Civil Engineering Reference
In-Depth Information
wood-based materials. The movement of timber and wood-
based materials with time, varying moisture content and tem-
perature change are also signifi cant considerations in design.
The following describes the infl uence of these factors together
with the durability of timber and wood-based materials.
Tension
Compression
Emean
Parallel to grain (0
o
)
14.0 N/mm
2
21.0 N/mm
2
11.0 kN/mm
2
Perpendicular to
grain (90
o
)
0.5 N/mm
2
2.5 N/mm
2
0.37 N/mm
2
Table 19.2 Characteristic strengths of C24 timber at 0
o
and 90
o
to the
grain (BSI, 2009). Permission to reproduce extracts from BS EN 338 is
granted by BSI
19.3.2 Grading of timber and wood-based products
All sawn timber used structurally should be traceable by the
marking of each piece with an appropriate stamp describing
the grade, the source of the material and how it was graded
(visually or by machine with reference to an approved person
or machine). If this is not possible, for example, for aesthetic
reasons, then there should be documentation recording this in-
formation. The head standard for grading is BS EN 14081-1
(
Timber Structures: Strength Graded Structural Timber with
Rectangular Cross Section - Part 1: General Requirements
)
and for the visual grading of softwoods and hardwoods BS
4978 (
Visual Strength Grading of Softwood: Specifi cation
) and
BS 5756 (
Specifi cation for Tropical Hardwoods for Structural
Use
). Note that the traceability of the grading procedures also
provides a step towards providing sustainable forestry and
establishing the chain of certifi cation.
Glued laminated timber is manufactured in accordance with
BS EN14080 from laminates graded. Laminated veneer lum-
ber (LVL) is manufactured in accordance with BS EN 14279.
Board materials such as particle board, oriented strand
board (OSB), hardboard and plywood are manufactured to a
particular standard for the material and grade all of which are
identifi ed in the standard for the strength values BS EN12369
(
Wood-Based Panels: Characteristic Values for Structural
Design
).
In design there is no differentiation between the strengths
radially and tangentially to the grain - a single value is given
(
Table 19.2
).
These orientation differences are refl ected also in board
materials where the direction of the face grain veneers in ply-
wood and the orientation of the fl akes in oriented strand board
infl uences strength and stiffness.
19.3.4 Moisture content and service classes
Timber and wood-based materials are moisture sensitive.
Expressed very simply, solid timber is comprised of a series of
interconnecting cells and when a tree is felled there is water in
the cells and in the cell walls. The ratio of the weight of water
in the timber to the weight of oven dried wood fi bre (the mois-
ture content usually expressed as a percentage) is in excess of
100% at this stage. When the timber is dried, the free water
in the cells is lost fi rst until at a particular moisture content
(known as the
fi bre saturation point
) any further drying takes
moisture from the cell walls. Typically the fi bre saturation
point is around 25% and drying below this causes shrinkage
The next moisture content threshold for solid timber is 20% -
above this the timber is 'wet', liable to decay, diffi cult to glue
and has lower tabulated strength and stiffness values.
Moisture classifi cations are defi ned in both BS 5268 and
EC5 as Service Classes. Timber with a moisture content over
20% is in Service Class 3. Between 12% and 20% in Service
Class 2 and 12% and below in Service Class 1. Examples of
these Service Classes in a building are given in
Table 19.3
(taken from the UK National Annex Table NA.2).
There is no differentiation between SC1 and SC2 regarding
strength properties but they do affect creep defl ection.
19.3.3 The infl uence of grain direction on the
properties of timber
Timber is an orthotropic material, i.e. it has different strength
and stiffness values in each of the three principal axes - longi-
tudinally parallel to the grain of the timber, perpendicular to
the grain in the radial direction normal to the growth rings and
perpendicular to the grain but tangential to the growth rings
(
Figure 19.5
).
Type of construction
Service class
Cold roofs
2
Warm roofs
1
perpendicular
radial
perpendicular
tangential
Intermediate fl oors
1
Ground fl oors
2
Timber-frame walls - internal and party walls
1
2
- external walls
External uses where member is protected from
direct wetting
2
External uses, fully exposed
3
Table 19.3 Service classes (BSI, 2004). Permission to reproduce
extracts from UK NA BS EN 1995-1-1 is granted by BSI
Figure 19.5 Grain directions in sawn timber
