effects of climate and climate variations on strength
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Eff t f li tEffects of climateand
li t i ticlimate variationson
t thstrength
Martin Häglund
2008-09-23
Martin Häglund, Div. of Structural Engineering, Lund University1
Outline
W d d M i t• Wood and Moisture• Moisture induced stress (MiS)
C id ti f i t ti• Consideration of moisture as an action– an external “load” to be combined with other loads
Martin Häglund, Div. of Structural Engineering, Lund University2
Varying climate
100Stockholm Outdoor and Indoor RH, starting 1 Jan
80
90
60
70
RH [%]
40
50
20
30
MA filtered over 14 days Indoor, 20oCO td
0 200 400 600 800 1000 12000
10
(assumed 3g/m2 indoor moisture production) Outdoor
Martin Häglund, Div. of Structural Engineering, Lund University3
RHdays
Varying climateMoisture content variation in Glulam (90*100*600 m3) in unheated
room southern Sweden.
Humid during winter
Dry during winter
(Alpo Ranta-Maunus)
Martin Häglund, Div. of Structural Engineering, Lund University4
Varying climate• Climate variation
– ”Noisy” behavior– “Regular” oscillations are on a daily and annual basis– Regular oscillations are on a daily and annual basis
Frequency analysis:
Seasonal variation
T= 1 year
Daily variationT= 1 dayT= 1 day
Martin Häglund, Div. of Structural Engineering, Lund University5
[day-1]
Varying climate
Physical relation between vapour content and temperature
Martin Häglund, Div. of Structural Engineering, Lund University6
Temperature [degree Celcius]
Varying climate
Heated room (Espoo)
Sheltered environment (Kirkkonummi)( )
Martin Häglund, Div. of Structural Engineering, Lund University7 (Alpo Ranta-Maunus)
Varying climate
• Moisture penetration in timberp– Wood is basically a low-pass filter
• Swift variations are damped• Seasonal variations penetrate, although somewhat phase-shifted (cf. surface p , g p (
and middle)
“illustrative cross-section”
Martin Häglund, Div. of Structural Engineering, Lund University8
Varying climate
Another illustrative example of moisture penetration in wood
Martin Häglund, Div. of Structural Engineering, Lund University9
(Alpo Ranta-Maunus)
Properties of wood
• Principal propertiesHygroscopic– Hygroscopic
• Tries to be in balance with the surrounding climate.• Anisotropic
M t i l t f ti f di ti i t t t t t t• Material parameters functions of direction, moisture content, temperature etc.
– Low strength perpendicular to grain
Picture: Courtesy of Johan Jönsson, Lund University, 2005
Martin Häglund, Div. of Structural Engineering, Lund University10
Properties of woodNormalized rank: Comparison between compression, tension and bending
strength at three different moisture contents.
(Alpo Ranta-Maunus)
Martin Häglund, Div. of Structural Engineering, Lund University11
Properties of wood
• Dry wood stronger than wet wood• Rule of thumb maximum strength when MC around 10%Rule of thumb, maximum strength when MC around 10%• High temperatures are negative
– Above 60 oC, degrading of wood starts, permanent effect, g g , p
N dj t t f b di d t i d t MC (EN384)• No adjustment for bending and tension due to MC (EN384)– Compression is adjusted 3% per every percentage point difference in MC
Martin Häglund, Div. of Structural Engineering, Lund University12
Transport of moisture
• Transient transport of moisture– Drying / wetting - until equilibrium is reached
• Resistance to transportSurface (boundary) / Body (the domain)– Surface (boundary) / Body (the domain)
• Continuity equationCo u y equa o– Fickian law
– Boundary θ−= θgradDq
– Domain ⎟⎠⎞
⎜⎝⎛
∂θ∂
∂∂
=∂∂
θ xD
xtw
Martin Häglund, Div. of Structural Engineering, Lund University13
⎠⎝ ∂∂∂ xxt
Moisture in wood
Different situations• Outdoor
– Sheltered / unsheltered– Temperature, relative humidity, sun exposure
I d• Indoor– Relative humidity (mainly)
• Location – ”normal” or humid/dry conditions (e.g. public baths)Location normal or humid/dry conditions (e.g. public baths)
Moisture gradients and changes important• Moisture gradients and changes important– Restraint of hygroexpansion– Absolute values not as important (beside the risk for rot and decay)p ( y)
Martin Häglund, Div. of Structural Engineering, Lund University14
Moisture induced stress• Effect of hysteresis, isoterms • Influence of scanning curves (variation of RH cause little effect on
MC)MC)
Martin Häglund, Div. of Structural Engineering, Lund University16
Moisture induced stress• Stress models
– Assumption that different types of stress are additive
umscetotal ε+ε+ε+ε=ε &&&&&
Hygro expansiveelastic
Viscoelastic creep Mechano-sorptive
Hygro-expansive
Viscoelastic creep known to be negligible to MS creep:
u)uum(Etotal &&&&
& α+β+σ+σ
=ε
Martin Häglund, Div. of Structural Engineering, Lund University17
Moisture induced stress
• Classic illustrative figure showing effect of mechano-sorption (note: small clear wood specimens)
Martin Häglund, Div. of Structural Engineering, Lund University18
Moisture induced stress
Stress developmentMC
Moisturevariation in air
60
80
100
RH
[%] Uneven moisture
distribution
MC
0 200 400 600 800 100020
40
days x [m]
U t i t hStress
Uneven hygro-expansion Restraint causes
compression/tension
Unrestraint hygro-exp.
compression/tension
x [m]Due to uneven MC but also to variation of radial and tangential wood directions
Martin Häglund, Div. of Structural Engineering, Lund University19
Moisture induced stress
Stress development
MoisteningEquilibriumt
Drying
pressure
no stress
pressure
tension
(rough schematic distribution of stress over a glulam cross-section)
Martin Häglund, Div. of Structural Engineering, Lund University20
Moisture induced stress• Induced stresses at the surface and in the middle during 1000 days for Sturup
together with indoor variation in relative humidity (starting January 1st).
Martin Häglund, Div. of Structural Engineering, Lund University21
Moisture induced stress• 23 years calculation: Stress range and CDFs (max tension)
Martin Häglund, Div. of Structural Engineering, Lund University22
Moisture induced stress• Combination of moisture induced stress and mechanical loading
– Detailing (fasteners), curved beams, tapered beams, notched beams …
h
90σ
M
h
M
M3 r
rbh2M3
90 =σr
Martin Häglund, Div. of Structural Engineering, Lund University23
Moisture induced stress
• Experiments show effect of moisture on tensile capacity (Jönsson J. 2005)
Martin Häglund, Div. of Structural Engineering, Lund University24
ExternalExternal
Moisture induced stressExternal stressExternal stress Moisture
inducedstress
Moisture inducedstress
(From Jönsson)
+++ + -+
- -+
- -+
- -+
- -+
- -+
- = Very non-uniform stress distribution+ =
2,5
Stress (MPa)
stress distribution
Stress (MPa)
”uniform” stress distribution
1,5
2,0
,5
Mean stress
Combined stress2,0
2,5
Mean stress
External stress
Combined stress
Drying phase0,5
1,0
Internal stress
External stress
1,0
1,5
Drying phase RH 80 40%
Moistening phase1 0
-0,5
0,0
Sli
0,0
0,5
Internal stress
Moistening phase RH 40 80%
Martin Häglund, Div. of Structural Engineering, Lund University25
-1,0
1 2 3 4 5 6 7 8 9 10 11
Slice-0,5
1 2 3 4 5 6 7 8 9 10 11
Slice
Moisture induced stress(From Jönsson)(From Jönsson)
Curved beam
when exposing the beam to when exposing the beam to moisture induced stressesmoisture induced stresses +
-The beam usedin the testin the test,cross-section 90x280 where the lamelleas are 20 mm thick
860
SFS-screw
F Fare 20 mm thick SFS-screw
280
R=121520.7o
110 110280
239
Martin Häglund, Div. of Structural Engineering, Lund University26
160
2060
Moisture induced stress
Rh 40Rh 40--80% and 8080% and 80--40%, (no screws)40%, (no screws) Can clearly see an effect on the strength when
i t i d d i th
(From Jönsson)
moistening and drying the beam.
In the moistening
Failed in shear, meaning that the tensile strength perpendicular to grain is not the limiting factor
More reasonable shape of the curve, if the shear failure was prevented
1.40
1.60
1.80 Bending test, drying phaseBending test, mean value, seasoned in RH 40%
In the moistening phase the tensile stresses are added in the inner part
0.80
1.00
1.20
ess (
MPa
)
Bending test, mean value, seasoned in RH 80%
in the inner part, leading to a very uniform stress distribution
7.179.037.1
=
0 20
0.40
0.60
Stre
Bending test, moistening phase
distribution.
Leading to lower strength
Failure mode: Tension perpendicular to grain
0.00
0.20
0 5 10 15 20 25 30 35 40
Day of test
Failure mode: Tension perpendicular to grain
Martin Häglund, Div. of Structural Engineering, Lund University27
Moisture induced stress– Superposition of stress distribution– Wetting and drying effect differs
• Drying leads to more uniform combination of stresses – BETTER!• Wetting leads to the opposite
2,0
2,5
Stress (MPa)
20
2,5
Stress (MPa)
External stress stress
1,0
1,5 Mean stress External stress
Combined stress
1,5
2,0
Mean stress Combined stress
0,0
0,5
Internal stress 0,5
1,0 Drying phase
-1,0
-0,5
Slice -0,5
0,0
Slice
Internal stress stress
Moistening phase
Martin Häglund, Div. of Structural Engineering, Lund University28
1 2 3 4 5 6 7 8 9 10 11 1 2 3 4 5 6 7 8 9 10 11
Slice
Consideration of moisture as an action
Martin Häglund, Div. of Structural Engineering, Lund University29
Consideration of moisture as an action
Consideration of moisture in building codes
Martin Häglund, Div. of Structural Engineering, Lund University30
Consideration of moisture as an action
Consideration of moisture in building codes
Martin Häglund, Div. of Structural Engineering, Lund University31
DOL effects due to moisture and loading time
Martin Häglund, Div. of Structural Engineering, Lund University32
(P Hoffmeyer, Timber Engineering, Wiley 2003)
Consideration of moisture as an action
• Moisture induce stress in timber structures– Results in non-uniform stress field– Strength perpendicular to grain is “weak”
( ) ( )kk f∑∑• General design criteria ( ) ( )
Mmod
1iQiiQi
1iGiGi
fkγ
≤σψγ+σγ ∑∑==
• Two general design options– Should the action be considered on the resistance side, or on the action
side ?side ?• Today it is on the resistance side in form of strength reduction factor
Martin Häglund, Div. of Structural Engineering, Lund University33
Consideration of moisture as an action
M i t ff t i t dMoisture effects in current codes• Considered by service classes based on expected
ilib i i t l lequilibrium moisture levels• Time dependent variation of moisture exposure is not
consideredconsidered
A more precise characterisation is desired to impro eA more precise characterisation is desired to improve the way moisture is considered
Martin Häglund, Div. of Structural Engineering, Lund University34
Consideration of moisture as an action• How should effects of actions be combined?
– Factors ψi to consider effect of concurrent actions and their combined effect
– Summation of individual annual design values not correctVariation in time different for t ex snow wind and MiS• Variation in time different for t ex snow, wind and MiS.
– Two (or more) loading process(es) are not prone to reach their maximum loads within a given reference period (e.g. 1 year suitable in order to have (“fairly”) stationary processes at the same instance of time.
( ) ( ) d
k
QiiQi
k
GiGifk≤σψγ+σγ ∑∑( ) ( )M
mod1i
QiiQi1i
GiGi kγ
≤σψγ+σγ ∑∑==
Martin Häglund, Div. of Structural Engineering, Lund University35
Consideration of moisture as an action
• Different typs of loads have their typical variation in timeC i idi l d ti• Coinciding loads or time separated loads
• Moisture induced stress predominant during summerpredominant during summer season
Martin Häglund, Div. of Structural Engineering, Lund University36
(From Nowak & Collins, 2000)
Summary
• Strength of wood depends of MC– Compression strength is most effected
• Load bearing capacity is reduced due to moisture variation
D t b th h i t th d i d d i t di t ( hi h– Due to both change in strength and induced moisture gradients (which cause eigen-stress due to restraint of hygro-expansion)
• Eurocode uses kmod as a strength reduction factor to consider moisture impact
• Should it instead be considered as an external load?
Martin Häglund, Div. of Structural Engineering, Lund University37
Thank you for your attentionThank you for your attention
AcknowledgementJohan Jönsson (div. of Structural Engineering, Lund University) for sharing of experimental data and results.
Alpo Ranta-Maunus: Timber Engineering, 2003, Wiley, Chapter 9
Martin Häglund, Div. of Structural Engineering, Lund University38
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