Building Construction Handbook, Eighth Edition

Joist and Beam Sizing ~ design tables and formulae have limitations,

therefore where loading, span and/or conventional joist spacings are

exceeded, calculations are required. BS 5268: Structural Use Of

Timber and BS EN 338: Structural Timber † Strength Classes, are both

useful resource material for detailed information on a variety of

timber species. The following example serves to provide guidance on

the design process for determining joist size, measurement of

deflection, safe bearing and resistance to shear force:-

Total load (W) per joist = 5 m ¾ 0„4 m ¾ 2„25 kN/m^2 = 4„5 kN

or:5 m span4„5 kN = 0„9 kN=m

Bending moment formulae are shown on page 544

BM =WL 8 =fbd

2

6

Where: W = total load, 4„5 kN (4500 N)

L = span, 5 m (5000 mm)

f = fibre stress of timber, 7„5 N/mm^2

b = breadth of joist, try 50 mm

d = depth of joist, unknown

Transposing:

WL

8 =

fbd^2

6

Becomes:

d=

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

6WL

8fb

s

=

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

(^6) ¾ (^4500) ¾ 5000
(^8) ¾7„5¾ 50
s
¼212 mm
Nearest commercial size: 50 mm ¾ 225 mm
Timber Beam Design