Page 17 - Caswell Firesafe Manual 2018
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Factors affecting Bearer specification
It is essential to take the progressive weakening of the steel drop rods and bearers into consideration when
using a fire resisting ductwork system. The criteria used to calculate the support system is based upon the
following maximum stress levels for unprotected steel.
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Fire Duration Max Tensile Stress σ (N/mm ) Approx TemperatureºC
30 Minutes 30N/mm 2 842ºC
60 Minutes 15N/mm 2 945ºC
120 Minutes 10N/mm 2 1049ºC
240 Minutes 6N/mm 2 1153ºC
On horizontal ductwork it is also necessary to consider the distance between supports. DW144 stipulates
support centres should be a maximum of 3m for standard ductwork however all CASWELL FIRESAFE ®
support systems are based on 1.5m maximum centres for horizontal bearers. This helps to guard against
possible sagging of the ductwork between the supports at elevated temperatures.
Vertical ducts are affected differently as the ductwork is in compression rather than tension. The maximum
support centres for vertical (riser) ducts is 5m or 8x the smallest side of the duct. A riser duct of 600mm x
300mm in section would therefore need supports at 2.4m maximum centres to reduce the buckling of the
duct under fire conditions.
Riser supports and non-standard horizontal supports are calculated on a project basis as it is necessary to
establish the exact installation detail in advance and it is unlikely that any two riser installations would be
identical.
The weight of each ductwork specification is carefully calculated and then cross-checked by manufacturing
and weighing actual sections of duct. The weight is included in the calculation to establish a suitable bearer
and drop rod size. The weight of the selected bearer/rods themselves are then added to the equation to
obtain a final specification.
In principle, support systems are based on a number of known factors such as the rating of the system, the
physical duct weight (load) and the location of the drop rods in relation to the side of the duct (span).
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