1. Design of Tension Members in Timber

2. 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

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

4. Design values for Douglas fir glulam (MPa)

5. Glulam tension membersAn 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 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

8. 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)

9. 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

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 (%)

13. 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

15. 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

16. 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

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

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

20. 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

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

22. 3 or more members sharing
Load sharing KH
3 or more members sharing
Type 1 system
KH = 1.1
< 600mm

23. 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

24. Platform construction