Loading a Cantilevered Fan Scaffold on a City Centre Facade
A scaffold is to be erected on a six-storey office block for facade cleaning and minor repairs. The elevation is 24 m long and 18 m high. Because the pavement below is only 1.4 m wide and a bus lane runs alongside it, the designer proposes a fan scaffold: a standard scaffold founded at ground level, with a cantilevered fan projecting 2.5 m out over the pavement at each lift, starting at 6 m and repeating every 3 m up to 18 m. The fan is to be tied back to the building at 3.0 m centres. The designer's loading schedule states 'light duty, 1.5 kN/m², one lift loaded at a time'. The client wants the fan to also support a temporary debris net and a 1.0 m high toe board. The scaffold contractor queries whether the tie-back and fan brackets are adequate.
1. Evaluate the designer's loading assumption against the proposed use. What load cases must the design check include, and why is 'one lift loaded at a time' problematic?
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Load cases to check:
- Permanent: self-weight of standards, ledgers, fan brackets, decking, toe boards, netting and net supports.
- Variable (imposed): the 1.5 kN/m² light-duty load, applied to the fan deck and the inner deck.
- Wind: the fan and netting present a large sail area. The netting increases the effective wind area and must be included, not ignored.
- Eccentric moment: the fan projects 2.5 m from the standards, so the imposed and wind loads create a large overturning moment about the standards. The tie-backs at 3.0 m centres must resist this moment plus the direct tension/compression from wind.
- Debris loading: the toe board and netting will trap debris; a nominal debris load should be considered.
Why 'one lift loaded at a time' is problematic: It is a management control, not a structural one. In practice, operatives on a fan scaffold will load multiple lifts simultaneously (materials staged at several levels). The structure must be designed for the realistic worst case, or the control must be enforced by physical means (e.g. limiting access) and recorded in the method statement. Relying on a verbal instruction is not acceptable for a cantilevered structure with a large eccentric moment.
2. The tie-backs are specified at 3.0 m centres. Assess whether this is adequate and describe the checks you would require on the tie-back anchors and the building structure.
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Tie-back spacing: 3.0 m centres is sparse for a cantilevered fan with a 2.5 m projection. Each tie-back must resist the overturning moment from the fan above it plus wind. I would require the designer to calculate the tension in each tie-back and confirm the anchor capacity with a factor of safety. In practice, ties at every standard (typically 2.0 m or less) and at every lift are more likely to be adequate, and the designer should justify any wider spacing.
Anchor checks:
- The anchors must be tested (pull-out test) or be of a type with a known, certified capacity in the substrate.
- The substrate must be checked: is it masonry, concrete, or cladding? Cladding cannot be relied on.
- Anchors must be installed by a competent person and recorded.
Building structure: The tie-back loads are transferred into the building frame. The designer must confirm the frame can take the horizontal and vertical components, particularly at the top of the fan where the moment is greatest. A building surveyor or structural engineer should sign off.
Netting and toe board: These add wind area and debris load. The design must be revised to include them, and the tag and handover documentation updated.