Distorti on of Sawn Timber Timber
Authors Rainer Grohmann
[email protected] grohmann@fh-r osenheim.de
Daniel Aléon
[email protected] daniel-aleon@f cba.fr
Gordon Knaggs
[email protected] gordonknaggs@ei rcom.net
Rastislav Lagana
[email protected] lagana@vsld. tuzvo.sk
Carlos Baso
[email protected] [email protected]
Content 1.
Introduction
2.
Overview about types of distortion
3.
Fact sheets containing -
4.
Reasons for distortions Methods to measure distortions Classification of distortions Minimizing distortions
Appendix
1. Introd Introd uction ucti on This leaflet is a part of the total outcome of COST Action E53 “Quality “Quality control for wood and wood products”, products”, which has been funded by the European Commission. Working Group 2 in COST E53 has been supported by European Drying Group, Group, a pool of specialists in the field of timber drying. Many practitioners who process or use sawn timber have to deal with distortions of the dried material. Often information regarding this problem is missing. The aim of t his leaflet is to supply information about the issue ‘distortion of sawn timber’ to those who have to face this problem in their daily work. The information starts with an overview about the various types of distortion. It also provides further information about the reasons, on how to measure, how to classify and how to minimize distortions. The leaflet is not product related. It is intended to handle ‘distortions’ in a more general manner. Nevertheless, users of this paper can easily find their own method to classify their specific wooden products in terms of distortion.
Distorti on of Sawn Timber
2. Overview
Cupping Twist
Types of Distortion
Bow
Spring or Crook
Distortion of Sawn Timber Fact sheet – Cuppi ng
page 3/14
Fact sheet 1
Cupping
Cupping is a …
deformation across the width of a sawn timber.
Definition
curvature perpendicular to the face of a board. convex deformation towards the pith of the tree. concave deformation towards the bark of the tree.
Cupping occurs mainly on flat sawn boards.
It is inherent in the wood due to anisotropy of shrinkage and swelling. In tangential direction wood shrinks twice as much as in radial direction.
Due to the curvature of the annual rings, in a flat sawn timber the bark face is nearer to the tangential direction than the pith face. So the shrinkage of the bark face will be higher than the shrinkage of the pith face. As a consequence the board will cup as it dries.
Causes
During shrinkage the growth rings tend to become straighter: as shown below:
Changes in the level of cupping can occur due to MC/EMC changes in service.
Re-sawing in the width of case-hardened boards (deeping) can cause boards to cup.
Cupping is not c aused by improper drying!
Distortion of Sawn Timber Fact sheet – Cuppi ng
page 4/14
The height of the curvature across the width is measured on the concave face with a 0,1mm scaled
how to
device, 100 mm long, at the place where - by eye -
measure
cupping is maximal.
For appearance grading of softwoods according to EN 1611:
Maximum cup in % of the width
G2/G4-0
G2/G4-1
G2/G4-2
G2/G4-3
G2/G4-4
3
3
3
5
5
Cupping can also be expressed as mm depth per 100 mm width measured on the concave side of a board. Example for classification could be as follows: Quality level
100 mm width
STANDARD
4.0 mm cup
SUPERIOR
2.0 mm cup
how to classify
If the width is less than 100 mm, check the table and the method in the appendix.
Example for a product-oriented classification:
Maximum cup in % of the width
wall studs
floor beams
panels
2
2
2
Distortion of Sawn Timber Fact sheet – Cuppi ng
page 5/14
Put heavy top load on stack, 180 - 1000 kg per m², e.g. made of concrete weight and
Use more piling stickers than usual
If the timber is prone to develop stresses, an increase in both temperature and relative humidity can be helpful.
Do not over-dry. Try to bring the moisture content as close to the target value as possible.
Use rift or quarter sawn patterns instead of flat sawn boards.
Rift and quarter sawing patterns
Types of sawn boards
How to minimize
Cupping cannot be avoided completely! But it c an be reduced, if the recommended measures are taken.
Distortion o f Sawn Timber Fact sheet – Twis t
page 6/14
Fact sheet 2
Twist Definition After the drying process previously straight boards are formed like propeller blades.
Spiral grain orientation along the axis of a stem
Cells are not oriented parallel to the longitudinal axis of a log.
Causes
Inherent in the tree/in the wood
Often found in juvenile timber of Spruce and other species.
Boards cut close to the pith or including the pith can be severely affected.
The drying pr ocess is not the reason for twist, although it occurs mainly during th e drying proc ess!
Put one front side tight on an even underlayer.
Measure maximum height h in a distance of 2000 mm from front side, see picture.
How to
Express twist in mm height per 25 mm width over a 2000 mm length of board.
X
measure
or Express twist in % of the width: twist
x in mm width in mm
x 100 % .
Distortion o f Sawn Timber Fact sheet – Twis t
page 7/14
For grading of structural softwoods according to EN 14081-1, EN 518, EN 519: Quality level Maximum twist over a length of 2000 mm
≤
C18
> C18
2 mm / 25 mm width
1 mm / 25 mm width
For appearance grading of softwoods according to EN 1611: Quality level
G2/G4-0
G2/G4-1
G2/G4-2
G2/G4-3
G2/G4-4
8
8
8
10
20
Maximum twist in % of the width over a length of 2000 mm
How to
For timber in joinery table 1 in EN 942 gives the following limits
classify Quality level twist in mm/m
J2
J5
not
not
allowed allowed
J10
J20
J30
J40
J50
≤ 10
≤ 10
≤ 10
≤ 20
≤ 20
Please note that EN 942 does not account for width.
Due to huge variety of requirements depending on the final product, no general classification system for twist can be given. On demand contracting parties could fix their special requirements following the example shown the table below: Any other purposes Maximum twist over a length of 2000 mm
STANDARD
SUPERIOR
X mm / 25 mm width
Y mm / 25 mm width
Distortion o f Sawn Timber Fact sheet – Twis t
page 8/14
Put heavy top load on stack, 180 - 1000 kg per m², e.g. weight made of concrete.
Use more piling stickers than usual.
If the timber is prone to develop stresses, increasing both temperature and relative humidity during kiln drying can be helpful.
How to
Avoid over-drying! Try to bring the moisture content as close to the target value as possible.
minimize
Apply or extend the conditioning phase in order to relax stresses.
Avoid small diameter logs and boxed heart / included pith.
Twist cannot be avoided completely! But it c an be reduced, if the mention ed measures are taken.
Distortion of Sawn Timber Fact sheet – Bow
page 9/14
Fact sheet 3
Bow
Definition
Timber forms a curvature over the long axis at right angles to the t hickness of the board.
Bad piling: -
bearers and piling stickers not aligned vertically
-
bearers and stickers too far apart
Curved form of the trunk
Timber contains reaction wood:
Causes
-
compression wood in softwoods
-
tension wood in hardwoods
-
both can shrink longitudinally unlike normal wood
Distorted grain around knots
Re-sawing timber containing reaction wood
Re-sawing timber containing stresses and / or casehardening
Maximum deviation in mm from the longitudinal axis on the concave side, measured over a length of 2 m on boards placed on edge, expressed as mm / 2 m
How to measure
Width
Bow 2m
Thickness
Distortion of Sawn Timber Fact sheet – Bow
page 10/14
EN 942 and most hardwood standards do not specify limits for bow as these products are normally re-sawn or machined prior to use.
For structural softwoods according to EN 14081-1, EN 518 and EN 519: Quality level
≤
Maximum bow over a
classify
< 10 mm
For appearance grading of softwoods according to EN 1611: Quality level
G2/G4-0
G2/G4-1
G2/G4-2
G2/G4-3
G2/G4-4
4
4
4
10
30
Maximum bow in mm / 2 m
> C18
< 20 mm
length of 2000 mm
How to
C18
Use a product-oriented classification according to the following table: product
wall stud
floor beam
panel
6
length in mm 300
50
Maximum bow in mm / entire length
Measure bow after sawing, not after drying!
Stickers should be positioned in vertical alignment at the appropriate centres over bearers
Where bowing is the result of improper stacking, it may be possible to reduce the bow by re-stacking and applying a steaming or stress relief schedule.
How to minimize
Pre-sort, rejecting pieces containing excessive reaction wood.
Use more piling stickers than usual.
Use heavy top load on stack, 180 - 1000 kg per m², e.g. weight made of concrete.
Do not over-dry. Try to bring the moisture content as close to the target value as possible.
If the timber is prone to develop stresses increasing both temperature and relative humidity during drying can be helpful.
Apply a stress relief or conditioning period at the end of the kiln run, particularly when timber is to be re-sawn.
Distortion o f Sawn Timber Fact sheet – Spring or
page 11/14 Crook
Fact sheet 4
Spring or Crook Definition The board is curved in its own plane.
The most frequent cause is curvature of the log:
Causes
Timber containing reaction wood: -
compression wood in softwoods
-
tension wood in hardwoods
-
leads to locally different shrinking behavior compared to normal wood
Distorted grain around knots
Internal growth stresses, for example when fast grown
Due to immediate stress relief, spring often occurs right after sawing
Rarely: internal stresses due to incorrect drying operation, for example too rapid drying. Spring or crook is not normally caused by the drying pro cess!
Distortion o f Sawn Timber Fact sheet – Spring or
page 12/14 Crook
Measure maximum deviation after sawing in mm over a length of 2 m
Express result height h in mm / 2 m:
How to measure 2m
For structural softwoods according to EN 14081-1, EN 518 and EN 519: Quality level
≤
Maximum spring over a length of 2 m
> C18
< 12 mm
< 8 mm
For appearance grading of softwoods according to EN 1611:
how to
Quality level
classify
< 45 mm thickness ≥ 45
mm
thickness
C18
G2/G4-0
G2/G4-1
G2/G4-2
G2/G4-3
G2/G4-4
10
10
15
50
50
10
10
10
20
20
EN 942 and most hardwood standards do not specify limits for spring as these are normally re-sawn or machined prior to use.
For special agreements use table shown in fact sheet 2 ‚Twist‘.
Distortion o f Sawn Timber Fact sheet – Spring or
page 13/14 Crook
Best practice to minimize the problem is to orientate the log before sawing to produce bow, not spring. Bow can be more easily corrected than spring, by correct piling (see ‘bow’).
If reaction wood or growth stresses are present, use appropriate sawing patterns to avoid normal and reaction wood on both sides of one board.
how to
Reject logs containing too much reaction wood.
If internal stresses due to inappropriate drying are the reason for spring, apply a conditioning phase after the drying phase and / or
minimize
carry out an extended storage in a climate controlled hall.
Avoid over-drying. Try to bring the moisture content as close to the target value as possible.
If the timber is prone to develop stresses, an increase in both temperature and relative humidity can be helpful.
Apply a stress relief or conditioning period at the end of the kiln run, particularly when timber is to be re-sawn.
A steaming process before drying can be helpful in reducing stresses due to growth factors.
Distortion of Sawn Timber Fact sheet - Appendix
page 14/14
Appendix Cup values for widths differing from 100 mm (in mm): Quality level
50
60
70
80
90
100
STANDARD
1.0
1.4
2.0
2.6
3.2
4.0
SUPERIOR
0.5
0.7
1.0
1.3
1.6
2.0
Cup values were calculated according to the following formula (EDG recommendation):
cup x
w cup100 100
2
x
where cup100 is a maximum cup value for width of 100 mm and
w is
the width of a board.
See also documents which may contain information on deformations:
EN 975-1
EN 1611-1
EN 14081-1
EN 518
EN 519
EN 942
EDG recommendation: Assessment of drying quality of timber
