BS 476 Part 20

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Summary of BS 476: Part 20: 1987 Method for determination of the fire resistance elements of construction (general principle)


PurposeThe objective of determining fire resistance, as described in this Part and the associated Parts of BS 476, is to assess the behaviour of a specimen of an element of building construction when subjected to defined heating and pressure conditions. The method provides a means of quantifying the ability of an element to withstand exposure to high temperatures, by setting criteria by which the loadbearing capacity, the fire containment (integrity) and the thermal transmittance (insulation) functions can be adjudges.

BS 476: Part 20 provides guidance on the general principles of the test method for fire resistance and should be used in conjunction with BS 476: Part 21 to 23.

Test specimenThe test specimen and any associated construction forming a test construction are identical to, or representative of, the element of building construction that is to be evaluated.

For non-separating elements, separating elements that are only required to resist fire from one side in use, and for vertical symmetrical separating elements, only one specimen is tested. For vertical asymmetrical separating elements that are required to resists fire from either side, the element is tested from each side using a separate specimen.

Wherever possible, test specimens are full-sized elements of building construction and unless it is impossible, the individual components making up a test specimen are also full-sized. Where the above is not possible, the minimum size of the elements shall be:

Non-separating elements:
Vertical:
3 m high
Horizontal:
4 m high

Separating elements:
Vertical:
3 m high by 3 m wide
Horizontal:
4 m span by 3 m wide

For non-loadbearing elements, the test construction is mounted as in practice. For loadbearing elements, the test loading is selected to represent the condition applicable to the element in practice.

ApparatusThis consists essentially of:

1.
A furnace of a design appropriate to the type of specimen being tested to allow:
a.
vertical separating elements to be exposed to the heating and pressure conditions on one face.
b.
Horizontal separating elements to be exposed to heating and pressure conditions from the undersides.
c.
Beams normally to be exposed to the heating and pressure conditions on three faces, i.e. the soffit and two sides, but provision should be made to enable beams to be tested with all four faces exposed.
d.
Free standing columns to be exposed to the heating and pressure conditions equally on all four faces. The furnace is fuelled by either natural gas or LP Gas and has thermocouples linked to associated temperature measuring devices for determining the temperature at specific positions. A pressure sensing probe is also sited within the furnace.
2.
Loading equipment. A test loading is applied to the specimen either by dead weights or by a hydraulic or mechanical jacking system, or by a combination of both.
3.
Specimen support systems to apply restraint at the edges or ends of the specimen.
4.
Monitoring equipment consisting if fixed specimen surface thermocouples, a roving surface thermocouple, a cotton pad for monitoring permeability, gap gauges for monitoring impermeability, deformation measuring devices and a radiometer.

MethodA representative sample of the element is exposed to a specified regime of heating under positve pressure and the performance of the test sample is monitored on the basis of criteria described in the Standard. Fire resistance of the test element is expressed as the time for which the appropriate criteria have been satisfied. The times so obtained are a measure of the adequacy of the construction in a fire but have no direct relationship with the duration of a real fire.
Performance criteriaThe fire resistance of the test construction is assessed against one or more of the criteria for loadbearing capacity, integrity and insulation, whichever are relevant to the elements used in practice.

1.
Loadbearing capacity – failure is deemed to have occurred when the specimen fails to support the test loading.

2.
Integrity – failure is deem to occur when collapse or sustained flaming on the unexposed face occurs, or the criteria given for impermeability are exceeded.

3.
Impermeability – failure has occurred when flames and or hot gases cause flaming or glowing of a cotton pad, or a 6 mm diameter gap is formed with a length greater than 150 mm or a 25 mm diameter gap is formed.

4.
Insulation – failure has occurred if:
a.
the mean unexposed face temperature increase by more than 140°C above its initial value, or
b.
the temperature recorded at any position on the unexposed face is more than 180°C above the initial mean temperature.

The standardized temperature / time conditions used I this evaluation are representative of only one possible fire exposure condition at the fully developed fire stage and the method does not quantify the behaviour of an element, for a precise period of time, in a real fire situation. In addition, due to restrictions of size and the absence of surrounding construction, the laboratory test cannot reproduce the actual behaviour pattern of an element in a fire. However, test data can provide a basis for making engineering evaluations.