Ceilings

'Flat' ceilings and ceiling diaphragms

One of the common practices now used to achieve a high quality level ceiling is to suspend the ceiling using furring channels 'clipped' to the bottom chords of trusses or to ceiling joists, or to use a full suspended ceiling system.

These systems offer the advantages of overcoming differential levels in truss cambers and bottom chord levels, but do present other issues that need to be considered by designers and certifiers including bottom chord restraint to trusses, and adequacy of ceiling diaphragms to transfer racking forces.

Lateral Restraint

For the design of trusses, one of the key design inputs is spacing of bottom chord restraints. For direct fixed ceilings, or where ceiling battens are direct fixed (nailed or screwed to bottom chords), it would be usual to assume bottom chord restraint at 450 - 600 mm centres. This assumption will not be valid where clipped or suspended ceilings are used. For clipped or suspended ceilings, a more appropriate assumption for bottom chord lateral restraint would be the spacing of the actual bottom chord ties (restraints), as input by the truss detailer. (These may be at bottom chord panel points or at 3.0 m maximum centres, if required to act as binders as well. (Refer AS 1684)).

Ceiling Diaphragms

As was the case with the old Tradac manuals, and whilst not explicitly stated, the underlying assumption for the bracing wall spacing requirements given in the current AS 1684, is that ceiling linings are direct fixed to bottom chords/ceiling joists or fixed to ceiling battens, which in turn are direct fixed to bottom chords/ceiling joists with nails or screws.

The ability of suspended ceiling systems to act as diaphragms, and to be able to adequately distribute racking forces to bracing walls, has been questioned by the Cyclone Testing Station. Unless appropriate engineering design and certification is provided to justify the use of suspended ceilings to act as diaphragms, it is recommended that alternative bracing systems be designed and incorporated to provide the necessary transfer of racking forces from the roof structure to the bracing walls.

Alternative systems could include:-

• crossed metal strap bracing attached to the upper or lower side of bottom chords, or

• structural sheet bracing fixed to bottom chords, or

• horizontal wind girders (horizontal trusses or beams).