means of finger joints (Fig. 6.23). The laminat-
ing process also allows the construction of
elements which have a curved profile (Figs 6.3,
6.20 and 6.24).
The general quality and strength of
laminated timber is higher than that of sawn-
timber for two principal reasons. Firstly, the
use of basic components which have small
cross-sections allows more effective seasoning,
with fewer seasoning defects, than can be
achieved with large sawn-timber elements.
Secondly, the use of the finger joint, which
causes a minimal reduction in strength in the
constituent boards, allows any major defects
which are present in these to be cut out. The
allowable stresses of laminated timber are
therefore higher than for equivalent sawn
timber. Laminated timber elements are
produced in three standards of finish: architec-
tural (machined, sanded and free from
blemishes), industrial (machined and sanded
but with blemishes permitted) and economy
(untreated after manufacture), but there is no
structural difference between these categories.6.6.3.3 Plyweb beams
Plyweb beams are built-up-beams of rectangu-
lar-box or l-cross-section in which solid timber
flanges are used in conjunction with a web ofgluing together smaller solid timber members
of rectangular cross-section, usually either
38 mm or 50 mm in thickness (Fig. 6.22). The
direction of the grain is the same for all
laminations and is usually parallel to the axis
of the element. The obvious advantage of the
process is that it allows the manufacture of
solid elements with much larger cross-sections
than are possible in plain sawn timber. Very
long elements are also possible because the
constituent boards are joined end-to-end by
Fig. 6.23 The glued finger joint
is an essential part of the technol-
ogy of laminated timber. It allows
the creation of very long sub-
elements from which major
defects, such as knots, have been
removed [Photo: TRADA].203Timber structuresFig. 6.22 Typical cross-section of a laminated timber
element [Photo: TRADA].