Rainer Grohmann [email protected] Daniel …coste53.net/downloads/Literature/Distortion of Sawn Timber... · Distortion of Sawn Timber page 3/14 Fact sheet – Cupping Fact

Distortion of Sawn Timber

Authors

Rainer Grohmann [email protected] Daniel Aléon [email protected]

Gordon Knaggs [email protected] Rastislav Lagana [email protected]

Carlos Baso [email protected]

Content

1. Introduction

2. Overview about types of distortion

3. Fact sheets containing

- Reasons for distortions - Methods to measure distortions - Classification of distortions - Minimizing distortions

4. Appendix

1. Introduction

This leaflet is a part of the total outcome of COST Action E53 “Quality control for wood

and wood products”, which has been funded by the European Commission. Working

Group 2 in COST E53 has been supported by European Drying 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 this 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.


2. Overview

Types of Distortion

Bow

Spring or Crook

Cupping

Twist


page 3/14 Fact sheet – Cupping

Fact sheet 1

Cupping

Definition

Cupping is a …

deformation across the width of a sawn timber.

curvature perpendicular to the face of a board.

convex deformation towards the pith of the tree.

concave deformation towards the bark of the tree.

Causes

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.

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 caused by improper drying!



how to

measure

The height of the curvature across the width is

measured on the concave face with a 0,1mm scaled

device, 100 mm long, at the place where - by eye -

cupping is maximal.

how to

classify

For appearance grading of softwoods according to EN 1611:

G2/G4-0 G2/G4-1 G2/G4-2 G2/G4-3 G2/G4-4

Maximum cup

in % of the width 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

If the width is less than 100 mm, check the table and the method in the appendix.

Example for a product-oriented classification:

wall studsfloor

beams panels

Maximum cup

in % of the width 2 2 2



How to

minimize

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

Cupping cannot be avoided completely!

But it can be reduced, if the recommended measures are taken.

Distortion of Sawn Timber page 6/14 Fact sheet – Twist

X

Fact sheet 2

Twist

Definition After the drying process previously straight boards are formed like propeller blades.

Causes

Spiral grain orientation along the axis of a stem

Cells are not oriented parallel to the longitudinal

axis of a log.

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 process is not the reason for twist,

although it occurs mainly during the drying process!

How to

measure

Put one front side tight on an even underlayer.

Measure maximum height h in a distance of 2000 mm from front side, see

picture.

Express twist in mm height per 25 mm width over a 2000 mm length of board.

or

Express twist in % of the width: %100xmminwidth

mminxtwist .


How to

classify

For grading of structural softwoods according to EN 14081-1, EN 518, EN 519:

Quality level ≤ C18 > C18

Maximum twist over a

length of 2000 mm 2 mm / 25 mm width 1 mm / 25 mm width


Quality level G2/G4-0 G2/G4-1 G2/G4-2 G2/G4-3 G2/G4-4

Maximum twist in % of

the width over a length of

2000 mm

8 8 8 10 20

For timber in joinery table 1 in EN 942 gives the following limits

Quality level J2 J5 J10 J20 J30 J40 J50

twist in mm/m not

allowed

not

allowed≤ 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 STANDARD SUPERIOR

Maximum twist over a

length of 2000 mm X mm / 25 mm width Y mm / 25 mm width


How to

minimize

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.

Avoid over-drying! Try to bring the moisture content as close to the target

value as possible.

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 can be reduced, if the mentioned measures are taken.

Distortion of Sawn Timber page 9/14 Fact sheet – Bow

Fact sheet 3

Bow

Definition Timber forms a curvature over the long axis at right angles to the thickness of the

board.

Causes

Bad piling:

- bearers and piling stickers not

aligned vertically

- bearers and stickers too far apart

Curved form of the trunk

Timber contains reaction wood:

- 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

How to

measure

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

Thickness

Width Bow

2m

Distortion of Sawn Timber page 10/14 Fact sheet – Bow

How to

classify

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:


Maximum bow over a

length of 2000 mm < 20 mm < 10 mm



Maximum bow

in mm / 2 m 4 4 4 10 30

Use a product-oriented classification according to the following table:

product wall stud floor beam panel

Maximum bow

in mm / entire length 6

300

mminlength 50

How to

minimize

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.

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 of Sawn Timber page 11/14 Fact sheet – Spring or Crook

Fact sheet 4

Spring or Crook

Definition The board is curved in its own plane.

Causes

The most frequent cause is

curvature of the log:

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 process!


How to

measure

Measure maximum deviation after sawing in mm over a length of 2 m

Express result height h

in mm / 2 m:

how to

classify

For structural softwoods according to EN 14081-1, EN 518 and EN 519:


Maximum spring over a

length of 2 m < 12 mm < 8 mm



< 45 mm

thickness 10 10 15 50 50

≥ 45 mm

thickness 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‘.

2 m


how to

minimize

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.

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

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 page 14/14 Fact sheet - Appendix

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):

2

100 100

xx

wcupcup

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

Documents

Rainer Grohmann [email protected] Daniel …coste53.net/downloads/Literature/Distortion of Sawn Timber... · Distortion of Sawn Timber page 3/14 Fact sheet – Cupping Fact