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1、BRITISH STANDARD BS 5268-7.7: 1990 Structural use of timber Part 7: Recommendations for the calculation basis for span tables Section 7.7 Purlins supporting sheeting or decking UDC 624.011.1 + 674.038.5 + 691.1.11:692.421.2.001.24 Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:41 GMT+0
2、0:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.7:1990 This British Standard, having been prepared under the direction of the Civil Engineering and Building Structures Standards Policy Committee, was published under the authority of the Board of BSI and comes into effect on 31 July 1990 BSI 01-2000
3、The following BSI references relate to the work on this standard: Committee reference CSB/32 Draft for comment 88/13315 DC ISBN 0 580 18047 6 Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Civil Engineering and Building Structures Stand
4、ards Policy Committee (CSB/-) 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
5、) Department of the Environment 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 Brit
6、ain Inc. Institute of Wood Science 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 Timbe
7、r Trade Federation Amendments issued since publication Amd. No.Date of issueComments Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:41 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.7:1990 BSI 01-2000i Contents Page Committees responsibleInside front cover Forewordii 1Scope1 2Def
8、initions1 3Symbols2 4Design considerations3 5Permissible spans5 6Bearing length10 7Information to be given in span tables10 Appendix A Sample calculations for a purlin supporting sheeting or decking11 Appendix B Specimen span tables for purlins supporting sheeting or decking12 Figure 1 Roof construc
9、tion1 Figure 2 Bearing length, permissible effective and permissible clear span10 Table 1 Permissible clear spans for purlins supporting sheeting or decking, roof slope 30, uniform snow load 0.75 kN/m2: SC 3, regularized sizes13 Table 2 Permissible clear spans for purlins supporting sheeting or deck
10、ing, roof slope 30, uniform snow load 0.75 kN/m2: redwood/whitewood, SS grade, basic sizes14 Table 3 Permissible clear spans for purlins supporting sheeting or decking, roof slope 30, uniform snow load 0.75 kN/m2: spruce-pine-fir, joist and plank no. 2 grade, CLS sizes15 Publications referred toInsi
11、de back cover Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:41 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.7:1990 ii BSI 01-2000 Foreword This Section of BS 5268 has been prepared under the direction of the Civil Engineering and Building Structures Standards Policy Committee.
12、 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 range of components. Experience has shown that different interpretations of these principles have led to inconsistencies i
13、n 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 preparation of span tables. Part 7 is intended to ensure that different organizations produce span tables on a consisten
14、t 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 procedures may produce adequate designs. This Section deals with purlins supporting sheeting or decking on pitched roofs. Other Sections of BS 526
15、8-7 published or in preparation are as follows. Section 7.1: Domestic floor joists; Section 7.2: Joists for flat roofs; Section 7.3: Ceiling joists; Section 7.4: Ceiling binders; Section 7.5: Rafters; Section 7.6: Purlins supporting rafters. BS 5268-2 gives grade stresses for very many combinations
16、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 basis of the calculations. The solution of the design equations for many combinations of geometry an
17、d 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 prepare their own span tables or computer programs, Appendix A gives a sample calculation. Appendi
18、x B gives span tables for three typical species/grade combinations. Although the presentation of span tables is not covered in BS 5268-7, it is recommended that tables for predetermined purlin centres and loading follow this format. A British Standard does not purport to include all the necessary pr
19、ovisions 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 obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 16
20、, 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 inside front cover. Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:41 GMT+00:00 2006, Uncontrolled
21、 Copy, (c) BSI BS 5268-7.7:1990 BSI 01-20001 1 Scope This Section of BS 5268 recommends a calculation basis for the permissible clear span of purlins supporting sheeting or decking in pitched roof construction. The recommendations apply to purlins formed from solid timber and supported by external o
22、r internal walls. The major axis of the purlin is perpendicular to the roof slope (see Figure 1); other orientations of the purlin are not covered by this standard. The uniform and concentrated loads of BS 6399-1 are considered. Provision is made for a uniformly distributed snow load derived from BS
23、 6399-3. This Section of BS 5268 is applicable to the species, stress grades or strength classes of timber given in BS 5268-2. NOTEThe titles of the publications referred to in this standard are listed on the inside back cover. 2 Definitions For the purposes of this Part of BS 5268 the definitions g
24、iven in BS 5268-2 and BS 6100-4.1 to BS 6100-4.4 and BS 6100-2.1 apply together with the following. 2.1 grade stress stress that can safely be permanently sustained by material of a specific section size and of a particular strength class or species and grade 2.2 load-sharing system assembly of piec
25、es or members that are constrained to act together to support a common load Figure 1 Roof construction Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:41 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.7:1990 2 BSI 01-2000 2.3 permissible stress stress that can safely be sustained
26、by a structural material under a particular condition 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
27、stress 2.5 bearing length length at each end of the purlin 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
28、 span lowest value of effective span found from the calculations for bending strength, shear strength and deflection 2.9 permissible clear span permissible unsupported span of a purlin, measured between the faces of the supports at its two ends NOTEPermissible clear span is equal to permissible effe
29、ctive span less the minimum bearing length. 2.10 point load concentrated load required by BS 6399-1, regarded as acting at a point for calculation purposes 2.11 purlin beam parallel to the eaves giving support to sheeting or decking on pitched roofs 3 Symbols For the purposes of this Section of BS 5
30、268, the following symbols apply. NOTEThe symbols used in this standard are in accordance with ISO 3898, published by the International Organization for Standardization, supplemented by the recommendations of CIB-W18-1, “Symbols for use in structural timber design,” published by the International Co
31、uncil for Building Research Studies and Documentation, which takes particular account of timber properties. The symbols used are: aDistance (notional bearing length) bBreadth of purlin EModulus of elasticity FTotal load per metre length, applied to purlin in the direction perpendicular to the roof s
32、lope FdDead load per square metre applied by mass of roofing material (excluding purlin self weight), measured on slope FiImposed uniformly distributed load per square metre, measured on plan FjSelf weight of purlin per metre length FpPoint load GShear modulus Licensed Copy: sheffieldun sheffieldun,
33、 na, Mon Dec 04 16:11:41 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.7:1990 BSI 01-20003 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 separated by commas.
34、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 consistent with the re
35、commendations 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 effective span (see 14.7 of BS 5268-2:1988). NOTEA sa
36、mple calculation is given in Appendix A and Table 1 to Table 3 in Appendix B contain specimen span tables. 4.2 Qualifying assumptions The design calculations recommended in this Section of BS 5268 apply to single-span purlins supporting sheet roofing materials or timber decking. The major axis of th
37、e purlin is perpendicular to the rafter slope. The component of load in the plane of the roofing is assumed to be carried by the roof construction other than the purlins; it is essential that the type of roofing and its fixings are capable of acting in this manner. The uniformly distributed dead and
38、 imposed loads are as given in BS 6399-1 and BS 6399-3 for pitched roofs. hDepth of purlin ISecond moment of area KModification factor (always with a subscript) LEffective span LadmPermissible effective span LclPermissible clear span MBending moment sSpacing of purlins, centre-to-centre, measured on
39、 slope wDeflection zSection modulus Roof slope (pitch) Density Stress Shear stress cCompression mBending admPermissible clClear gGrade maxMaximum par or Parallel (to the grain) tra or Perpendicular (to the grain) Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:41 GMT+00:00 2006, Uncontr
40、olled Copy, (c) BSI BS 5268-7.7:1990 4 BSI 01-2000 The purlins are treated as solid timbers acting alone, with no provision for load sharing. In accordance with 14.7 of BS 5268-2:1988, the minimum value of modulus of elasticity is used in the calculations. Lateral support should be provided in accor
41、dance with 14.8 of BS 5268-2:1988. Where stress reversal can occur, lateral restraint of the upper and lower edges of the purlin should be considered. The bearing length required at each end of the purlin, calculated in accordance with 5.5, may not be sufficient for practical construction purposes.
42、4.3 Loading The design calculations provide for roof loads which consist of the following. a) Imposed load 1) For a roof slope of 30 or less: a uniformly distributed snow load (e.g. 0.75 kN/m2) measured on plan or a 0.9 kN concentrated vertical load, whichever governs the design. 2) For a roof slope
43、 greater than 30 and not exceeding 75: an imposed load obtained by linear interpolation between the values at 30 roof slope (e.g. 0.75 kN/m2) and zero at 75 roof slope. No concentrated load is applied. b) Dead load. Dead load per square metre on slope Fd (in kN/m2) to provide for the mass of roofing
44、 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 weight of the purlin. The timber densities (in kg/m3) given in Tables 9 and 92 of BS 5268-2:1988 should be used. The concentrated load in accordance wit
45、h 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 stress or deflection. However, the effect of deflection under the point load need be considered only when it would affect the finishes. The imposed distributed load
46、should be considered as a medium term load. The imposed point load should be considered as a short term load as given in Table 8 of BS 5268-3:1985. 4.4 Design loads Three loading conditions should be considered. a) A uniform imposed load condition, the loading consisting of uniformly distributed imp
47、osed load, dead load and member self weight. This loading should be considered as medium term. b) A point imposed load condition, the loading consisting of a point imposed load plus uniformly distributed dead load and member self weight; this condition may be omitted for roof slopes greater than 30.
48、 This loading should be considered as short term. c) A longer term load condition, the loading consisting of uniformly distributed dead load and member self weight with no imposed load. This loading should be considered as long term. NOTEWind uplift may be the governing design loading when the light
49、er types of roof cladding are to be used, and should be checked when appropriate. For the uniform imposed load condition, the total load per metre length of purlin, F(in kN/m) resolved perpendicular to the roof slope, is given by the equation For the point imposed load condition, the load resolved perpendicular to the roof slope from the point imposed load is given by: For the long term load condition, i.e. dead load and self weight alone, F (in kN/m) resolved perpendicular to the roof slope is given by the equation (1) Fp cos = 0.