Tie Pattern on a Partially Clad Facade
You are supervising erection of a 12 m high independent scaffold along a building facade. The design specifies ties at every second standard and every second lift, giving a 4.8 m horizontal by 4.0 m vertical tie grid. The facade has a 3 m wide section of glass curtain wall where the owner refuses drilled ties, and a 2 m wide downpipe zone where ties cannot be fixed. The scaffold is fully sheeted on the windward face for weather protection. The crew has erected the first four lifts and proposes to omit ties across the 5 m combined obstruction and rely on 'the sheeted face acting as a diaphragm'. Wind loading on the sheeted face is significant, and the design notes a maximum unbraced panel size but does not address the obstructions.
1. Assess whether the proposed omission of ties is acceptable and explain the load path consequences.
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Omitting ties across a 5 m section is not acceptable without a design check. Ties transfer horizontal wind load from the sheeted face back to the building; without them, the wind load must travel through the scaffold's own bracing to adjacent tied bays. The design's maximum unbraced panel size is the limit for that load path, and a 5 m wide by 4 m high untied zone with full sheeting likely exceeds it. The 'diaphragm' claim is not a substitute for a designed tie or buttress; sheeting is not a structural diaphragm unless the design says so. The correct process is: stop erecting in the affected zone, refer the obstruction to the designer with the actual dimensions and the sheeting wind pressure, and obtain a revised tie arrangement. Options include a designed buttress or raker, a framed return, or ties relocated around the obstructions with additional ties either side. Do not proceed on the crew's assumption.
2. What controls would you put in place while the tie design is resolved?
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Isolate the affected zone: do not erect above the last fully tied lift in that section, and physically barricade or tag the incomplete area. Record the status on the scaffold tag and in the site diary, and inform the principal contractor in writing that the scaffold is incomplete and not to be loaded in that zone. Check that the erected portion is stable for the wind load it can see: verify the ties either side of the obstruction are the designed type and are tight, and that bracing is complete. If the designer requires temporary props or additional ties to make the erected portion safe, install them before leaving the site. Re-inspect after any significant wind event. Only remove the controls when the revised design is erected, inspected and the handover certificate is updated. The key judgement is that an incomplete scaffold is a live structure, not a paused one, and must be left in a stable, clearly identified condition.