Background. Moisture content, causes of distortion and its problems. Robert Kliger. Steel and Timber Structures Chalmers University of Technology

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1 Moisture content, causes of distortion and its problems Robert Kliger Magnus Bäckström, Marie Johansson Steel and Timber Structures Chalmers University of Technology Background Robert Kliger Kampen om skogen! Stål och träbyggnad Photo by: Bob Ericson

2 Good timber quality for builders means: no distortion Distortion modes Twist Spring/Crook Bow Quantification Distortion After sawing, after drying and after different moisture changes Cause Material properties Sawing pattern Modelling Statistical models Analytical models Numerical (FE) model

3 Norway and sitka spruces 3 trees, over 1 battens Top log Middle log - Sawing pattern - Material parameters - Distortion measured at different MC - Unloaded at all times Butt log Parameters caused by sawing Position of stud relative to the pith butt, middle & top logs Skew sawing Annual ring orientation Wane Pith Drying parameters All studs were dried without restraint (in a commercially-used kiln chamber) to 18% MC (Drying time 97 hours and dried temperature ºC)

4 Moisture-related twist [ ] 1 Green 18% RH ABS/TW/green ABS/TW/kiln 1 Sawn Sawing pattern Norway spruce Moisture-related twist RH: 18% 1% 18% [ ] Green 18% RH [ ] ABS/TW/green ABS/TW/kiln 1 Sawn ABS TW III [ ] ABS TW IV [ ] ABS TW V [ ] Planed Sawing pattern Norway spruce Longitudinal variation green and 18% MC, studs close to the pith Twist [ ] Bow [mm] Spring [mm] G 18% G 18% G 18% ABS/TW/green ABS/TW/kiln Logs: butt middle top AB /BO/green 1 3 ABS/BO/kiln ABS/SP/green ABS/SP/kiln

5 Twist Twist is zero directly after sawing The parameters that have an effect are: Annual ring curvature 1/r n Spiral grain angle θ Tangential shrinkage strain s Moisture content Describe an analytical model for twist Twist develop during drying Twist is highly dependent on moisture content and shows reversibility Significant parameters are Spiral grain angle, annual ring curvature, tangential shrinkage Twist - analytical model cf. Stevens and Johnston s model 3 Y = -,1 +,99 * X; R^2 =,73 Tw ist 1% mc [ ] Twist 1% mc [ ] Analytical model Twist = l r 2sθ ( 1 s) n +

6 Spiral Grain Angle (SGA) Grain angle - radial variation Grain angle [ ] 2. pith -2. Left-handed spiral grain angle (SGA) Right-handed spiral grain angle (-SGA) - Distance from the pith [cm] Grain angle - radial variation Grain angle [ ] Distance from the pith [cm]

7 Twist model Stevens and Johnston s model for twisting of one cylindrical wooden shell (196) Considers both tangential and longitudinal shrinkage Calculates the twist for cylindrical shells and calculates the average (resulting) twist y θ x

8 Test of the model The model has been tested for 1 studs of dimension of 4 x 9 x 2 mm of Norway spruce Twist change between 18% MC and 1% MC Material unloaded at all times

9 Material parameters Position of stud in relation to the pith, Radial variation in longitudinal shrinkage Radial variation in spiral grain angle A fixed value of tangential shrinkage 1 Measured twist [ ] y = 1,3x - 2,17 R 2 =, Calculated tw ist [ ] Objectives to understand bow and spring Explain the mechanisms behind bow and spring in terms of material parameters Describe an analytical model for bow and spring Suggest which material parameters to use to create sorting criteria

10 Photo from Timell, T.E Bow and spring Bow and spring develop directly after sawing due to residual stresses in the material Not possible to create a statistical model based on material data with any accuracy Difference in the longitudinal shrinkage between two faces of the studs due to moisture changes explains changes in spring or bow Analytical model - difference in the longitudinal shrinkage between two faces of the studs Bow Spring

11 Shrinkage/swelling properties LT HT.2.1. Strain in the longitudinal direction caused by a change in equilibrium MC from 9% to 3% RH Measurement of bow and spring Example of bow 1 Bow [mm] Length [mm] Bow meas. at 16% MC Bow meas. at 12% MC

12 Example of bow 1 Bow [mm] Length [mm] Bow meas. at 16% MC Bow meas. at 12% MC Material and methods 12 studs of Norway spruce Distortion measured at different moisture contents below fibre saturation point Material unloaded during moisture change Axial (longitudinal) shrinkage measured Variation in axial (longitudinal) shrinkage Based on the variation in axial (longitudinal) shrinkage 1 x 1 x 2

13 Model bow and spring.6.4 α Model bow and spring α Model bow and spring α

14 Length [mm] Measured Delta Bow Calculated Delta Bow Marie Johansson, Chalmers 1.. Bow [mm] Length [mm] Measured Delta Bow Calculated Delta Bow Conclusions It was possible to model the change in bow and spring rather accurately using the variation in longitudinal shrinkage The model was better for bow than for spring It was possible to model both S-shaped and evenly curved bow and spring

15 Consequences of drying timber to the MC according to the present standards If the requirements are set for too high MC If timber is not used within 1 day (mounted in a structure) If timber is prone to twist Twist for studs vs. MC for different percentiles Twist [ ] th 7th Median (th) Twist will double in value if MC decreases by 1% 2th 1th MC [%] Simulation of additional drying of three products which fulfil the requirements for twist at 2% MC Board Wall stud Floor beam

16 Comparison between the measured and predicted twist at 2% MC [mm] Board 22 x 1 Stud x 1 Beam x 2 Nordic Timber 9 8. Analytical model FE model Measured (at 18% MC) Illustration of the simulated shapes as a result of drying distortion to 14% MC Board Wall stud Floor beam 22 x 1 x 1 x 2 L = 3 m L = 2. m L = 4.2 m Comparison between requiremets, measured and predicted twist at 14% MC [mm] Board 22 x 1 Stud x 1 Beam x 2 Requirements (at 14% MC) 1 1 Analytical model FE model Measured (at 14% MC)

17 Conclusions The deformation of sawn timber during and after the drying process is the most important reason for downgrading Timber for indoor applications should be delivered kiln dried to an MC no higher than 14% ± 2% % higher than expected final MC How to improve straightness? In terms of twist: measure grain angle improve sawing green gluing In terms of bow and spring: knowledge of residual stresses NDT to obtain shrinkage properties How to improve straightness? In general: improve drying dry to expected MC for right end use measure distortion on line improve handling and storage

18 Missing knowledge? Bow and spring NDT Residual stresses Shrinkage data Material data using new drying methods and at high temperature