SAFETY & SECURITY 67 The reality of fire safety
The question for fire resistance in cavity closers is how to achieve tested ‘safer alignment and confinement’ onsite, says John Shillabeer from Cavity Trays
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uildings should be constructed to eliminate or minimise as much as possible, all potential fire and fire safety risks likely to arise. In theory this should be a fairly straightforward undertaking, but in practice the building envelope can present both designer and builder with considerations that are not always immediately obvious. Cavity wall construction relies on the external leaf (that can become saturated with rain) from not coming into contact with the inner leaf that is required to be kept dry. Wall ties are shaped to structurally link the skins and prevent rain tracking across to the inner leaf. The cavity successfully separates and isolates the two. But in the event of a fire, both air and fire can be drawn through the cavity within all the external walls of the building, fuelling combustion.
The question arises of whether there is an optimum way of protecting the external envelope at one of the most frequently occurring features – where it is punctuated by window and door openings? Such openings present the designer and contractor with the usual heat-loss and damp-transference considerations, but with multi-occupancy and numerous other structure categories, also the additional important requirement to protect against fire and fire transference.
Fire-resistant cavity closers Fire-resistant cavity closers are an accepted way of addressing window and door reveals. A typical closer contains a non- combustible thermal and fire-resistant core that straddles the cavity, effectively linking both skins. Such closers provide passive fire protection. Applied to all four sides of an opening, the completed installation provides a level of fire resistance, commonly 30 or 60 minutes. To establish the number of minutes, closers are subjected to testing at a fire research facility. These do a good job – one can accept they want to get it right, and accordingly best practice construction is witnessed. Products interface squarely,
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correctly and consistently. But if one looks at actual onsite building tolerances, they are different, prompting the question of whether currently-accepted site tolerances adversely affect the performance of fire- resistant cavity closing?
Are such tolerances compatible with those required with a fire-resistant closer, bearing in mind closer and masonry must interface consistently and without gaps, to perform? Construction tolerances accepted by various authoritative bodies vary, but in general building, the following are commonly witnessed;
(External skin) • ±8 mm maximum deviation in any length of wall up to 5 metres
• ±8 mm storey height (up to 3 metres) • 8 mm max, plumb in any storey up to 3 metres
• 4 mm max, external reveals over 1 metre
(Internal skin) • 3 mm max, out of level over 1.5 metres – 5 mm beyond 1.5 metres
• 3 mm max, reveal is permitted to be 3 mm out of vertical over 1.5 m – 5 mm beyond 1.5 metres.
• 5 mm out of square is permitted for reveals up to 250 mm deep.
Optimum performance of any fire- rated closer is dependent on correct
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The relationship between a fire resistant closer and masonry should match those under which the test results were achieve
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