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Scaffold Basic Design Example

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Construction Safety Division 2017
1
SCAFFOLD BASIC DESIGN EXAMPLE
Figure 1. Basic scaffolding working platform.
Let us design the components of a medium type scaffold with the configuration
1
as shown in Figure 1 and for the general building works (brickwork, window and mullion fixing, rendering and plastering)
2
.
Table 1. Loading conditions.
Nominal load: 0.5 kNm
-2
Imposed load: 2.0 kNm
-2
Wind load: Lateral load:
1. Platform
The platform is of 1.8 m wide, and may be considered to be made up of 4 planks of 450 mm width
3
.
1
Refer reg. 94(b). Construction of tubular scaffold.
2
Refer Table 1. Service loads for working platform, BS 1139-5:1990, Guidelines for Approval of Design Scaffolding.
3
Refer reg. 87(1)(b). Planks must not less than 200 mm, but if thickness > 50
mm, width must ≥ 150
mm.
Construction Safety Division 2017
2
1.1. Plank thickness
The putlogs
4
are the secondary beams for the platform. Span for platform = Spacing of putlogs, i.e. 1.2 m. The planks extends over three putlogs.
Figure 2. Platform rests on three putlogs.
Given the bending strength of timber platform
5
is 16 Nmm
-2
, and a safety factor of 2, allowable bending stress
6
max
is 8 Nmm
-2
and allowable shear stress
allow
is 4 Nmm
-2
. For simply supported beam carrying a uniformly distributed load, maximum bending moment
M
is at mid-span and equal to
8
2
wL M
(1) where
-1-2
kNm5.4m8.1kNm5.2
w
7
and
m2.1
L
kNm81.0
8m2.1kNm5.4
2-1
M
(2) Section modulus or elastic modulus
S
is functions of geometry only
8
and relates stress and internal moment during elastic or recoverable bending.
maxmax
y I M S
(3) where
32-33
mm250,101
Nmm8 Nmm101081.0
S
and
2
max
d y
4
Putlog or bearer means that part of the scaffold upon which the platform rests (reg.2).
5
Refer Table A.5, BS EN 12811-2:2004.
6
Table 2 of BS 5975:2008+A1:2011 specifies 7.20 Nmm
-2
for D30 and 10.0 Nmm
-2
for D40 hardwoods. Table 5 of the same standards classify
keruing
, karri. opepe, merbau, teak, jarrah and iroko and typical tropical hardwoods.
7
This is a load per unit width of the plank.
8
Benham, Crawford & Armstrong, Mechanics of Engineering Materials, 2
nd
edition, 1996. pp. 137.
Construction Safety Division 2017
3
Figure 3. Cross-section of a timber plank, where
y
is the distance from the neutral plane, and
y
is maximum at the surface.
Given for rectangular cross-section, moment of inertia
I
12
3
bd I
(4) Substituting eqn. (4) into eqn. (3), and solving for
d
gives
bS d
6
(5)
mm37.18
mm800,1
mm250,1016
3
d
(6)
USE 450 mm x 20 mm PLANK
Note: Width of the plank can be varied, but thickness must not be less than 20 mm. For example, if 450 mm x 20 mm is not commercially
available, can use 300 mm x 25 mm (12’ x 1’) plank, but would require 6 planks.
1.2. Plank deflection
Assume, for wood
9
, Young’s Modulus
-26-2
kNm1010 Nmm10,000GPa10
E
Now, consider plank is 300 mm x 25 mm, and applying eqn. (4) gives
4646
3
m10344.2mm10344.2
12mm25mm800,1
I
(7)
9
Table 2 of BS 5975:2008+A1:2011 specifies modulus of elasticity of hardwoods ranges from 7.6 Nmm
-2
to 12 Nmm
-2
.
Construction Safety Division 2017
4
Figure 4. Deflection under cases (a) and (b).
Under case Figure 4(a), deflection is given by
EI wL
4
009150.0
(8) Under case Figure 4(b), deflection is given by
EI wL
4
005416.0
(9) Assume only one span is loaded with imposed load, but nominal load over both spans. Applying eqns. (8) and (9) gives,
mm7285.0m10285.7
m10344.2kNm1010
m2.1m8.1kNm5.0
009150.0
4462-6
4-2dead
(10)
mm7278.1m1073.1
m10344.2kNm1010
m2.1m8.1kNm0.2
005416.0
3462-6
4-2live
(11)
mm46.27278.17285.0
livedead total
The maximum deflection for platform units shall not exceed 1/100 of the span length when suppporting the intended loads
10
. i.e.
10
In BS EN 12811-1:2003, the elastic deflection of platform unit shall not be exceed 1/100 of its span. In Scaffold Safety Handbook, Saudi Aramco, 2001, pp. 29, and OSHA US pp. 3, the limit is 1/60 of the span length.

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