STRUCTURAL DESIGN FOR ARCHITECTURE

Timber structures

Fig. 6.20

Tongued-and-

grooved decking

elements are used

here in conjunction

with laminated

timber frames. The

use of thick cross-

sections for the

decking elements

allows the elimin-

ation of secondary

structural elements

such as purlins.

6.6.2 Composite boards

6.6.2.1 Introduction
Composite boards are manufactured products
composed of wood and glue (Fig. 6.21). They
are intended to exploit the advantages of
timber while at the same time minimising the
effects of its principal disadvantages, which are
variability, dimensional instability, restrictions
in the sizes of individual components and
anisotropic behaviour.
There are two basic types of composite
board: laminated boards (also called
plywoods), in which the constituent parts are
relatively large and are either veneers or strips
of wood, and particle boards, in which the
constituent pieces of timber are very small
fibres or particles.

6.6.2.2 Laminated boards
The components of laminated boards are
assembled by gluing such that the direction of
the grain is different in adjacent layers. This,
together with the high level of glue impregna-
tion, reduces variability and improves dimen-
sional stability in the plane of the board. It

also reduces the tendency of the board to split

in the vicinity of nails and screws.

Laminated boards are subdivided into two

categories: veneer plywood and core plywood.

Veneer plywood is defined as plywood in which

all the plies are made up of veneers, up to

7 mm thick, orientated with their grain parallel

to the surfaces of the panel. Core plywood is

plywood which has a relatively thick core and

which may contain layers with grain normal to

the plane of the board. Batten board, block-

board and laminboard are included in this

second group (Fig. 6.21).

The plywood which has been most exten-

sively used as a structural material is veneer

plywood. A large number of types of this are

produced by the timber industry but few are

considered suitable for structural use. The

principal structural plywoods which are used in

the UK are listed in BS 5268. These are usually

available in sheets measuring 2440 mm by

1220 mm and in a range of thicknesses and

surface qualities. The good in-plane rigidity

and out-of-plane bending strength of veneer

plywood make it suitable for a variety of struc-

tural applications, usually in some form of 201