Design of Tension Members in Timber

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					Design of Tension Members in Timber
        Tension member
Tr =  An ft (KDKHKStKT) KZt
where
 = 0.9
An = net area                    Ag    An
ft = tension strength
KD = load duration factor
KH = system factor (load sharing)
KSt = service condition factor (dry/wet)
KT = treatment factor (chemicals)
KZt = size factor
Design values for structural joists and planks (MPa)
Design values for
Douglas fir glulam
     (MPa)
Glulam tension members
           An      Ag



T                                T

    Tr    = minimum of
           An ftn (KDKHKStKT)
    and    Ag ftg (KDKHKStKT)
Glulam tension members
                    Zone C
                    Zone D




T                                       T
           An      Ag

Probability of a defect in zone C is much
  smaller than in zone D, therefore
                 ftn > ftg
         Tension member
Tr =  An ft (KDKHKStKT) KZt
where
An = net area
KD = load duration factor
KH = system factor (load sharing)
KSt = service condition factor (dry/wet)
KT = treatment factor (chemicals)
KZt = size factor
                                Duration of load effect
                                          KD
                       100
                             KD = 1.15
                             Wind load       most design cases
                       90                      (~3 months)
Specified strength %




                       80                  KD = 1.0
                                                              creep of the lignin matrix
                               Live load
                       70


                                 KD = 1.0 – 0.5 log (D/L) ≥ 0.65
                       60
                                                                                     KD = 0.65
                       50                                                            Dead load

                               2000        4000        6000           8000 1 year   10,000

                                              Load duration (hours)
         Tension member
Tr =  An ft (KDKHKStKT) KZt
where
An = net area
KD = load duration factor
KH = system factor (load sharing)
KSt = service condition factor (dry/wet)
KT = treatment factor (chemicals)
KZt = size factor
                                              Effect of moisture content
% of strength at 12% moisture content




                                        100
                                                        impact strength




                                                                                                  fibre saturation point
                                        80                                modulus of elasticity


                                                                            modulus of rupture

                                        60




                                                                          crushing strength
                                        40
                                              12   15            20                 25                            28       30

                                                        moisture content of wood (%)
         Tension member
Tr =  An ft (KDKHKStKT) KZt
where
An = net area
KD = load duration factor
KH = system factor (load sharing)
KSt = service condition factor (dry/wet)
KT = treatment factor (chemicals)
KZt = size factor
     Chemical treatment of wood
               against
           decay and fire




Typically no effect, except    Fire retardant has a
when wood surface is incised   weakening effect on strength
                               of wood
         Tension member
Tr =  An ft (KDKHKStKT) KZt
where
An = net area
KD = load duration factor
KH = system factor (load sharing)
KSt = service condition factor (dry/wet)
KT = treatment factor (chemicals)
KZt = size factor
         Size effect
               A,L
   Tr                           Tr
               5A,L
< 5Tr                           < 5Tr

        Length effect
               A,L
   Tr                           Tr

              A,2L
< Tr                                 < Tr
        (not covered in code)
Examples of tension members where a
     length effect would apply
         Tension member
Tr =  An ft (KDKHKStKT) KZt
where
An = net area
KD = load duration factor
KH = system factor (load sharing)
KSt = service condition factor (dry/wet)
KT = treatment factor (chemicals)
KZt = size factor
Load sharing systems




               Bridging
               (load sharing)




           Closely spaced members
           (load sharing)
            Load sharing KH

3 or more
members
 sharing




                         Type 1 system
                            KH = 1.1
              Load sharing KH
150mm edge nailing       9.5mm min. sheathing
300mm intermediate




 3 or more
 members
  sharing

                             Type 2 system
                                KH = 1.4
  Platform
construction

				
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posted:12/4/2011
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