Scaffold Basic Design Example

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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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