Light steel framed walls and floors

6.10

6.10.7Steel and fixings

Steel and fixings shall be suitable for the intended use. Issues to be taken into account include:

  1. steel grade
  2. protection against corrosion
  3. connections and fixings
  4. holes and notches.

Steel grade

Steel should be grade:

  • S280 to BS EN 10346
  • S350 to BS EN 10346, or
  • S390 steel with a nominal yield strength of 390N/mm2 (when compliant with BS EN 10346).

Protection against corrosion

Steel should be pre-galvanised in accordance with BS EN 10346 (minimum 275 g/m2 zinc coating). Joists should not be altered without the approval of the designer. Welded or flame cut edges should be cleaned and treated with a zinc-rich paint to prevent corrosion.

The junction between the ground floor joists and their support should be designed to maintain the durability of the floor. Light steel floor joists and ring beams in ground floors should be galvanised to 450g/m2. Alternatively, they can be galvanised to 275 g/m2 with additional protection of a two-coat bitumen based coating to BS 1070, BS 3416 or BS 6949, or have a two-coat liquid asphaltic composition applied. Ring beams to ground floors should be totally protected, and joists protected for 300mm adjacent to an external wall support or ring beam.

Connections and fixings

Where two metals are to be joined, they should either be:

  • compatible and not cause bimetallic corrosion, or
  • isolated from each other.

Connections should be:

  • properly installed
  • securely made by clinching, crimping or by one of the methods detailed in Table 4
  • justified in accordance with BS EN 1993-1-3 or a test method acceptable to NHBC.

Table 4: Types of connections

Type of connectionRelevant standard
CleatsBS EN 1993-1-1
Countersunk bolts (tightened to the correct torque)BS 4933
Rivets including self-piercing rivetsManufacturer’s recommendations
ScrewsBS 5427
Welded connectionsBS 5135 (using welding wire to BS 2901)
Zinc-plated boltsBS 4190

Holding-down devices

Holding-down devices should be suitable for the environment they will be exposed to, and manufactured from:

  • mild steel with zinc coating to BS 729 or BS 706
  • stainless steel to BS EN 10095 (suitable for most environments).

Holes and notches

Joists should not be altered without the approval of the designer. Drilling or punching through the web should be carried out within recognised limits.

Unstiffened holes through the web should:

  • have a minimum spacing equal to the depth of the member, and be a minimum of 1.5x the depth from the end of the member
  • have rectangular holes not exceeding 40% of the overall depth of the member; the length of the hole should not exceed 3x the depth of the hole
  • limit the diameter of circular holes to a maximum 60% of the depth of the member.

End notching of light steel joists may be required for the interconnection of trimming joists and should be in accordance with the design. Notches elsewhere in the span are not acceptable.
Also see Clause 6.10.19.