Design of Tension Members in Timber
Tension member Tr = An ft (KDKHKStKT) KZt = 0.9 An = net area 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
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 An = net area 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 (~3 months) 90 creep of the lignin matrix 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)
Tension member Tr = An ft (KDKHKStKT) KZt An = net area 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 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 (%)
Tension member Tr = An ft (KDKHKStKT) KZt An = net area 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 when wood surface is incised Fire retardant has a weakening effect on strength of wood
Tension member Tr = An ft (KDKHKStKT) KZt An = net area 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 Length effect A,L Tr Tr 5A,L < 5Tr < 5Tr A,L Tr Tr (not covered in code)
Examples of tension members where a length effect would apply
Tension member Tr = An ft (KDKHKStKT) KZt An = net area 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
3 or more members sharing Load sharing KH 3 or more members sharing Type 1 system KH = 1.1 < 600mm
3 or more members sharing Load sharing KH 150mm edge nailing 300mm intermediate 9.5mm min. sheathing 3 or more members sharing Type 2 system KH = 1.4 < 600mm
Platform construction