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1、BRITISH STANDARD BS 5268-4.1: 1978 Incorporating Amendment Nos. 1 and 2 Structural use of timber Part 4: Fire resistance of timber structures Section 4.1: Recommendations for calculating fire resistance of timber members UDC 691.11:674.049.3:620.1.08 Licensed Copy: sheffieldun sheffieldun, na, Fri D
2、ec 01 14:02:00 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-4.1:1978 This British Standard, having been prepared under the direction of the Civil Engineering and Building Structures Standards Committee,was published under the authority of the Executive Board on 29 December 1978 BSI 03-1999 The
3、 following BSI references relate to the work on this standard: Committee reference CSB/32 Draft for comment 75/10262 ISBN 0 580 10384 6 Code Drafting Committee CSB/32 The structural use of timber ChairmanMr J G Sunley British Woodworking FederationMr B Perkins Mr B A Spurgeon Mr E C Ozelton Mr G H C
4、owley Department of the Environment: Building Research Establishment,Mr W T Curry Princes Risborough LaboratoryDr J W W Morgan Department of the Environment: Housing and ConstructionMr F A Lefever Department of the Environment (PSA)Mr R H Cutts Greater London CouncilMr J O A Korff Incorporated Assoc
5、iation of Architects and SurveyorsMr E Downes Mr D F Brough Institute of Wood ScienceMr I D G Lee Institution of Civil EngineersMr R F Marsh Institution of Structural EngineersMr P O Reece OBE Mr P J Steer Mr D W Gardner Dr L G Booth Mr J A Baird National Building AgencyMr R F Pontin National Federa
6、tion of Building Trade EmployersMr P B R Johnson National House-Building CouncilMr M J V Powell Royal Institution of Chartered SurveyorsMr D G Burcnam Timber Research and Development AssociationMr H J Burgess Timber Trade FederationMr H Gordon Craig Mr G O Hutchison Trussed Plate Manufacturers Assoc
7、iationMr M Macdonnell Co-optedDr W W L Chan Mr H P Stone Mr R W Wands SecretaryMr C B Corbin Drafting sub-committee CSB/32/6 ChairmanMr P J Steer British Woodworking FederationMr J Riley Mr K Thurman Mr P G Grimsdale Department of the EnvironmentMr W H Cutmore Department of the Environment: Building
8、 Research Establishment (Fire Research Station)Mr F C Adams Greater London CouncilMr A S Brown Mr G J Sansom Institution of Structural EngineersMr J Ollis Mr P J Steer Royal Institution of Chartered SurveyorsMr D G Burcham Timber Research and Development AssociationDr G S Hall Co-optedMr R Highet Am
9、endments issued since publication Amd. No.Date of issueComments 2947July 1979 6192March 1990Indicated by a sideline in the margin Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 14:02:00 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-4.1:1978 BSI 03-1999i Contents Page Code Drafting Commi
10、tteeInside front cover Forewordii 1Scope1 2References1 3Definitions1 4Behaviour of timber in fire1 5Design considerations2 Figure 1 Radius of arris rounding4 Figure 2 Sections built up with metal fasteners5 Figure 3 Columns built into walls6 Figure 4 Columns abutting on walls6 Table 1 Notional rate
11、of charring for the calculation of residual section1 Publications referred toInside back cover Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 14:02:00 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-4.1:1978 ii BSI 03-1999 Foreword BS 5268 supersedes the earlier code of practice CP 112 “T
12、he structural use of timber”. It is intended that BS 5268 will have the following Parts Part 1: Limit state design; Part 2: Code of practice for permissible stress design, materials and workmanship; Part 3: Code of practice for trussed rafter roofs; Part 4: Fire resistance of timber structures; Sect
13、ion 4.1: Recommendations for calculating fire resistance of timber members; Section 4.2: Recommendations for calculating fire resistance of timber stud walls and joisted floor constructions; Part 5: Preservative treatments for constructional timber; Part 6: Code of practice for timber frame walls; S
14、ection 6.1: Dwellings not exceeding three storeys1); Part 7: Recommendations for the calculation basis for span tables; Section 7.1: Domestic floor joists1); Section 7.2: Joists for flat roofs1); Section 7.3: Ceiling joists1); Section 7.4: Ceiling binders1); Section 7.5: Domestic rafters;1) Section
15、7.6: Purlins1); Section 7.7: Purlins supporting sheeting or decking. The recommendations of Part 1 of BS 5268, preparation of which has not yet started, may entirely supersede those of Part 2 of BS 5268 after a limited number of years. Section 4.1 of BS 5268-4 gives information for the calculation o
16、f fire resistance. Such calculations are possible because, in fire, the behaviour of timber is predictable with regard to the rate of charring and loss of strength. It is also free from rapid changes of state and has very low coefficients of thermal expansion and thermal conductivity. Timber treatme
17、nts including impregnation to retard the surface spread of flame should not be assumed to affect the charring rate. Section 4.2 will deal with timber stud walls and joisted floor constructions. Fire resistance relates to complete elements of construction and not to individual materials; the appropri
18、ate test is described in BS 476-201). The stability (resistance to structural failure), integrity and insulation criteria may all be applicable and the performance of an element is expressed in terms of the periods of time that the appropriate criteria are satisfied. The methods given in this code f
19、or assessing by calculation the fire resistance of timber members, in relation to stability criteria, use stress modification factors (see 5.1.2 and 5.2.2) which have been arrived at empirically and checked against the results of a number of fire resistance tests conducted in accordance with the app
20、ropriate British Standard. 1) The test methods specified in BS 476-8 have been revised and replaced by BS 476-20 to BS 476-23. The calculation methods of BS 5268-4 rely essentially on data gathered from tests to BS 476-8. In due course data will become available from the revised testing procedures w
21、hich will then be incorporated into Section 4.1 of BS 5268. The definitions and terminology of BS 476-20 will be maintained until these data are available. Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 14:02:00 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-4.1:1978 BSI 03-1999iii It is
22、 current practice during the fire resistance test in accordance with BS 476-20 in the case of compression members, to apply an axial load only, because the limitations of the existing test equipment preclude other loading arrangements. The information given in this Part of the code, dealing with the
23、 assessment of fire resistance of compression members, relates to fire resistance tests, where limited loading arrangements have to be used. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct applica
24、tion. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i to iv, pages 1 to 6, an inside back cover and a back cover. This standard has been updated (see copyright date) a
25、nd may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 14:02:00 GMT+00:00 2006, Uncontrolled Copy, (c) BSI iv blank Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 14:02:00 GMT+00:
26、00 2006, Uncontrolled Copy, (c) BSI BS 5268-4.1:1978 BSI 03-19991 1 Scope This Section of BS 5268 gives recommendations for assessing the fire resistance of flexural tension and compression members of solid or glued laminated timber and their joints. 2 References The titles of the standards publicat
27、ions referred to in this Section of BS 5268 are listed on the inside back cover. 3 Definitions 3.1 For the purposes of this section of BS 5268 the definitions given in BS 565, BS 4422, BS 5268-2 and BS 6100-4 apply, together with the following additional definitions. residual section the section of
28、the uncharred timber that would be left after a given period of exposure to the fire conditions described in the test in BS 476-20, assuming a steady rate of charring, with allowance for accelerated charring at exposed arrises where recommended herein (see 4.3) 3.2 In addition, the definition of sta
29、bility used throughout Part 4 is based on that given in BS 476-20, which for the purpose of this section of BS 5268 is taken as: a) the ability to sustain the applied load throughout the period of the fire test and, b) also, in the case of flexural members, ability to resist deflection during the fi
30、re test, to NOTEIt should be noted that this may differ from the normal structural engineering interpretation. 4 Behaviour of timber in fire 4.1 General. For the purposes of this British Standard, charring can be assumed to occur at a steady rate in the fire resistance test described in BS 476-20. T
31、he timber beneath the charred layer does not lose significant strength because the thermal conductivity is low. These characteristics make it possible to predict the performance in a fire resistance test of certain flexural tension and compression members thus reducing the need for testing. The crit
32、eria of BS 476-20 are applicable to elements of building construction as follows. a) Flexural members (beams): stability, (strength and deflection). b) Compression members (columns): stability. c) Tension members: stability. Where members are built into, or form part of, a fire resisting constructio
33、n, the insulation and integrity requirements may also be applicable. 4.2 Resistance to charring 4.2.1 Solid members. Calculation of residual section of solid members should be based on the values given in Table 1. These values should be modified in the case of fully exposed columns and tension membe
34、rs as set out in 5.2.2 a) and 5.3.2 a) respectively. Table 1 Notional rate of charring for the calculation of residual section NOTELinear interpolation or extrapolation for periods between 15 min and 90 min is permissible. Notional charring rates for particular species and longer periods of time not
35、 presently included in Table 1 may be established by an appropriate authority. 4.2.2 Glued laminated members. The charring rates given in 4.2.1 may be applied to members laminated with the following thermosetting phenolic and aminoplastic synthetic resin adhesives: resorcinol-formaldehyde, phenol-fo
36、rmaldehyde, phenol-resorcinol-formaldehyde, urea-formaldehyde, and urea-melamine-formaldehyde. Where other adhesives are used, guidance should be sought from an appropriate authority. 4.2.3 Finger joints. Finger joints manufactured in accordance with the requirements of BS 5291 using adhesives speci
37、fied in 4.2.2 may be considered to char at the rates given in Table 1. span 20 - - SpeciesCharring in 30 min Charring in 60 min mmmm a) All structural species listed in Appendix A of BS 5268-2:1989 except those noted in items b) and c)2040 b) Western red cedar2550 c) Hardwoods having a nominal densi
38、ty not less than 650 kg/m3 at 18 % moisture content1530 Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 14:02:00 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-4.1:1978 2 BSI 03-1999 4.2.4 Sections built up with metal fasteners (see Figure 2). The charring rates in 4.2.1 may only be appli
39、ed to the section as a whole if metal fasteners on which the structural performance of the built-up member depends are fully protected from the effects of fire (see 5.4.2). Where such protection is not given, local structural weaknesses may occur and the member can only be assessed for fire resistan
40、ce by applying the residual section calculation, assuming charring on all faces of each component of the built-up member, or by conducting a fire resistance test. 4.3 Increased rate of charring on exposed arrises. Arrises will become progressively rounded during fire exposure. The radius of this rou
41、nding is equal to the depth of charring and the centre lies equidistant from the two aspect faces at a distance of twice the charring depth (see Figure 1). For periods of fire exposure not exceeding 30 min, where the least dimension of the rectangular residual section is not less than 50 mm, roundin
42、g is insignificant and may be disregarded. 5 Design considerations 5.1 Flexural members 5.1.1 Stability criteria a) Strength. The residual section should be such that the member will support the appropriate loads that would be applied if the component were tested in accordance with the requirements
43、of BS 476-20 to either the maximum permissible design load or the loads based on those which the member is required to support in normal service. b) Deflection. The deflection under the appropriate design load should not exceed 1/20 of the clear span. Consideration should be given to the effect of d
44、eflection on the stability and integrity of other parts of the structure. 5.1.2 Assessment of fire resistance a) Residual section. The residual section should be computed by subtracting from the appropriate faces the notional amount of charring assumed to occur during the required period of fire exp
45、osure, making allowance for the rounding on the exposed arrises, where necessary. b) Strength. The load-bearing capacity of a flexural member should be calculated in accordance with normal practice, using the residual section and stresses of 2.25 permissible long-term dry stresses given in BS 5268-2
46、, when the minimum initial breadth of the section is 70 mm or greater and 2.00 permissible long term dry stress, when this dimension is less than 70 mm. c) Deflection. Deflections should be calculated using the residual section and the dry value of the modulus of elasticity taking the mean or minimu
47、m values, as used in the original design. The resulting deflection should not exceed the limit defined in 5.1.1. 5.2 Compression members 5.2.1 Stability criterion. The residual section should be such that the member will support the appropriate loads such as would be applied if the component were te
48、sted in accordance with the requirements of BS 476-20, to either the maximum design compressive load or loads based on those which the member is required to support in normal service. 5.2.2 Assessment of fire resistance a) A column that is exposed to the fire on all faces including a column which ab
49、uts on or forms part of a wall that does not have fire resistance, as in Figure 3(b) and Figure 4(b), should be assumed to char equally on all faces during the whole period of fire exposures. To determine the residual section of such columns, the rates of charring given in Table 1 should be multiplied by 1.25. Where a column abuts on or forms part of a wall which provides fire resistance from either side not less than that of the column, charring on all faces is unlikely. Calculations should therefore be based on c