Steel Structure Bolt Grades 8.8 and 10.9 Explained

Bolt head markings such as 8.8 or 10.9 are a compact specification. They are also frequently misread as a size or a torque value.

Reading the marking

The first number is roughly one tenth of the nominal tensile strength in MPa. The second number is the ratio of yield strength to tensile strength, expressed in tenths.

• 8.8 — nominal tensile strength around 800 MPa, yield at about 80% of tensile.
• 10.9 — nominal tensile strength around 1000 MPa, yield at about 90% of tensile.

So a higher second number means the bolt keeps more of its strength before yielding, not that it is a different diameter.

Why the grade matters in a steel frame

Connections are designed around a bolt grade. Substituting a lower grade changes the capacity of the connection; substituting a higher grade without checking can change the failure mode and the required tightening procedure.

Two practical consequences:

1. The bolt, nut and washer must match. A 10.9 bolt with an unsuitable nut does not give a 10.9 connection.
2. Tightening method follows the grade and the connection type. Bearing connections and slip-critical connections are not tightened to the same criterion.

Grades and structural bolt sets

Structural bolt sets are normally supplied as a matched assembly. For high-strength structural bolting, the set typically includes the bolt, a nut of matching class and hardened washers.

Mixing components from different batches or suppliers is a common cause of inconsistent tightening results, even when each part is individually correct.

What the marking does not tell you

• It does not define the dimensions. Diameter, length, thread pitch and head geometry come from the dimensional standard or the drawing.
• It does not define the coating. A 10.9 bolt can be black, plated or galvanized. Coating affects friction and therefore tightening.
• It does not define the tightening torque. Torque depends on the connection design, the coating and the lubrication condition.

Procurement checklist

When ordering structural bolts, state:

1. Bolt grade and the standard the dimensions follow.
2. Diameter, length and thread length — or the grip length and plate thicknesses.
3. Whether a matched nut and washer set is required.
4. Coating and its specification.
5. Any test certificate or marking requirement.
6. Packing by connection mark, if site issue is by drawing.

A caution on certificates

A certificate should describe the properties that were actually tested for that batch. If a document cannot be produced for the specific size and grade, the correct response is to say so — not to issue a generic sheet. Buyers should check that the certificate references the batch they received.

Grade markings are a starting point for verification, not proof on their own.