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1、BRITISH STANDARD BS 5268-7.2: 1989 Structural use of timber Part 7: Recommendations for the calculation basis for span tables Section 7.2 Joists for flat roofs UDC 624.011.1 + 674.038.5 + 691.1.11:692.526:694.5:001.4 Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:00 GMT+00:00 2006, Unc
2、ontrolled Copy, (c) BSI BS 5268-7.2:1989 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 Board of BSI and comes into effect on 30 June 1989 BSI 01-2000 The following BSI ref
3、erences relate to the work on this standard: Committee reference CSB/32 Draft for comment 86/10303 DC ISBN 0 580 16587 6 Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Civil Engineering and Building Structures Standards Committee (CSB/-
4、) to Technical Committee CSB/32 upon which the following bodies were represented: British Woodworking Federation Building Employers Confederation Chartered Institute of Building Department of the Environment (Building Research Establishment, Princes Risborough Laboratory) Department of the Environme
5、nt for Northern Ireland Department of the Environment (Housing and Construction Industries) Department of the Environment (Property Services Agency) Health and Safety Executive Incorporated Association of Architects and Surveyors Institute of Clerks of Works of Great Britain Inc. Institute of Wood S
6、cience Institution of Civil Engineers Institution of Structural Engineers International Truss Plate Association National House-building Council Royal Institute of British Architects Royal Institution of Chartered Surveyors Timber Research and Development Association Timber Trade Federation Coopted m
7、embers Amendments issued since publication Amd. No.Date of issueComments Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:00 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.2:1989 BSI 01-2000i Contents Page Committees responsibleInside front cover Forewordii 1Scope1 2Definitions1 3S
8、ymbols1 4Design considerations2 5Permissible spans3 6Bearing length8 7Information to be given in span tables8 Appendix A Sample calculations for a flat roof joist9 Appendix B Specimen span tables for flat roof joists10 Figure 1 Bearing length, permissible effective and permissible clear span8 Table
9、1 Permissible clear spans for roof joists without access, imposed load 0.75 kN/m2: SC3, regularized sizes11 Table 2 Permissible clear spans for roof joists without access, imposed load 0.75 kN/m2: redwood/whitewood, SS grade, basic sizes12 Table 3 Permissible clear spans for roof joists without acce
10、ss, imposed load 0.75 kN/m2: spruce-pine-fir, joist and plank no. 2 grade, CLS sizes13 Publications referred toInside back cover Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:00 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.2:1989 ii BSI 01-2000 Foreword This Section of Part 7
11、of BS 5268 has been prepared under the direction of the Civil Engineering and Building Structures Standards Committee. The general principles for the design of structural timber components are given in BS 5268-2 and using these principles it is possible for span tables to be prepared for a wide rang
12、e of components. Experience has shown that different interpretations of these principles has led to inconsistencies in span tables prepared by different compilers. It is the purpose of BS 5268-7 to eliminate these differences by recommending the design equations and the loading to be used in the pre
13、paration of span tables. Part 7 is intended to ensure that different organizations produce span tables on a consistent basis in the future, and is not necessarily intended for use by designers for individual designs carried out in their day-to-day work, where simplified equations may produce adequat
14、e designs. Section 7.2 deals with joists for flat roofs. Other sections of BS 5268-7 published or in preparation are as follows. Section 7.1: Domestic floor joists; Section 7.3: Ceiling joists; Section 7.4: Ceiling binders; Section 7.5: Rafters; Section 7.6: Purlins supporting rafters; Section 7.7:
15、Purlins supporting sheeting or decking. BS 5268-2 gives grade stresses for very many combinations of species and grade and it is considered impractical to publish in a British Standard span tables for all possible combinations of species, grades and sizes. BS 5268-7 is therefore restricted to the ba
16、sis of the calculations. The solution of the design equations for many combinations of geometry and material is most conveniently undertaken by computer. A program written by the Timber Research and Development Association (TRADA) was used to prepare Appendix A and Appendix B. For users wishing to p
17、repare their own span tables or computer programs Appendix A gives a sample calculation. Appendix B gives span tables for three typical combinations of species and grade. Although the presentation of span tables is not covered in BS 5268-7, it is recommended that tables for predetermined flat roof j
18、oist centres and loading follow this format. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligatio
19、ns. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 14, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the
20、inside front cover. Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:00 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.2:1989 BSI 01-20001 1 Scope This Section of BS 5268 recommends a calculation basis for the permissible clear span for joists for flat roofs and roofs with a slope
21、of up to 10 The recommendations apply to joists at a maximum spacing of 610 mm centre-to-centre, this being the maximum spacing for which the “load-sharing” assumption may be adopted as described in BS 5268-2. The method of calculation makes no allowance for any contribution of the roof decking to t
22、he load resistance of the joists although it is assumed that the decking should be capable of providing lateral load distribution. It does not cover the design of joists taking account of a structural contribution by sheet material supporting the roofing where such action can be provided by adequate
23、 design of its attachments as in a stressed skin panel roof. The uniform and concentrated loads recommended in BS 6399-1 and BS 6399-3 are considered. This Section of BS 5268 is applicable to the species and grades of timber given in BS 5268-2. NOTEThe titles of the publications referred to in this
24、standard are listed on the inside back cover. 2 Definitions For the purposes of this Section of BS 5268, the definitions given in BS 6100-4.2 to BS 6100-4.4, BS 6100-2.1 and BS 5268-2 apply, together with the following. 2.1 grade stress stress that can safely be permanently sustained by material of
25、a specific section size and of a particular strength class or species and grade 2.2 load-sharing system assembly of pieces or members that are constrained to act together to support a common load 2.3 permissible stress stress that can safely be sustained by a structural material under a particular c
26、ondition NOTEFor the purposes of this Section of BS 5268 it is the product of the grade stress and the appropriate modification factors for section size, service and loading. 2.4 strength class classification of timber based on particular values of grade stress 2.5 bearing length length at each end
27、of the joist in contact with the support 2.6 notional bearing length bearing length required for the calculation of permissible clear spans 2.7 effective span span from centre-to-centre of the minimum bearing lengths at each end 2.8 permissible effective span lowest value of effective span found fro
28、m the calculations for bending strength, shear strength and deflection 2.9 permissible clear span permissible unsupported span of a joist, measured between the faces of the supports at its two ends NOTEPermissible clear span is equal to permissible effective span less the notional bearing length. 2.
29、10 point load concentrated load referred to in BS 6399-1, that is regarded as acting at a point for calculation purposes 3 Symbols For the purposes of this Section of BS 5268, the following symbols apply. NOTEThe symbols used are in accordance with ISO 3898, published by the International Organizati
30、on for Standardization, supplemented by the recommendations of CIB-W18-1 “Symbols for use in structural timber design”, published by the International Council for Building Research Studies and Documentation, which takes particular account of timber properties. The symbols used are: aDistance (notion
31、al bearing length) bBreadth of joist EModulus of elasticity FTotal load per metre length FdDead load per square metre applied by mass of ceiling and roofing materials (excluding joist self weight) FjSelf weight of joist per metre length FpPoint load GShear modulus hDepth of joist Licensed Copy: shef
32、fieldun sheffieldun, na, Mon Dec 04 16:11:00 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.2:1989 2 BSI 01-2000 The following subscripts are used: a) Type of force, stress, etc. b) Significance c) Geometry It is recommended that where more than one subscript is used, the categories should be
33、separated by commas. Subscripts may be omitted when the context in which the symbols are used is unambiguous except in the case of modification factor K. 4 Design considerations 4.1 General The design calculations recommended by this Section of BS 5268 are based on engineers bending theory and are c
34、onsistent with the recommendations of BS 5268-2. The design method ensures that the permissible bending and shear stresses, as given in BS 5268-2, are not exceeded and that the deflection due to bending and shear does not exceed the recommended limit of 0.003 times the span. NOTEA sample calculation
35、 is given in Appendix A and Table 1 to Table 3 in Appendix B contain specimen span tables. 4.2 Qualifying assumptions The calculations given in this Section of BS 5268 apply to systems of at least four roof joists, at a maximum spacing of 610 mm centre-to-centre, and having roofing adequate to provi
36、de lateral load distribution. Because load sharing takes place the load sharing modification factor K8 and the mean modulus of elasticity should be used. Lateral support should be provided in accordance with 14.8 of BS 5268-2:1988. The bearing length required at each end of the joist, calculated in
37、accordance with 5.5, may not be sufficient for practical construction purposes. 4.3 Loading The design calculations provide for flat roof loads which consist of the following. a) Imposed load1) 1) with access: 1.5 kN/m2 uniformly distributed, or a concentrated load of 1.8 kN, whichever governs the d
38、esign; 2) without access: 0.75 kN/m2 uniformly distributed, or a concentrated load of 0.9 kN, whichever governs the design. The concentrated load, in accordance with BS 6399-1, is taken as a point load for calculation purposes. The point load is assumed to act in the position which produces maximum
39、stress or deflection. The imposed distributed load should be considered as a medium term load for both roofs with access and those without access. The imposed point load should be considered as a short term load, as given in Table 8 of BS 5268-3:1985. b) Dead load. Dead load per square metre Fd (in
40、kN/m2) to provide for the mass of ceiling and roofing materials, insulation, etc. Weights of materials are given in BS 648. c) Self weight. Self weight per metre length Fj (in kN/m) to provide for the mass of the joists. The timber densities (in kg/m3) given in Table 9 and 92 of BS 5268-2:1988 shoul
41、d be used. 4.4 Design loads For roofs with or without access three loading conditions should be considered. a) A uniform imposed load condition, the loading consisting of uniformly distributed imposed load, dead load and member self weight. This loading should be considered as medium term. lSecond m
42、oment of area KModification factor (always with a subscript) LEffective span LadmPermissible effective span LclPermissible clear span MBending moment sSpacing of joists, centre-to-centre wDeflection ZSection modulus Density Stress Shear stress cCompression mBending admPermissible clClear gGrade maxM
43、aximum tra or Perpendicular (to the grain) 1) The numerical values are examples of imposed loads, including snow load, specified in BS 6399-1. For other snow loads reference should be made to BS 6399-3. Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:00 GMT+00:00 2006, Uncontrolled Copy
44、, (c) BSI BS 5268-7.2:1989 BSI 01-20003 b) A point imposed load condition, the loading consisting of a point imposed load plus uniformly distributed dead load and member self weight. This loading should be considered as short term. c) A long term load condition, the loading consisting of uniformly d
45、istributed dead load and member self weight with no imposed load. This loading should be considered as long term. With access For the uniform imposed load condition F (in kN/m) is given by the equation For the point imposed load condition Fp = 1.8 kN acting together with uniform dead load and self w
46、eight (in kN/m) For the long term load condition, i.e. dead load and self weight alone, F (in kN/m) is given by the equation Without access For the uniform imposed load condition F (in kN/m) is given by the equation For the point imposed load condition Fp = 0.9 kN acting together with uniform dead l
47、oad and self weight (in kN/m) For the long term load condition, i.e. dead load and self weight alone, F (in kN/m) is given by the equation In equations (1) to (6) The value of Fj (in kN/m) may be found from the equation where For the calculation of spans under loading incorporating a point load, the
48、 combined effect of uniform and point loads may be obtained using the equivalent uniformly distributed load F. F (in kN/m) is given by the following equations. In bending strength calculations In shear strength calculations In deflection calculations: For bending deflection For shear deflection In e
49、quations (8) to (11) 5 Permissible spans 5.1 General The permissible effective span of a timber joist subjected to the applied loads given in 4.3 should be the shortest effective span resulting from calculations for bending strength, shear strength and deflection, as given in 5.2, 5.3 and 5.4. The permissible clear span should be calculated as the permissible effective span less the notional bearing length, calculated in accordance with 5.5. (1) (2) (3) (4) (5) (6) sis the joist spacing (in mm); Fdis the dead load (in kN/m2); Fjis the self weight