached.

Pre-loaded HSFG

High-Strength Friction Grip (HSFG) bolts are a specialized type of fastener used in critical joints. These require even higher torque levels and must be installed according to BS EN 1090-2 execution standards to ensure the joint never slips.

Pro Tip: Bolt Length

Always ensure your bolts are long enough to show at least **two full threads** beyond the nut once tightened. Conversely, ensure the bolt isn't so long that the nut runs out of thread ('bottoms out') before the connection is tight.

Bolts & Fasteners FAQ

Project Disclaimer

Torque values provided are for estimation based on standard UK industry averages for Grade 8.8 and 10.9 fasteners in 'as-supplied' condition. The specific torque and bolt specification for any structural connection must be provided by a qualified structural engineer.