STRUCTURAL DESIGN FOR ARCHITECTURE

Timber structures

Fig. 6.40 Stressed-skin

deck in which composite

action takes place

between joists and

boarding. This extends

the span range of the

traditional joisted floor.

6.7 Structural forms for timber

6.7.1 Floor and roof decks
Perhaps the simplest type of timber structure
is the traditional floor or roof deck in which
sawn-timber beams (joists) are spaced at close
centres (450 mm to 600 mm) supporting a deck
of relatively thin boarding (Fig. 6.39).
Traditionally, the boarding was of machined
solid timber boards which had tongued-and-
grooved edges so as to spread the effect of
concentrated load. In recent years plywood,
blockboard and even chipboard have been
used in this role. These provide the deck with
greater resistance to in-plane loads than
tongued-and-grooved forms of boarding, and
give it the ability to act more effectively as
diaphragm bracing, and therefore to contribute
to the general stability of the structure.
Timber decks are normally used in conjunc-
tion with loadbearing-wall vertical elements
(either of timber or masonry) but can also be
used with skeleton frames. The range of spans
for which the traditional joisted deck is
normally used is 3.5 m to 6 m. Tables 6.10 and

6.11 give the spans which are possible for the

standard range of sawn-timber sizes.

Two comparatively recent developments in

this form of construction are composite decks,

in which the joists and sheeting act together in

T- or box-beam configurations, and the use of

built-up-beams in place of sawn timber to form

the joists. If plywood is used to form a floor or

roof surface and is nailed and glued to the

joists, a T- or box-beam structure is created

which is stronger, and can span further, than a

traditional boarded deck of equivalent dimen-

sions (Fig. 6.40). The resulting element is

called a 'stressed-skin' deck and this form of

structure allows spans of up to 9 m to be

achieved with the largest sizes of sawn-timber

joist. Its use therefore extends the span range

of flat timber decks made with sawn-timber

elements. A good structural connection

between the web members (internal stringers)

and the skin is required if effective composite

action is to develop, however, and the joint

should preferably be made by dense nailing

and gluing. The assembly of the deck is best

carried out under factory conditions and 215

Plywood splice

Upper skin

Lower skin

End blocking

Splice plate

Internal Nogging

longitudinal

web member

External longitudinal

web member