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Modification Factors used in Timber Calculations



Introduction

The timber calculations reviewed in the timber design pages on this website are based on BS EN 5288 Part 2.     This standards includes modification factors used to allow for operating conditions and loading scenarios.    This page provides some guidance in assessing these factors



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.






Modifying factors
K2     by which stresses and moduli for service classes 1 and 2 should be multiplied to obtain stresses and modulii applicable to service class 3
K3   for duration of loading
K4   for bearing stress
K5   for notched beams
K6   for shapes other than rectangular
K7   width factor for bending stresses for widths other than 300mm
K8   for load sharing applications
K9   used to modify the minimum modulus of elasticity for trimmer joists and lintels
K12    for compression members
K13    for the effective length of spaced columns
K14     width factor for tensile loaded members for widths other than 300mm
K15, K16, K17, K18, K19 and K20    for single grade glued laminated members and horizontally
K27, K28 and K29    for vertically glued laminated members
K30, K31 and K32    for individually designed glued end joints in horizontally glued laminated members

Information on correction factors






      K2

Modification factor, K2, by which stresses and moduli for service classes 1 and 2 should be multiplied to obtain stresses and moduli applicable to service class 3.

Note:.. The properties provided in BS 5368:2007 for the different timbers are generally applicable to Service class 1 & 2.    Please refer to notes on service class Service Class

PropertyValue of K2
Bending parallel to grain0,8
Tension parallel to grain0,8
Compression paralle to grain0,6
Compression parallel to grain0,6
Shear parallel to grain0,9
Mean & Min Modulus of Elasticiy0,8





      K3
Duration of LoadValue of K3
Long Term (e.g.permanently imposed )1,00
Medium term (e.g. dead + temporary imposed1,25
Short Term (e.g. dead + imposed + wind1,5
Very short term (e.g. dead + wind1,75







      K4

The grade stresses for compression perpendicular to the grain apply to bearings of any length at the ends of a member, and bearings 150 mm or more in length at any position.    However for bearings less than 150 mm long located 75 mm or more from the end of a member, as shown in figure belwo thee grade stress should be multiplied by the modification factor, K4



length of BearingValue of K4
101,74
151,67
251,53
401,33
501,20
751,14
1001,10
150+1,00





      K5

This factor is applied for members taking load at notched ends as shown below






      K6

Grade bending stresses primarily apply to solid timber members of rectangular cross-section.    For other shapes of cross-section the grade bending stresses should be multiplied by the modification factor, K6, where

K6 = 1.18 for solid circular sections;
K6 = 1.41 for solid square sections loaded on a diagonal.





      K7

The grade bending stresses provided in the tables is generally based on sections 300mm deep a constant (K7)is used when other thicknesses are used.






      K8

In construction applications where four or more members are sharing the imposed load e.g. rafters , joists, trusses etc. with suitable provisions for distributing the load e.g. boarding, battens etc the admissibles stresses and the modulus shoul be obtained using the following rules .

  • The appropriate grade stresses should be multiplied by the load sharing modification factor, K8 = 1.1.
  • The mean modulus of elasticity should be used to calculate deflections and displacements under both dead and imposed load. However if the loading results from mechanical plant and equipment, for storage, or for floors subject to vibrations, then the minimum modulus of elasticity should be used.






      K9

The members which are sharing the load, as described above, the minimum modulus of elasticity hould be modified by the factor, K9, when calculating the deflection deflections


No Pieces Value of K9
Softwoods Hardwoods
11,001,00
21,141,06
31,211,08
4 or more1,241,10





K14

The grade compressive stress listed in the tables are based on a width of 300mm. For timber members of other sizes a factor K14 should be used.

K14 = 1,17 for solid timber with a width less than 72mm

K14 = (300 h.)0,11 for solid members having a width h greater than 72mm







K15,K16,K17K18,K19,K19

Modifcation factors for horizontally glued single grade laminates


Strength ClassNumber of Laminations Bending Parallel
to Grain
Tension parallel to grain Compression parallel to grainCompression normal to grain Shear parallel to grain> Modulus of Elasticity
K15 K16K17K18K19K20
C27,C30,C35, C40
D50,D60,D70
4 or More1,11,11,041,491,491,00
C16,C18,C22, C24
D30,D35,D40
41,261,261,041,552,341,07
51,341,34
71,391,39
10 or more1,421,42





K27,K28,K29

Modification factors for vertically glued single grade laminates




Number of laminationsBending, Tension and shear parallel to grain
K27
Mod of elasticity and compression parallel to grain
K28
Compression normal to grain
K29
Softwoods HardwoodsSoftwoods HardwoodsSoftwoods and Hardwoods
21,111,061,141,061,10
31,161,081,211,081,10
41,191,101,241,101,10
51,211,111,271,111,10
61,231,121,291,121,10
71,241,121,301,121,10





K30,K31,K32
TimberBending, parallel to grain Tension, parallel to grain Compression, parallel to grain
K28K31K32
Softwood1,631,631,43
Hardwood1,321,321,42
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Last Updated 8/05/2009