Light steel framed walls and floors

6.10

6.10.4Load-bearing walls and floors

Walls and floors shall be designed to support and transfer loads safely and without undue movement. Issues to be taken into account include:

  1. structural walls
  2. resistance to racking forces
  3. structural floors.

Structural walls

The structural design of the building should ensure adequate resistance to loadings including dead loads, imposed loads, wind loads and snow loads, in accordance with:

  • BS EN 1991-1-1
  • BS EN 1991-1-3
  • BS EN 1991-1-4.

Individual studs should generally:

  • be a minimum of 36mm wide
  • have a maximum spacing of 600mm.

Alternative stud arrangements should be agreed with NHBC.

Lintels should be:

  • provided to any opening in load-bearing panels where one or more studs is cut or displaced to form the opening, but are not required where an opening falls between studs
  • securely fixed to supporting studs to ensure that loads are fully transferred.

At openings, additional studs may be required to provide support or fixing points for cladding, and wall linings.

Multiple studs should be included to support multiple joists, unless otherwise specified by the designer.

Where panels are diagonally braced with a flat strip, the brace should be fixed to each stud at the intersection to minimise bowing in the bracing member.

Account should be taken of uplift forces, and proper holding-down devices should be provided to resist uplift where necessary.
The anchorage for holding-down devices should have sufficient mass to resist the uplift forces (See Clause 6.10.15).

Timber wall plates should be:

  • fixed to the head rail of wall panels onto which timber roof trusses bear
  • sized (including the head rail), to permit single timber trusses to be positioned at any point between studs.

Movement joints should:

  • prevent load transfer onto a chimney or flue (a joint should be constructed between the frame and any chimney or flue)
  • not be bridged by non load-bearing walls.

Resistance to racking forces

Methods to resist racking forces should comply with:

  • design to BS EN 1993-1-1, or
  • be tested to BS EN 594.

Plywood sheathing should:

  • have performance characteristics determined in accordance with BS EN 13986 Table 7
  • be suitable for use in humid conditions to BS EN 636
  • be at least 5.5mm thick
  • be appropriate to the exposure of the building.

Cement bonded particle board sheathing should be in accordance with BS EN 634 and BS EN 13986.
Oriented strand board should be OSB3 to BS EN 300 and have a minimum thickness of 8mm. Proprietary sheathing materials should be in accordance with Technical Requirement R3.

Wall panels may provide resistance to racking forces using one or more of the following techniques:

  • internal bracing
  • crossed flat bracing
  • internal sheathing board
  • external lining board
  • rigid frame action.

Structural floors

Floors should:

  • be designed to resist loading in accordance with BS EN 1991-1-1
  • have a maximum joist spacing of 600mm centres
  • have suitably sized trimmers around floor openings.

Light steel joists should be fixed to supporting walls by either:

  • web cleats
  • direct attachment to wall studs, or
  • bearing onto the supporting structure (bearing stiffeners may be required).

The in-service performance of light steel joists should be controlled by the following four serviceability criteria.

Static criteria for the maximum permissible deflection of a single joist due to:

  • imposed load, limited to (span/450).
  • dead and imposed loads, limited to the lesser of (span/350) or 15mm.

Dynamic criteria:

  • The natural frequency of the floor should be limited to 8Hz for dead load plus 0.2 x imposed load. This can be achieved by limiting the deflection of a single joist to 5mm for the given loading.
  • The deflection of the floor (i.e. a series of joists plus the floor decking) when subject to a 1kN point load should be limited to the values in Table 2.

Table 2: Deflection with point loads of 1kN

Span (m)Maximum deflection (mm)
3.51.7
3.81.6
4.21.5
4.61.4
5.31.3
6.21.2

The deflection of a single joist is dependent on the:

  • overall floor construction
  • number of effective joists that are deemed to share the applied 1kN point load (typical values are given in Table 3).

Table 3: Typical values

Floor configurationNumber of effective joists: 400mm joist centres 600mm joist centres
Chipboard, plywood or oriented strand board 2.52.35
Built-up acoustic floor43.5

Ground floor construction

Provision should be made to prevent ground moisture affecting light steel floors. This can be achieved by either:

  • 50mm concrete or 50mm fine aggregate on a polyethylene membrane laid on 50mm sand blinding, or
  • 100mm concrete.

Where necessary, oversite concrete should be protected against sulfate attack by the use of a polyethylene sheet DPM, not less than 1200 gauge (0.3mm) or 1000 gauge where assessed in accordance with Technical Requirement R3, properly lapped.

Floors should have a 150mm minimum void below the floor which is ventilated by:

  • openings on at least two opposite sides
  • 1500mm2 per metre run of external wall or 500mm2 per m2 of floor area (whichever provides the largest area).

Where there is shrinkable soil, heave can occur. The minimum underfloor void ventilation requirement should be increased as follows:

  • High potential – 150mm (300mm total)
  • Medium potential – 100mm (250mm total)
  • Low potential – 50mm (200mm total)