effects of climate and climate variations on strength

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


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


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)


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


[day-1]

Varying climate

Physical relation between vapour content and temperature


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”


Varying climate

Another illustrative example of moisture penetration in wood


(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


Properties of woodNormalized rank: Comparison between compression, tension and bending

strength at three different moisture contents.

(Alpo Ranta-Maunus)


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


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


⎠⎝ ∂∂∂ 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)


Moisture induced stress


Moisture induced stress• Effect of hysteresis, isoterms • Influence of scanning curves (variation of RH cause little effect on

MC)MC)


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



• Classic illustrative figure showing effect of mechano-sorption (note: small clear wood specimens)



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



Stress development

MoisteningEquilibriumt

Drying

pressure

no stress

pressure

tension

(rough schematic distribution of stress over a glulam cross-section)


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).


Moisture induced stress• 23 years calculation: Stress range and CDFs (max tension)


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



• Experiments show effect of moisture on tensile capacity (Jönsson J. 2005)


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%


-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


160

2060


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


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


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



Consideration of moisture in building codes


DOL effects due to moisture and loading time


(P Hoffmeyer, Timber Engineering, Wiley 2003)


• 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



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


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 ∑∑==



• 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


(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?


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


effects of climate and climate variations on strength

Documents

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d t i d t mc en384

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