Civil Engineering Reference
In-Depth Information
glued joints. All this supplementary information, collectively
known as Non-Contradictory Complementary Information
(NCCI), is given in PD 6693-1 (
Complementary Information
to Eurocode 5: Design of Timber Structures Part 1 - General:
Common Rules and Rules for Buildings
).
The Eurocodes presume a minimum level of academic
qualification so no mention will be found in EC5 of matters
that would be presumed to have been taught in these studies.
For example, the effective length of compression members in
relation to the end conditions is not given in EC5 so this infor-
mation is included in the NCCI.
At the same time as the Eurocode was being drafted, the
necessary supporting standards were being prepared by the
Comité Européen de Normalisation
(CEN). CEN standards are
written with each aspect of a particular material such as manu-
facture, grading, testing, marking, etc., being set out in a sep-
arate document whereas the corresponding British Standard
would encompass all these facets in a single document. As a
result there are numerically many more CEN standards refer-
enced in EC5-1-1 than British Standards in BS 5268.
The EC5 design procedure starts with characteristic strength
values with modification factors giving generally a lower design
strength value whereas BS 5268 starts with the long term, low-
est grade stress that may be factored upwards to achieve a higher
permissible design stress. BS 5268 may therefore be considered
'fail safe' if modification factors are not used whereas EC5 is
'fail unsafe' if modification factors are not used.
A new code of practice - referring to new technology and
with unfamiliar wording - is always a problem for the user.
To compound this issue, EC5 will be an introduction to limit
states design for many of its readers.
Size: mm
75
100
125
150
175
200
225
250
275
300
22
25
38
47
63
75
100
150
200
250
300
Table 19.1 Preferred sizes for sawn timber (BSI, 2003). Permission to
reproduce extracts from BS EN 336 is granted by BSI
regularised and machined on four sides dimensions are defined
as 'target sizes' and any of the target sizes may be used in an
EC5 calculation.
Surfaced or machined dimensions are achieved by subtract-
ing 3 mm from sawn dimensions of 100 mm or less and 5 mm
from the dimensions greater than 100 mm.
BS EN336 does not preclude the use of 'non-standard' dimen-
sions for sawn, regularised or machined on four sides timber where
such dimensions may be required for a particular purpose.
Tolerances are set out in BSEN 336 as follows:
T1 is applied to sawn sizes and is -1 / +3 mm for dimensions 100 mm
or less and -2 / +4 mm for dimensions greater than 100 mm.
T2 is applied to surfaced or planed dimensions and is -1 / +1 mm
and -1.5 / +1.5 mm for finished dimensions 100 mm or less and
greater than 100 mm respectively.
So starting with a 100 × 47 sawn timber section the full
dimensional specification would be 100(T1) × 47 (T1) for
sawn timber, 97 (T2) × 47(T1) for timber regularised in the
width and 97 (T2) × 44 (T2) for 'machined on four sides'.
All these dimensional measurements are assumed to be
at a moisture content of 20%. Adjustment for differences in
dimension due to a moisture content different from 20% may
be made.
In addition to the dimensions given above, there are sizes ori-
ginating in North America - American Lumber Sizes (ALS) and
Canadian Lumber Sizes (CLS) - that are surfaced on four sides
with arris rounding (makes handling easier). The common sizes
are 38 mm × 89 or 140 mm and many European mills also prod-
uce these sizes as they are widely used in house construction.
The length of a timber section is limited firstly by the tree size
from which it is obtained and secondly by the presence of any
unacceptable defects, as defined by the strength grading rules
(see Section 19.4), that may have to be cut out. The lengths
19.2 Timber and timber products
19.2.1 Introduction
Timber is available in many forms - softwood or hardwood
either solid, laminated, veneered as in plywood and laminated
veneered lumber (LVL) or reconstituted as in oriented strand
board (OSB), particle board and fibreboard, usually in preferred
sizes and formats as set out in European Standards or dictated by
commercial production. The use of commonly available species
and standard products provides an economic basis for design.
All these materials have defined mechanical properties for
use in structural applications but the facility to 'work' a par-
ticular material can further influence choice. For example, cer-
tain hardwoods have excellent strength properties but the need
to reset and resharpen tools more frequently can offset this
attribute. Aesthetic factors such as surface texture and colour
may also influence the choice of a particular material.
19.2.2 Solid timber
BS EN336 (
Structural Timber: Sizes, Permitted Deviations
)
sets out dimensions and tolerances for sawn, regularised and
'machined on four sides' timber (
Table 19.1
). These sawn,
