Questioning Safety: Scaffolding #5 – Stability
Is the scaffold stable?
Simple scaffold load calculations typically contemplate downward vertical (live load) forces only. But even the strongest scaffolds can be overturned if they are not stable against horizontal or even upward forces (Yes, upward!) such as:
- Wind forces (Horizontal and Uplift) especially from enclosures, tarps, etc.
- Eccentric loads from cantilevers or brackets
- Unbalanced live loads
- Dangerous settlement
- Workers climbing, pushing, or pulling
Scaffolds must be made stable against these forces by either connecting/tying the scaffold to another structure or, made stable by their own “height to base ratio.”
If the scaffold is freestanding – is it stable to stand on its own?
Independent (or freestanding) scaffolds may be considered stable on their own by limiting the maximum platform height to a ratio of the smallest base dimension. The “height to base ratio” varies by the regulatory authority, usually 3 to 1 or 4 to 1.
VERY IMPORTANT: Maximum height to base ratios may only be used to stabilize scaffolds that are exposed to little or no horizontal/uplift forces. Any independent scaffolds potentially subjected to these destabilizing forces MUST have additional measures to ensure their stability.
If ties are used – are they correct?
Another strategy to stabilize a scaffold is to anchor it to another structure, typically a building, wall, etc.
Scaffold ties must have several essential characteristics, including:
- Proper spacing, both vertically and horizontally following local regulations.
- Ties must be installed at both scaffold ends and as close as possible to the scaffold “node points” to ensure maximum strength and to minimize post bending.
- Ties must resist both “push” and “pull” forces (this rules out merely using rope or wire.)
- Ties must be strong enough to resist four times the force likely to be applied.
- Some form of horizontal bracing must occur at the tie locations to accurately transmit forces.
- Tie anchors may require pull testing, especially if they connect to possibly questionable structures like brick walls.
Field Questions
- Is the scaffold a freestanding (independent) structure or does it rely on another structure(s) for stability?
- What possible forces could affect the scaffold’s stability?
- If the scaffold relies only on the height-to-base ratio for stability – are you sure that no other forces (e.g., wind, eccentric loads, unbalanced, etc.) can make it unstable?
- If ties are used, do they resist both compression (push) and tension (pull) forces?
- Are the ties appropriately spaced according to code and regulation?
- Are ties placed at the node points of the scaffold?
- Are ties strong enough to provide the proper safety factors?
- Are horizontal braces installed to correspond to the ties?
- Does the scaffold appear unstable in any way?
- Do scaffold users clearly understand the methods used (and any limitations) of the scaffold stability methods?
