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