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Design of Tension Members in Timber

Design of Tension Members in Timber. Tension member. T r =  A n f t (K D K H K St K T ) K Zt where  = 0.9 A n = net area f t = tension strength K D = load duration factor K H = system factor (load sharing) K St = service condition factor (dry/wet)

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Design of Tension Members in Timber

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  1. Design of Tension Members in Timber

  2. Tension member Tr =  Anft (KDKHKStKT) KZt where  = 0.9 An = net area 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 Ag An

  3. Design values for structural joists and planks (MPa)

  4. Design values for Douglas fir glulam (MPa)

  5. An Glulam tension members Ag T T Tr = minimum of  An ftn (KDKHKStKT) and  Ag ftg (KDKHKStKT)

  6. 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

  7. 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

  8. creep of the lignin matrix Duration of load effectKD 100 KD = 1.15 Wind load most design cases (~3 months) 90 KD = 1.0 80 Specified strength % Live load 70 KD = 1.0 – 0.5 log (D/L) ≥ 0.65 60 KD = 0.65 Dead load 50 2000 4000 6000 8000 10,000 1 year Load duration (hours)

  9. 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

  10. Effect of moisture content 100 impact strength modulus of elasticity 80 fibre saturation point % of strength at 12% moisture content modulus of rupture 60 crushing strength 40 12 15 20 25 28 30 moisture content of wood (%)

  11. 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

  12. Chemical treatment of wood againstdecay and fire Fire retardant has a weakening effect on strength of wood Typically no effect, except when wood surface is incised

  13. 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

  14. Size effect A,L Tr Tr 5A,L < 5Tr < 5Tr Length effect A,L Tr Tr A,2L < Tr < Tr (not covered in code)

  15. Examples of tension members where a length effect would apply

  16. 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

  17. Load sharing systems Bridging (load sharing) Closely spaced members (load sharing)

  18. Load sharing KH 3 or more members sharing Type 1 system KH = 1.1 < 600mm

  19. Load sharing KH 9.5mm min. sheathing 150mm edge nailing 300mm intermediate 3 or more members sharing Type 2 system KH = 1.4 < 600mm

  20. Platform construction

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