Light steel framing

6.10

6.10.4Structural design of load-bearing floors and walls

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

  1. structural floors
  2. structural walls
  3. overall stability.

Structural floors

Floors should:

  • „be of the correct type
  • „be fitted in the specified location
  • „have suitably sized trimmers around floor openings
  • „have a typical maximum joist spacing of 600mm, although greater spacings may be applied when designed by an engineer in accordance with Technical Requirement R5 or covered by an SCI system-specific Stage 1 assessment with the project-specific application reviewed and checked by an NHBC registered LSF certifier.

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

  • „web cleats
  • „‘Z’ or ‘L’ hangers
  • „a track connection
  • „direct attachment to wall studs, or
  • „bearing onto the supporting structure (bearing stiffeners may be required).

Joist support cleats should:

  • „be of the correct type
  • „be fitted in the specified location
  • „use fixings as specified in the design.

Where required, web stiffeners should be properly fitted.

Where joists are fitted directly to light steel wall studs, pre-drilled holes should be correctly aligned before making the final connection.

Fixing holes should not be enlarged, and additional holes should not be cut without prior approval of the designer.

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

Light steel ground floor construction

Provision should be made to prevent ground moisture affecting light steel floors. This can be achieved by covering the ground below the floor with either:

  • „50mm oversite concrete or 50mm fine aggregate on 1200 gauge (0.3mm thick) polyethylene membrane laid on 50mm sand blinding, or
  • 100mm oversite concrete on a compacted clean, inert hardcore bed. Where necessary, this concrete should be protected against sulfate attack by the use of a lapped polyethylene DPM, not less than 1200 gauge (0.3mm thick) or 1000 gauge where assessed in accordance with Technical Requirement R3.

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).

See Chapter 4.2 ‘Building near trees’ for definitions of high, medium and low volume change potential.

For concrete ground floors refer to Chapters 5.1 ‘Substructure and ground-bearing floors’ and 5.2 ‘Suspended ground floors’.

Concrete upper floors

Concrete floors may be used with LSF and may be constructed using either thin precast units or in-situ concrete placed on steel decking. The deflection of simply supported composite floors should be limited to take account of the long-term effects of creep and shrinkage. Composite floors should be appropriately propped until the concrete reaches the required strength and should not be overloaded during construction. Guidance can be found in Section 6.3 of SCI publication P402 ‘Light steel framing in residential construction’.

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.

Further guidance on deflection limits can be found in SCI guidance P402 ‘Light steel framing in residential construction’.

Individual studs should generally:

  • „be sized to meet structural requirements, allowing for board fixings at joints and construction tolerances
  • „have a maximum spacing of 600mm
  • consider deflection if not designed to carry vertical loading from the primary structure.

Alternative stud arrangements should be agreed with NHBC.

Lintels, including trussed 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 wall ties, 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. Alternatively, bracing may be tensioned using alternative methods where included in the scope of the Stage 1 certification.

Appropriate 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 6.10.10).

Where roof trusses sit directly on a top track, the design should consider all loads, such as:

  • „wind uplift
  • „lateral support
  • „vertical loading (assuming that trusses may be offset from studs).

Where included in the design, 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.

Allowance for movement, including at openings and penetrations, should:

  • „prevent load transfer onto services or flues
  • „consider elastic shortening of the LSF and movement potential of any panels, cladding or boards
  • be fully coordinated with the whole building design.

Overall stability

Methods to provide overall stability should either:

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

Wall panels may provide stability using one or more of the following techniques:

  • „internal bracing
  • „crossed flat bracing
  • external sheathing board in accordance with Clause 6.10.20
  • rigid frame action.

Internal lining boards can be used where demonstrated to be suitable for the purpose.