Transcript Timber Class Presentation

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
and
= minimum of
 An ftn (KDKHKStKT)
 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
Specified strength %
100
90
KD = 1.15
Wind load
KD = 1.0
80
70
60
most design cases
(~3 months)
creep of the lignin matrix
Live load
KD = 1.0 – 0.5 log (D/L) ≥ 0.65
KD = 0.65
Dead load
50
2000
4000
6000
Load duration (hours)
8000 1 year
10,000
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
100
impact strength
modulus of elasticity
80
modulus of rupture
60
fibre saturation point
% of strength at 12% moisture content
Effect of moisture content
crushing strength
40
12
15
20
25
moisture content of wood (%)
28
30
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
when wood surface is incised
Fire retardant has a
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
(not covered in code)
< Tr
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