STRUCTURAL DESIGN FOR ARCHITECTURE

Structural Design for Architecture

forms of roof construction, from which they

derive their name, but they allow a much more

economical use of timber to be made due to

the use of the triangulated form. They are

generally considered to be most suitable for

spans in the range 5 m to 11 m. Typical spans

for which trussed rafters are suitable are listed

in Table 6.8.

6.6.5 Large one-off trusses

Large trusses are used as the horizontal

elements in post-and-beam structures of both

the skeleton-frame and the loadbearing-wall

types. They are normally only specified for

situations where very high strength or rigidity

is required, either because a long span must

be achieved or because heavy loads are carried

Fig. 6.27 Trussed rafter. Trussed

rafters are lightweight triangulated

elements which are designed to be

used at close spacings to carry roof

cladding directly.

Toothed plate fastener

Main ties acting as ceiling joists

206

Rafter size Span

Pitch (degrees)

15 17½ 20 22½ 25 27½ 30 32½ 35

50X125 10.01 10.24 10.44 10.62 10.77 10.94 11.00 11.00 11.00

Table 6.8 Typical spans for fully triangulated trussed rafters

mm

38 X 75

38 X 100

38 X 125

44 X 75

44 X 100

44 X 125

50 X 75

50 X 100

m

6.03

7.48

8.80

6.45

8.05

9.38

6.87

8.62

m

6.16

7.67

9.00

6.59

8.23

9.60

7.01

8.80

m

6.29

7.83

9.20

6.71

8.40

9.81

7.13

8.97

m

6.41

7.97

9.37

6.83

8.55

9.99

7.25

9.12

m

6.51

8.10

9.54

6.93

8.68

10.15

7.35

9.25

m

6.60

8.22

9.68

7.03

8.81

10.31

7.45

9.38

m

6.70

8.34

9.82

7.14

8.93

10.45

7.53

9.50

m

6.80

8.47

9.98

7.24

9.09

10.64

7.67

9.66

m

6.90

8.61

10.16

7.35

9.22

10.81

7.78

9.80