BS-5268-7.1-1989.pdf

BRITISH STANDARD BS 5268-7.1: 1989 Structural use of timber Part 7: Recommendations for the calculation basis for span tables Section 7.1 Domestic floor 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 16:10:51 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS5268-7.1:1989 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 03-1999 The following BSI references relate to the work on this standard: Committee reference CSB/32 Draft for comment 84/11849 DC ISBN 0 580 16586 8 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 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 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 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 Development Association Timber Trade Federation Coopted members Amendments issued since publication Amd. No.Date of issueComments Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:10:51 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.1:1989 © BSI 04-1999i Contents Page Committees responsible Inside front cover Foreword ii 1Scope 1 2Definitions1 3Symbols 1 4Design considerations2 5Permissible spans3 6Bearing length 6 7Information to be given in span tables 6 Appendix A Sample calculation for a domestic floor joist 7 Appendix B Specimen span tables for domestic floor joists 9 Figure 1 Bearing length, permissible effective and permissible clear span 5 Table 1 Permissible clear spans for domestic floor joists: SC3: regularised sizes 9 Table 2 Permissible clear spans for domestic floor joists: redwood/whitewood, SS grade, basic sizes 10 Table 3 Permissible clear spans for domestic floor joists: spruce-pine-fir, joist and plank no. 2 grade, CLS sizes 11 Publications referred to Inside back cover Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:10:51 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.1:1989 ii © BSI 04-1999 Foreword This Section of Part 7 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 range 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 preparation 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 adequate designs. This Section deals with domestic floor joists. Other Sections of BS 5268-7, published or in preparation, are as follows. 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; Section 7.7: Purlins suppporting 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 basis 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 prepare 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 domestic floor joist 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 obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and 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 16:10:51 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.1:1989 © BSI 04-19991 1 Scope This Section of BS 5268 recommends a calculation basis for the permissible clear span for simply supported domestic floor joists of solid timber. 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 flooring to the load resistance of the joists where such action can be provided by adequate attachments between the elements as in a stressed skin panel floor. Only uniform loading is considered and concentrated or line loads applied by partitions, trimmers and other similar causes are excluded. 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 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 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 permissible stress stress that can safely be sustained by a structural material under a particular condition NOTEFor the purpose 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 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 from 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. 3 Symbols For the purposes of this Section of BS 5268, the following symbols apply. NOTEThe symbols used are generally 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 Council 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 and flooring materials (excluding joist self weight) FjSelf weight of joist per metre length GShear modulus hDepth of joist lSecond moment 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 ÔDensity ÖStress ÙShear stress Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:10:51 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.1:1989 2 © BSI 04-1999 The following subscripts are used: a) Type of force, stress etc. b) Significance c) Geometry tra or 8 Perpendicular (to the grain) 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 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 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 does not exceed the recommended limit of 0.003 times the span or 14 mm (see 14.7 of BS 5268-2:1988), whichever is smaller. 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 The calculations given in this Section apply to systems of at least four domestic floor joists, at a maximum spacing of 610 mm centre-to-centre and having adequate flooring to provide 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 accordance with 5.5, may not be sufficient for practical construction purposes. 4.3 Loading The design calculations provide for domestic floor loads which consist of the following.1) a) Imposed load 1) for an effective span equal to or greater than 2 400 mm, the imposed load is 1.5 kN/m2 uniformly distributed; 2) for an effective span less than 2 400 mm, the imposed load is 3.6 kN per metre width of floor (measured perpendicular to the span) uniformly distributed over the span. NOTEThis imposed load of 1.5 kN/m2 entirely fulfils the recommendations of BS 6399-1. The maximum imposed load of 3.6 kN per metre width is a more onerous load applied to ensure that very small joist sizes do not result from the calculations for small spans. The imposed load should be considered to be a long term load. b) Dead load. Dead load per square metre Fd (in kN/m2) to provide for the mass of ceiling and flooring materials, pugging, 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 joist. The timber densities (in kg/m3), given in Tables 9 and 92 of BS 5268-2:1988 should be used. 4.4 Design loads The total load per metre length of span, F, is found in different ways depending on whether the span is less than or greater than 2 400 mm. For spans equal to or greater than 2 400 mm F (in kN/m) is given by the equation For spans equal to or less than 2 400 mm F (in kN/m) is given by the equation where cCompression mBending admPermissible clClear gGrade maxMaximum 1) Concentrated or line loads applied by partitions, trimmers and other similar causes are excluded. (1) (2) sis the joist spacing (in mm); Lis the effective span (in mm); Fdis the dead load (in kN/m2); Fjis the self weight of joist (in kN/m). NOTEAt a span of 2 400 mm, equations (1) and (2) give the same value for F. Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:10:51 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.1:1989 © BSI 04-19993 The value of Fj (in kN/m) may be found from the equation where 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. 5.2 Limitation of bending stress From BS 5268-2, the permissible bending stress Bm,adm(in N/mm2) is given by the equation where Expanding the equation leads to the following equations. Effective span, L, 2 400 mm Bm,g× 1.0 × K7× 1.1 Effective span, L, 2 400 mm L 2 400 mm Ùg× 1.0 × 1.1 Effective span L 2 400 mm L 2 400 mm and for an effective span L 2 400 mm and for an effective span L 2 400 mm A further design equation is required for the 14mm limitation on deflection of spans greater than or equal to 2 400 mm: this is similar to equation (21) but with “0.003L” replaced by “14”. The deflection of spans less than 2 400 mm will be limited to less than 14 mm by equation (22). NOTEThe three design equations lead to the following polynomials in L. (14) (15) Wmax = 0.003 L(16) (17) 2) In addition to the deflection due to bending the shear deflection may be significant and has been taken into account. (18) (19) (20) E 16 - (21) (22) Limitation 0.003L (23) (24) Limitation 14 mm (25) Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:10:51 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-7.1:1989 © BSI 04-19995 5.5 Permissible clear spans The calculation of clear span requires the deduction of a notional bearing length from an effective span. The calculation of the notional bearing length to be deducted from the permissible effective span to produce the clear span is made after finding Ladm, the smallest of the effective spans for a given cross section, as limited by: a) bending stress, L $ 2 400 mm; b) bending stress, L # 2 400 mm; c) shear stress, L $ 2 400 mm; d) shear stress, L # 2 400 mm; e) deflection, limitation 0.003L, L $ 2 400 mm; f) deflection, limitation