STRUCTURAL DESIGN FOR ARCHITECTURE

A2.7.2 Tension elements

The area of cross-section required can be

determined from:

Areq = P/fapp (A2.13)

where:

Areq = area of cross-section required. If

the end connection is of the

bolted type the cross-section

selected should be increased by

20%

P = applied axial load

fap - permissible axial stress. This

will depend on the species of

timber. Typical values are:

softwood 4 N/mm^2

laminated timber 6 N/mm^2

hardwood 9 N/mm^2

A2.7.3 Bending elements

Solid rectangular sections

The section modulus required is found from:

Zreq = M/fpb (A2.14)

where:

Zreq = section modulus required (bd^2 /6

for a rectangular cross-section)

M = maximum applied bending

moment

fpb = permissible bending stress.

Typical values for this are,

softwood 6 N/mm^2

laminated timber 12 N/mm^2

hardwood 15 N/mm^2

The deflection is more likely to be critical than

with other materials and should be checked.

Because most timber beams carry distributed

loads, the requirement for adequate stiffness is

satisfied if the following relationship is

complied with:

/req = 4.34WL^2 /E (A2.15)

where:

/req = second moment of area of

cross-section [bdV\2 for a

rectangular cross-section)

W = total applied load

L = span

E = modulus of elasticity of timber.

Typical values for this are:

softwood 8000 N/mm^2

laminated timber 10000 N/mm^2

hardwood 15000 N/mm^2

A2.7.4 Compressive elements

Timber columns and other compressive

elements must be sized from the trial-and-

error procedure outlined in Section A2.4.3. The

permissible stress is determined by the

slenderness ratio. Typical values of permissible

stress are given in Table A2.7.

Table A2.7 Typical permissible compressive

stresses in timber

Slenderness

ratio fpc (N/mm^2 )

Le/r Softwood Laminated timber Hardwood

0 7 8 16

5 6.83 7.80 15.60

10 6.66 7.61 15.22

20 6.27 7.17 14.37

30 5.79 6.62 13.23

40 5.15 5.88 11.76

50 4.35 4.97 9.94

60 3.54 4.05 8.10

70 2.86 3.26 6.53

80 2.31 2.64 5.28

90 1.90 2.17 4.34

100 1.58 1.80 3.60

140 0.85 0.97 1.94

160 0.66 0.75 1.50

200 0.43 0.49 0.97

For rectangular cross-sections, r = 0.288t

t = the smaller of the cross-section dimensions.

Structural Design for Architecture

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