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1、BRITISH STANDARD BS 5268-7.3: 1989 Structural use of timber Part 7: Recommendations for the calculation basis for span tables Section 7.3: Ceiling joists 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, Uncontrol
2、led Copy, (c) BSI BS 5268-7.3:1989 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 02-1999 The following BSI reference
3、s relate to the work on this standard: Committee reference CSB/32 Draft for comment 86/10305 DC ISBN 0 580 16588 4 Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Civil Engineering and Building Structures Standards Committee (CSB/-) to T
4、echnical 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 Environment for
5、 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 Science
6、 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 members
7、 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 5268-7.3:1989 BSI 02-1999i Contents Page Committees responsibleInside front cover Forewordii 1Scope1 2Definitions1 3Symbols
8、1 4Design considerations2 5Permissible spans4 6Bearing length7 7Information to be given in span tables7 Appendix A Sample calculations for a ceiling joist8 Appendix B Specimen span tables for ceiling joists9 Figure 1 Roof construction3 Figure 2 Bearing length, permissible effective and permissible c
9、lear span7 Table 1 Permissible clear spans for ceiling joists: SC3: regularized sizes9 Table 2 Permissible clear spans for ceiling joists: redwood/whitewood, SS grade, basic sizes10 Table 3 Permissible clear spans for ceiling joists: spruce-pine-fir, joist and plank no. 2 grade, CLS sizes11 Publicat
10、ions referred toInside back cover Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:00 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.3:1989 ii BSI 02-1999 Foreword This Section of Part 7 of BS 5268 has been prepared under the direction of the Civil Engineering and Building Structur
11、es 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 range of components. Experience has shown that different interpretations of these principles has le
12、d 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 prepared to ensure that different organizations produce span
13、 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 adequate designs. This Section deals with ceiling joists. Other Sections of BS 5268-7, published or in
14、 preparation, are as follows. Section 7.1: Domestic floor joists; Section 7.2: Joists for flat roofs; Section 7.4: Ceiling binders; Section 7.5: Rafters; Section 7.6: Purlins supporting rafters; Section 7.7: Purlins supporting sheeting or decking. BS 5268-2 gives grade stresses for very many combina
15、tions 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 geome
16、try 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 calculation. App
17、endix 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 ceiling joist centres and loading follow this format. A British Standard does not purport to include al
18、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
19、 ii, pages 1 to 12, 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 16:11:00 GMT+00:00
20、2006, Uncontrolled Copy, (c) BSI BS 5268-7.3:1989 BSI 02-19991 1 Scope This Section of BS 5268 recommends a calculation basis for permissible clear span for ceiling joists with access at a maximum spacing of 610 mm centre-to-centre. It does not cater for the design of the ties of trussed rafters, wh
21、ich are dealt with in BS 5268-3. The uniform and concentrated loads recommended in BS 6399-1 are considered. This Section of BS 5268 is applicable to both single and continuous span ceiling joists. A typical arrangement is shown in Figure 1. This Section of BS 5268 is applicable to the species and g
22、rades 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 Section of BS 5268, the definitions given in BS 6100-4.1 to BS 6100-4.4, BS 6100-2.1 and BS 5268-2 apply, together with the
23、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 pieces or members that are constrained to act together to support a common load 2.3 permissib
24、le stress stress that can safely be sustained by a structural member of a particular cross section under the particular conditions of service and loading 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, s
25、ervice and loading. 2.4 strength class classification of timber based on particular values of grade stress 2.5 bearing length length at each end 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 s
26、pan span from centre-to-centre of the minimum bearing lengths at each end 2.8 permissible effective 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 joist, measured between
27、 the faces of the supports at its two ends NOTEPermissible clear span is equal to permissible effective span less the notional bearing length. 2.10 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
28、 Section of BS 5268, the following symbols apply. NOTEThe symbols used 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 Coun
29、cil for Building Research Studies and Documentation, which takes particular account of timber properties. The symbols used are: aDistance (notional bearing length) bBreadth of joist EModulus of elasticity FTotal load per metre length FdDead load per square metre applied by mass of ceiling materials,
30、 insulation etc. (excluding joist self weight) FjSelf weight of joist per metre length FpPoint load GShear modulus hDepth of joist ISecond moment of area KModification factor (always with a subscript) LEffective span LadmPermissible effective span LclPermissible clear span MBending moment Licensed C
31、opy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:00 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.3:1989 2 BSI 02-1999 The following subscripts are used: a) Type of force, stress etc. b) Significance c) Geometry tra or Perpendicular (to the grain) It is recommended that where more than one
32、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 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
33、 based on engineers bending theory and are consistent 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.
34、003 times the span. NOTEA sample calculation is given in Appendix A and Table 1 to Table 3 in Appendix B contain specimen span tables. 4.2 Qualifying assumptions Although the calculations given in this Section envisage systems of ceiling joists at a maximum spacing of 610 mm centre-to-centre, it is
35、assumed that lateral load distribution is not adequate to allow stresses to be increased for “load sharing”. For members acting alone, i.e. without load sharing, the use of minimum modulus of elasticity is recommended in BS 5268-2. However for ceiling joists long experience has indicated that satisf
36、actory performance can be achieved by the use of mean modulus of elasticity, and this practice is adopted in the equations for limiting deflection. For roof pitches greater then 20 the axial tension induced by rafter thrust when the ceiling joist is used to tie complementary rafter feet together may
37、 be ignored except that it should be considered in connection design. The importance of obtaining tensile continuity when the ceiling joist is acting as a tie is emphasized. The uniformly distributed dead and imposed loads are as given in BS 6399-1 for ceiling joists. Ceilings with access are assume
38、d. The 0.9 kN concentrated load is applied only once to the ceiling joist and not simultaneously in any other position. The design calculations given in this Section do not allow for water tank loads. Lateral support should be provided in accordance with 14.8 of BS 5268-2:1988. The bearing length re
39、quired at each end of the joist, calculated in accordance with 5.5, may not be sufficient for practical construction purposes. 4.3 Loading The design calculations provide for ceilings with access which consist of the following. a) Imposed load. 0.25 kN/m2 uniformly distributed, together with a conce
40、ntrated load of 0.9 kN which, 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 stress or deflection. The imposed distributed load should be considered as a long term load. The imposed point load s
41、hould 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 kN/m2) to provide for the mass of ceiling materials, insulation etc. Weights of materials are given in BS 648. c) Self weight. Self weight per metre length Fj (in kN/m) to
42、 provide for the mass of the joists. The timber densities (in kg/m3) given in Table 9 and 92 of BS 5268-2:1988 should be used. sSpacing of joists, centre-to-centre wDeflection ZSection modulus Density Stress Shear stress cCompression mBending admPermissible clClear gGrade maxMaximum Licensed Copy: s
43、heffieldun sheffieldun, na, Mon Dec 04 16:11:00 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.3:1989 BSI 02-19993 Figure 1 Roof construction Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:00 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.3:1989 4 BSI 02-1999 4.4 Design loa
44、ds Two loading conditions should be considered. a) A point and uniform imposed load condition, the loading consisting of a point imposed load plus uniformly distributed imposed load, dead load and member self weight. This loading should be considered as short term. b) A uniform imposed load conditio
45、n, the loading consisting of uniformly distributed imposed load, dead load and member self weight. This loading should be considered as long term. For the point and uniform imposed load condition Fp= 0.9 kN acting together with uniform imposed and dead loads and self weight (in kN/m) For the uniform
46、 imposed load condition, F (in kN/m) is given by the equation where 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 combined effect of uniform and point loads may be obtained using the equivalent uniformly dist
47、ributed load F. F (in kN/m) is given by the following equations. In bending strength calculations In shear strength calculations where the factor 1.25 is inserted to allow for continuity (see 5.1). In deflection calculations: For bending deflection For shear deflection In equations (4) to (7) Fp = 0
48、.9 kN L is the span (in mm). 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.
49、4. The permissible clear span should be calculated as the permissible effective span less the notional bearing length, calculated in accordance with 5.5. Ceiling joists may be single-span beams, i.e. supported only at both ends, or they may be supported within their length by binders or supporting walls. Both configurations are covered by the design equations which recognize that the greatest deflection and bending stress occur in the single-span case while the shear stress is greater for multi-span cases. 5.2 Limitation of bending stress Fr