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1、BRITISH STANDARD BS 5268-7.6: 1990 Incorporating Amendment No. 1 Structural use of timber Part 7: Recommendations for the calculation basis for span tables Section 7.6 Purlins supporting rafters UDC 624.011.1 + 674.038.5+691.1.11:692.421.2 + 694.5:001.4 Licensed Copy: sheffieldun sheffieldun, na, Mo
2、n Dec 04 16:11:35 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.6:1990 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 30
3、 April 1990 BSI 03-1999 The following BSI references relate to the work on this standard: Committee reference CSB/32 Draft for comment 87/10398 DC ISBN 0 580 17775 0 Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Civil Engineering and B
4、uilding Structures Standards 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, Prince
5、s Risborough Laboratory) 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 Clerk
6、s of Works of Great Britain 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 Devel
7、opment Association Timber Trade Federation Coopted members Amendments issued since publication Amd. No.Date of issueComments 6902February 1992Indicated by a sideline in the margin Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:35 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.6:1
8、990 BSI 03-1999i Contents Page Committees responsibleInside front cover Forewordii 1Scope1 2Definitions1 3Symbols4 4Design considerations5 5Permissible spans7 6Bearing length15 7Information to be given in span tables15 Appendix A Sample calculations for a purlin supporting rafters17 Appendix B Speci
9、men span tables for purlins supporting rafters18 Figure 1 Roof construction3 Figure 2 Bearing length, permissible effective span and permissible clear span15 Table 1 Permissible clear spans for two-span continuous purlins, roof slope 30, uniform snow load 0.75 kN/m2: SC3, regularized sizes19 Table 2
10、 Permissible clear spans for single span purlins, roof slope 30, uniform snow load 0.75 kN/m2: redwood/whitewood, SS grade, basic sizes20 Table 3 Permissible clear spans for two-span continuous purlins, roof slope 30, uniform snow load 0.75 kN/m2: spruce-pine-fir, joist and plank no. 2 grade, CLS si
11、zes21 Publications referred toInside back cover Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:35 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.6:1990 ii BSI 03-1999 Foreword This Section of BS 5268 has been prepared under the direction of the Civil Engineering and Building Stru
12、ctures Standards Policy 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 range of components. Experience has shown that different interpretations of these princi
13、ples 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 preparation of span tables. Part 7 is intended to ensure that different organizations p
14、roduce 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 procedures may produce adequate designs. This Section deals with purlins supporting rafters. Other Sections of BS
15、 5268-7, 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 7.7: Purlins supporting sheeting or decking. BS 5268-2 gives grade stresses for very
16、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 basis of the calculations. The solution of the design equations for many combinati
17、ons 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 prepare their own span tables or computer programs, Appendix A gives a sample ca
18、lculation. Appendix A 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 al
19、l 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 obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and
20、 ii, pages 1 to 22, 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 16:11:35 GMT+00:00
21、2006, Uncontrolled Copy, (c) BSI BS 5268-7.6:1990 BSI 03-19991 1 Scope This Section of BS 5268 recommends a calculation basis for the permissible clear span of purlins used in traditional pitched roof construction, i.e. not including trussed rafter roofs. The method of calculation is for purlins for
22、med from solid timber and supported by external or internal walls or by purlin struts. The major axis of the purlin is perpendicular to the rafter slope (see Figure 1); other orientations of the purlin are not covered by this Section of BS 5268. The uniform and concentrated loads of BS 6399-1 are co
23、nsidered. Provision is made for a uniformly distributed snow load derived from BS 6399-3. A typical example of roof construction is given in Figure 1. 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 BS 5268 the definitions given in BS 6100-4 and BS 6100-2.1 apply, together with the following. Reference should also be made to clause 2 of BS 5268-2:1988. 2.1 grade stress stress that can safely be permanen
25、tly 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 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
26、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 stress 2.5 bear
27、ing 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 span lowest va
28、lue 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 effective span less
29、 the minimum bearing length. Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:35 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.6:1990 2 BSI 03-1999 2.10 point load concentrated load required by BS 6399-1, regarded as acting at a point for calculation purposes 2.11 rafter member ex
30、tending the full distance from eaves to ridge; may be continuous or jointed over the purlin, i.e. non-continuous 2.12 purlin beam parallel to the eaves giving intermediate support to rafters Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:35 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS
31、 5268-7.6:1990 BSI 03-19993 Figure 1 Roof construction Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:35 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.6:1990 4 BSI 03-1999 3 Symbols For the purposes of this Section of BS 5268, the following symbols apply. NOTEThe symbols used ar
32、e generally in accordance with ISO 3898, published by the International Organization for Standardization supplemented by the recommendations of CIB-W18-1, published by the International Council for Building Research Studies and Documentation, which takes particular account of timber properties. The
33、symbols used are: The following subscripts are used: a) Type of force, stress etc. b) Significance aDistance (notional bearing length) bBreadth of purlin bsBreadth of supported members (rafters) EModulus of elasticity FTotal load per metre length, applied to purlin in the direction perpendicular to
34、the rafters FdDead load per square metre applied by mass of roofing material (excluding rafter and purlin self weights), measured on slope FiImposed uniformly distributed load per square metre, measured on plan FjSelf weight of purlin per metre length FsWeight of supported members (rafters) per metr
35、e length FpPoint load GShear modulus hDepth of purlin hsDepth of supported members (rafters) 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
36、on slope ssSpacing of supported members (rafters), centre-to-centre wDeflection ZSection modulus Roof slope (pitch) rDensity rsDensity of supported members (rafters) sStress tShear stress cCompression mBending admPermissible clClear gGrade maxMaximum Licensed Copy: sheffieldun sheffieldun, na, Mon D
37、ec 04 16:11:35 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.6:1990 BSI 03-19995 c) Geometry It is recommended that where more than one subscript is used, the categories should be separated by commas. Subscripts may be omitted when the context in which the symbols are used is unambiguous, exc
38、ept 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 recommendations of BS 5268-2. The design method ensures that the permissible bending and shea
39、r 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 sample calculation is given in Appendix A and Table 1 to Table 3 in Appendix B contain specim
40、en span tables. 4.2 Qualifying assumptions The calculations given in this Section of BS 5268 are for single-span or two-span continuous purlins providing intermediate support for rafters. The major axis of the purlin is perpendicular to the rafter slope. The horizontal thrust at the eaves is assumed
41、 to be transmitted by the ceiling joists to the complementary rafter. The rafters may be continuous or may consist of shorter lengths joined at the purlin. Since continuous rafters load the purlin more severely, this is the case considered. The formulae derived are based on rafters having two equal
42、spans. The uniformly distributed dead and imposed loads are as given in BS 6399-1 and BS 6399-3 for pitched roofs. It is assumed that the spacing of the rafters is such that the load applied by the rafters to the purlin may be taken as uniformly distributed. The 0.9 kN concentrated load is applied o
43、nly once to the purlin and not simultaneously in any other position. The purlins are treated as solid timber members 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 sh
44、ould be provided in accordance with 14.8 of BS 5268-2:1988. The bearing length required at each end of the binder, calculated in accordance with 5.6, may not be sufficient for practical construction purposes. 4.3 Loading The design calculations provide for roof loads which consist of the following.
45、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. The concentrated load is assumed to act in the position that produces maximum stress or deflection. However, t
46、he effect of deflection under the concentrated load needs to be considered only when it would affect the finishes. 2) For a roof slope 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 for a 75 roof s
47、lope. No concentrated load is applied. The imposed distributed load 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. b) Dead load. Dead load per square metre on slope, Fd (in kN/m2), to provide for the
48、 mass of roofing materials, insulation etc. Weights of materials are given in BS 648. par or Parallel (to the grain) tra or Perpendicular (to the grain) Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:35 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.6:1990 6 BSI 03-1999 c) Weight
49、 of rafters. Weight per metre length of rafter, Fs (in kN/m), to provide for the weight of the rafters supported by the purlin. The timber densities (in kg/m3) given in Tables 9 and 92 of BS 5268-2:1988 should be used. d) 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. 4.4 Design loads Three loading conditions should be considered. a) Uniform imposed load condition, the loading consisting of u