BS 476-20:1987 pdf free download.Fire tests on building materials and structures Part 20: Method for determination of the fire resistance of elements of construction (general principles).
0 Introduction
The 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 adjudged.
However, the standardized temperature/time conditions used in 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 (see A.1) and Appendix D. The principle of the method is as follows.
A representative sample of the element is exposed to a specified regime of heating 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.
The test data can be used directly to show compliance with fire resistance requirements in regulations, byelaws or other safety specifications. The tests can also be used to study the behaviour of constructions at high temperatures and obtain guidance on the effect of design features on fire resistance. It is intended to extend the application of test data by agreed procedures for interpolation and extrapolation. 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. A future Part of the standard will address itself to these aspects.
1 Scope
This Part of BS 476 describes a procedure for a laboratory test for the determination of fire resistance of elements of construction.
NOTE The titles of the pub1ication referred to in BS 476-20 are li8ted on the inside back cover.
c) measurements shall be made in essentially draught free conditions.
3.3.3 For tests on separating elements where the unexposed face temperature is only being determined for other reasons, e.g. for determining the time at which to discontinue the use of the cotton pad (see 10.3.2), the conditions a) and c) specified in 3.3.2 shall be complied with but the temperature rise in b) is permitted to increase by 15°C.
3.4 Loss of accuracy
Where the mean furnace temperature, pressure conditions and ambient conditions in the laboratory create more severe conditions than those specified in 3.1, 3.2 and]or 3.3, the actual test result achieved may still be accepted as valid (see items g), k) and m) of clause 12 and A.2.7).
4 Test specimen
4.1 General
The test specimen and any associated construction forming a test construction shall be identical to, or representative of, the element of building construction that is to be evaluated. The specimen and any associated construction shall not be in such a state at the start of the test that the specimen would fail the permeability criteria as defined by the use of the gap gauges [see 10.3.2 b)1.
NOTE The choice of the associated construction may be significant to the performance of the test specimen, and reference should be made to the guidance notes given in the appendices of BS 476.21 to BS 476.23 relevant to the element under test.
The test construction only represents the element of construction when the conditions of load. restraint and fixity used during the test are identical to, or typical of, those used in practice. If information is required to determine the behaviour of the construction under different end or edge conditions or to determine the effect of special aspects of the construction, which are not an essential or integral part of the element, then additional test(s) shall be carried out.
4.2 Design considerations
The test construction shall he designed so that the critical aspects of the element are incorporated in the specimen (see A.3.1). In designing a specimen and its associated construction the influence of joints, incorporated services, finishes, openings for access and vision, etc. shall be considered. Unless evidence exists indicating that these features will not have a deleterious effect, they shall be incorporated in the construction to be tested.
6.4 Instrumentation
6.4.1 Measuring and control equipment, to monitor the test construction, the laboratory and the furnace environment and the behaviour of the test sample during the test.
NOTE Details of the instrumentation needed for the measurement of temperature. pressure and load are given in Appendix C.
6.4.2 Monitoring equipment
6.4.2.1 Fixed specimen surface thermocouples, constructed as shown in Figure 3, and consisting of either type T, type N or type K thermocouple wires complying with BS 4937-4 or BS 4937-5, with a maximum diameter of 0.5 mm, brazed to a 12 mm diameter, 0.2 mm thick copper disc. Each thermocouple shall be provided with a 30 mm square, 2 ± 0.5 mm thick pad of insulation material. This insulation material shall have a dry density of 900 ± 90 kg/rn2, a thermal conductivity of 0.13 W/(rn ‘K)± 10% at 100 °C, and shall be of a material capable of withstanding temperatures of 400 °C without any change in mechanical or physical properties. The insulation pads shall be slotted or provided with holes to accommodate the thermocouple wires.
The wires from the thermocouples shall be connected either directly or via compensating leads to an appropriate temperature indicator (see C.2.2.4).
6.4.2.2 Roving surface thermocouple, with a measurement junction consisting of either type T, type N or type K thermocouple wires complying with BS 4937-4 or BS 4937-5, with a diameter of 1.0 mm, brazed to a 12 mm diameter, 0.5 mm thick copper disc (a copper/constantan wire thermocouple with a diameter of 0.5 mm may also be used). The wires of the thermocouple shall be bent normal to the rear face of the copper disc to allow them to be inserted into separate holes in a porcelain insulator as shown in Figure 4. The porcelain insulator shall have a diameter of 6 mm to 10 mm and shall contain two holes of approximate 2 mm diameter. The distance between the rear face of the copper disc and the end of the insulator shall be 6± 1 mm.
The wires from the thermocouple shall be connected to an appropriate temperature indicator (see C.2.2.5).
6.4.2.3 Cotton pad, used for monitoring of permeability and consisting of new, undyed and soft cotton fibres, without any admixtures of man-made fibres, and shall be 100 mm square by 20 mm thick and shall have a mass of between 3 g and 4 g. It shall be dried in an oven at 105 ± 5 °C for at least 30 mm and then stored in a desiccator to cool and until ready for use.
m) Temperature!time graphs of the furnace heating conditions (see A.2.3).
n) The fire resistance when adjudged by the appropriate criteria as given in clause 10. and expressed as specified in clause 11, including:
1) the rate of deflection when this is the criterion used to assess loadbearing capacity, including the value of d used in calculating the limiting rate;
2) the maximum deflection and the time and position at which it occurred, supported by adequate graphical data;
3) the mode of failure with respect to all criteria, but especially in respect to loss of loadbearing capacity;
4) temperature/time graphs of unexposed face
temperatures;
5) the position(s) at which the maximum
temperature rise was measured should this be
the cause of failure with respect to insulation;
6) the time at which the cotton fibre pad was
discontinued in favour of gap gauges;
7) if recorded, the irradiance with respect to
time with details of positioning/screening of
radiometers (see A.9.4);
8) if recorded, the temperature of any internal
components or cavities;
When a test is terminated before the occurrence of failure under any of the relevant criteria, this shall be stated.
o) The following statement:
“The results only relate to the behaviour of the specimen of the element of construction under the particular conditions of test; they are not intended to be the sole criteria for assessing the potential fire performance of the element in use nor do they reflect the actual behaviour in fires.”