Structural bolts: grades, sets and tightening

Property classes

Metric bolts are marked with a property class such as 4.6, 8.8 or 10.9. The first number × 100 is the nominal tensile strength in MPa; the first number × the second × 10 is the nominal yield strength. So an 8.8 bolt has about 800 MPa tensile and 640 MPa yield strength; a 10.9 bolt about 1,000 and 900 MPa. Nuts are marked with a matching class (8 or 10).

In American practice, structural bolts are specified to ASTM F3125, with grades A325 (≈ 120 ksi tensile) and A490 (≈ 150 ksi tensile).

Snug-tight or preloaded?

• Non-preloaded (snug-tight) bolts are tightened by the full effort of a worker with a normal spanner. They transfer load by bearing and shear, and are used in most PEB end-plate and purlin connections.
• Preloaded bolts (e.g. EN 14399 HR / HV systems) are tightened to a specified preload by the torque, combined or turn-of-nut method, so that the joint resists slip by friction or carries fatigue loads. They are supplied as matched sets (bolt, nut, washers) and must not be mixed between manufacturers.

How bolts are written

A typical bolt line reads M20 × 60 – 8.8 – HDG: diameter 20 mm, length 60 mm under the head, class 8.8, hot-dip galvanised. The required length depends on the grip (plates being joined), the washers and the nut, plus a few threads protruding.

Bolts in the BOM

Tekla models bolts as assemblies of bolt, nuts and washers. PEB Studio lists them by standard, size, length and grade, with the quantity per assembly, so purchasing can order sets directly and packing can control them by unit, limited to the BOM quantity plus an allowance.