Tie Spacing Conflict and Board Count on a 24 m Elevation
You are planning an independent tube-and-coupler scaffold along a 24 m long, 12 m high wall. The drawing specifies a 2.4 m bay, a 2.0 m lift, a five-board platform 1.0 m wide, and medium duty loading of 2.0 kN/m². The drawing note says ties are to be 'provided at 6 m horizontally and 6 m vertically'. The system manual in the site office states a maximum tie spacing of 4 m horizontally by 4 m vertically for this duty and height, and requires ties at the ends and at every alternate standard. The site has 2.4 m boards, 225 mm wide, and 600 boards in stock. The client wants the scaffold ready in six days; a 4 m spacing means more ties, more drilling and more anchors.
1. Calculate the number of bays, standards, lifts, and boards required, and state whether the 600 boards in stock are enough.
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Work from the drawing dimensions and show the arithmetic.
- Bays: 24 m ÷ 2.4 m = 10 bays. That gives 11 standard positions along each row.
- Standards: an independent scaffold has an inner and outer row, so 11 × 2 = 22 standards.
- Lifts: 12 m ÷ 2.0 m = 6 lifts, plus the working platform at each lift level. Confirm the top lift is at the working height and that guardrail height is added above it; the 6 lifts are the lift count, not the number of boarded levels unless the drawing says every lift is boarded.
- Boards per bay: platform width 1.0 m ÷ 0.225 m board width = 4.44, so 5 boards are needed to cover the width. Five boards per bay is consistent with the drawing.
- Boards for one boarded lift: 5 boards × 10 bays = 50 boards of 2.4 m length.
- For six boarded lifts: 50 × 6 = 300 boards.
- Allowance: add about 10% for cutting, damaged ends and replacements = 330 boards. 600 boards is therefore sufficient, provided the boards pass inspection for splits, knots and cracks.
If only alternate lifts are boarded, the count falls, but the plan should follow the drawing and the access needs.
2. Resolve the conflict between the drawing note (6 m × 6 m ties) and the manual (4 m × 4 m ties). Show the tie counts under each option.
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A drawing note does not override a proprietary system's tested tie spacing, and it certainly does not override a designed scaffold. The safe resolution is to follow the more restrictive verified requirement - the 4 m × 4 m manual spacing - unless a competent engineer provides a design that justifies 6 m spacing for this height, duty and anchor capacity, and the client accepts the design.
Counts:
- 4 m × 4 m: horizontally 24 ÷ 4 = 6 spaces; vertically 12 ÷ 4 = 3 rows; 6 × 3 = 18 tie positions, plus ties at the ends as required. With the 'every alternate standard' rule, check that the resulting pattern does not leave a standard un-tied beyond the permitted distance.
- 6 m × 6 m: horizontally 24 ÷ 6 = 4 spaces; vertically 12 ÷ 6 = 2 rows; 4 × 2 = 8 tie positions. That is less than half the number of ties, so the pull-out load per anchor roughly doubles.
Because the anchor capacity and the wall construction are not stated, I would not accept the 6 m note. I would raise a technical query, obtain written confirmation, and proceed with 4 m × 4 m and the anchor test records.
3. What planning and documentation would you complete before erection starts, and how would you communicate the tie change?
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Planning package:
- A marked-up scaffold layout showing bays, standards, lifts, tie positions, bracing, access points and the exclusion zone.
- A design/verification check for medium duty 2.0 kN/m², including the tie anchors, base bearing and the wall-to-scaffold gap.
- A tie test regime: sample anchors tested to the specified proof load, results recorded, failed anchors relocated or re-designed.
- A materials list with board count (330 including allowance), couplers, ties, base plates and toe boards.
- A six-day programme that sequences erection, tie installation, inspection and handover; ties must go in as the lift rises, not at the end.
Communication: issue a written technical query to the designer/client, keep the response on file, brief the erection crew at a toolbox talk, mark the revised tie spacing on the drawing revision, and record the change in the site diary. The inspection form must show the tie spacing actually installed, so the next inspector is not misled by the old drawing.