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Structural Design Using Timber



Introduction..... Relevant Standards..... Service Class..... Species of Trees..... Grade Stresses (strength class).....
Grade Stresses (timber species)-softwoods..... Grade Stresses (timber species)-imported hardwoods..... Grade Stresses (timber species)-temperate hardwoods.....



Introduction

The notes below relate to the factors involved in designing structures using timber.    The notes are primarily based on information included in BS 5268:part 2.

Commercial timbers include hardwoods and softwoods.    Hardwoods are produced from broad leaved deciduous trees and softwoods are produced from coniferous trees which have needle like leaves and are evergreen.

Structural timbers are catergorised by a strength class which combines the timber type (softwood or hardwood) and the strength grade.     The strength grade is allocated visually or results from testing by machine.

The strength grade classification identified in BS 5268 part 2 are
C14 to C40 for Coniferous = Softwoods and D30 to D70 for Deciduous = Hardwoods.
The numbers refer to the characteristic bending stress i.e ultimate strengths of the woods in N/mm2 before safety factors and margins for loading conditions are included.

BS 5268 identify grade stresses for the strength classes.    These stresses are modified by factors related to the design conditions to arrive at the permissible stresses.

Designs should be based either on the grade stresses for the strength classes, or on grade stresses listed for the individual species and grades

Timber used in construction is stress graded using either machine grading or visually graded.     Visual grading is carried out by approvided graders who examine the timber for defects e.g knots, wane, resin pockets and distortion.    Timber which is accepted for use is graded in one of two caterories GS (General Structural) or SS ( Special Structural) .

Visually graded hardwoods from tropical regions are designated with a (HS) grade ..HS = structural tropical Hardwood.    When derived from temperate regions the visually graded timber is designated using TH1,TH2, THA, or THB = structural temperate hardwood grades in accordance with the requirements of BS 5756:2007








Relevant Standards..For comprehensive list of standards Wood related Standards

BS 5268 -2 ;2002 Structural use of timber Part 2: Code of practice for permissible stress design, materials and workmanship

BS EN 1912:2004: Structural timber Strength classes- Assignment of visual grades and species

BS 4978:1996 Specification of Softwood Grades for Structural Use.

BS 5756:2007 Visual grading of hardwood. Specification








Service Class

Moisture has a significant effect on the mechanical properties of timber and the British standard allocates service class designations to allow for this

The permissible stresses used is generally alocated relevant to the service classes as identified below

a) Service class 1 is characterized by a moisture content in the materials corresponding to a temperature of 20 C and the relative humidity of the surrounding air only exceeding 65 % for a few weeks per year.    In such moisture conditions most timber will attain an average moisture content not exceeding 12 %.

b) Service class 2 is characterized by a moisture content in the materials corresponding to a temperature of 20 C and the relative humidity of the surrounding air only exceeding 85 % for a few weeks per year.     In such moisture conditions most timber will attain an average moisture content not exceeding 20 %.

c) Service class 3, due to climatic conditions, is characterized by higher moisture contents than service class 2.

Note: Design using timber sections greater than 100 thick or deep are generally based on service class 3 because of the difficulty in drying thicker sections.








Species of Trees

BS 5268-2 gives a series of strength classes which for design use can be considered as being independent of species and grade.    Guidance as to which species and grades satisfy the strength requirements for each class is given in BS EN 1912. For some applications it may be necessary to specify particular species (or exclude them) from within a strength class to take account of particular characteristics, e.g. natural durability, amenability to preservatives , glues and fasteners.    This may be particularly important for hardwood.

Table showing strength grades and tree species that have been assigned by grading organisations in different countries

Note; this table should be only considered as indicative and should not be used in design as a method of selecting a species which has a specified grade.

Strength Class Tree Species
C14Western Red Cedar,Douglas Fir
C16Hem Fir,
C22British Pine,spruce
C24Spruce, Poplar, Larch
C30 Spruce, Poplar, Larch,Pine (redwood)
C35 Douglas Fir
D30 Oak
D35 Beech
D40 Beech, Teak, Eucalyptus
D50 Keruing, Karri, Opepe
D60 Kapur
D70 Balau, Greenheard, Ekki





Grade stresses and Modulus of Elasticity values for some strength classes relating to service classes 1 & 2

Note: Service class 3 grade stresses are obtained be multiplying the stresses in the tables by the factor K2 ..ref Modifying factors



Strength Class Bending
parallel
to grain
Tension
parallel
to grain
Compression
parallel
to grain
Compression
perpendicular
to grain
Shear
parallel
to grain
Modulus of
elasticity
Characteristic
density
Average
density
Mean Min
σm,g,ll σt,g,ll σc,g,ll σc,g,l- σc,g,l- τg,ll Emean Emin ρk ρmean
N/mm2 N/mm2 N/mm2 N/mm2 N/mm2 N/mm2 N/mm2 kg/m3 kg/m3
C14 4,1 2,5 5,2 2,1 1,6 0,6 6800 4600 290 350
C16 5,3 3,2 6,8 2,2 1,7 0,67 8800 5800 310 370
C18 5,8 3,5 7,1 2,2 1,7 0,67 9100 6000 320 380
C22 6,8 4,1 7,5 2,3 1,7 0,71 9700 6500 340 410
C24 7,5 4,5 7,9 2,4 1,9 0,71 10800 7200 350 420
C27 9,5 6 8,2 2,5 2 1,1 11500 8200 370 450
C30 11 6,6 8,6 2,7 2,2 1,2 12300 8200 380 460
C35 12 7,2 8,7 2,9 2,4 1,3 13400 9000 400 480
C40 13 7,8 8,7 3 2,6 1,4 14500 10000 420 500
D30 9 5,4 8,1 2,8 2,2 1,4 9500 6000 530 640
D35 11 6,6 8,6 3,4 2,6 1,7 10000 6500 560 670
D40 12,5 7,5 12,6 3,9 3 2 10800 7500 590 700
D50 16 9,6 15,2 4,5 3,5 2,2 15000 12600 650 780
D60 18 10,8 18 5,2 4 2,4 18500 15600 700 840
D70 23 13,8 23 6 4,6 2,6 21000 18000 900 1080







Grade stresses for selected softwoods for service classes 1 and 2

Stresses applicable to timber 300 mm deep(or wide).
Service class 3 grade stresses are obtained be multiplying the stresses in the tables by the factor K2        ref modifying factors


Name Grade Bending stress Tension Compression Shear Modulus of Elasticity
parallel to
grain
parallel to
grain
parallel to
grain
perpendicular
to grain
parallel to grain Mean Minimum
σm,g,ll σt,g,ll σc,g,ll σc,g,l- τg,ll Emean Emin
N/mm2 N/mm2 N/mm2 N/mm2 N/mm2 N/mm2 N/mm2
Redwood/whitewood SS 7,5 4,5 7,9 2,1 0,82 10500 7000
GS; 5,3 3,2 6,8 1,8 0,82 9000 6000
British larch SS; 7,5 4,5 7,9 2,1 0,82 10500 7000
GS 5,3 3,2 6,8 1,8 0,82 9000 6000
British pine SS 6,8 4,1 7,5 2,1 0,82 10500 7000
GS 4,7 2,9 6,1 1,8 0,82 9000 6000
British spruce SS 5,7 3,4 6,1 1,6 0,64 8000 5000
GS 4,1 2,5 5,2 1,4 0,64 6500 4500
Douglas fir SS 6,2 3,7 6,6 2,4 0,88 11000 7000
GS 4,4 2,6 5,2 2,1 0,88 9500 6000
Parana pine SS 9 5,4 9,5 2,4 1,03 11000 7500
GS 6,4 3,8 8,1 2,2 1,03 9500 6000
Pitch pine SS 10,5 6,3 11 3,2 1,16 13500 9000
GS 7,4 4,4 9,4 2,8 1,16 11000 7500
western red cedar SS 5,7 3,4 6,1 1,7 0,63 8500 5500
GS 4,1 2,5 5,2 1,6 0,63 7000 4500
Douglas fir-larch SS 7,5 4,5 7,9 2,4 0,85 11000 7500
GS 5,3 3,2 6,8 2,2 0,85 10000 6000







Grade stresses for selected imported hardwoods for service classes 1 and 2

Stresses applicable to timber 300 mm deep (or wide)
Service class 3 grade stresses are obtained be multiplying the stresses in the tables by the factor K2        ref modifying factors


Name Grade Bending stress Tension Compression Shear Modulus of Elasticity
parallel to
grain
parallel to
grain
parallel to
grain
perpendicular
to grain
parallel to grain Mean Minimum
σm,g,ll σt,g,ll σc,g,ll σc,g,l- τg,ll Emean Emin
N/mm2 N/mm2 N/mm2 N/mm2 N/mm2 N/mm2 N/mm2
Ekki HS 25 15 24,6 5,6 3 18500 15500
Greenheart HS 26,1 15,6 23,7 5,9 2,6 21600 18000
Iroki HS 12,6 7,5 12,6 2,8 1,6 10600 8500
Jarrah HS 13,8 8,2 14,2 3,1 2 12400 8700
Kapur HS 18,1 10,9 18 4,1 1,9 19200 15800
Karri HS 17,1 10,3 15,2 3,9 1,7 17800 13500
Kempas HS 19,3 11,6 19,4 4,3 2,3 19100 16000
Keruing HS 16,2 9,7 16 3,6 1,7 19300 16100
Merbau HS 18,1 10,9 15,7 4,1 2,3 15900 11700
Opepe HS 17 10,2 17,6 3,8 2,1 14500 11300
Teak HS 13,7 8,2 13,4 3,1 1,7 10700 7400

The timber in the above table has visual strength grade HS = structural tropical Hardwood








Grade stresses for selected temperate hardwoods for service classes 1 and 2

Stresses applicable to timber 100 mm deep(or wide).
Service class 3 grade stresses are obtained be multiplying the stresses in the tables by the factor K2     ref modifying factors


Name Grade Bending stress Tension Compression Shear Modulus of Elasticity
parallel to
grain
parallel to
grain
parallel to
grain
perpendicular
to grain
parallel to grain Mean Minimum
σm,g,ll σt,g,ll σc,g,ll σc,g,l- τg,ll Emean Emin
N/mm2 N/mm2 N/mm2 N/mm2 N/mm2 N/mm2 N/mm2
Oak TH1 9.6 5.8 9.3 3 2 12500 8500
  TH2 7.8 4.7 8.4 3 2 10500 7000
  THA 12.6 7.6 10.5 3 2 13500 10500
  THB 9.1 5.5 9 3 2 12000 7500
Sweet chesnut TH1 7.6 4.5 8.3 2.3 2 11300 6300

The timber in the above table has visual strength grade TH1,TH2, THA, or THB = structural temperate hardwood grades in accordance with the requirements of BS 5756:2007






Marking

Every piece of strength graded timber should be clearly and indelibly marked with the following information.(See first link below)



  • Grade/strength class
  • Specification of species.
  • Number of relevant British Standard
  • Company and grader/machine used
  • Company logo
  • Timber condition


A typical stamp is shown below


Links Providing information on Wood
  1. Introduction to Structural Timber design to The Eurocodes..Very important document to latest codes :75 page document
  2. Strength Graded Timber Mark..Download showing identification to be shown on strength graded timber
  3. Wood Handbook -- Wood as an Engineering Material..Downloads ..Comprehensive Document (American ) Excellent
  4. Canadian Wood Council ..Excellent site on Wood Engineering - my words
  5. Timber Trade Federation ..The Timber Trade Federation is the official voice of the UK timber trade.
  6. Scottish Timber Trade Association..Home page with links to useful information pages.
  7. In situ strength grade..Notes on visual strength grading of timber
  8. BSW Timber..Large UK Sawmill homepage

This Page is being developed



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Last Updated 15/06/2012