Questioning Safety: Scaffolding #4 – Scaffold Load Ratings
Is the scaffold load rating calculated correctly?
Welcome to the North American scaffold industry’s “Dirty Little Secret.” That is, hardly ANY of the scaffolds erected have proper load ratings based on accurate calculations.
Ironic, isn’t it? Given that scaffolds are built precisely to hold a specific live load in the air – no one verifies the rating?
Why?
Most access scaffolds don’t have proper load ratings because most “designers” (aka the scaffold erectors) don’t know HOW to do them! Complete scaffold load calculations are not covered by many current training programs – let alone those who were trained 20 years ago.
Odds are, even IF your scaffolds have a tag showing a load rating – it merely says “Light Duty” and is not based on any specific calculation or manufacturer’s data.
Welcome to one of your most significant challenges!
Who is supposed to calculate the scaffold load rating?
Scaffold load ratings must be calculated by the “designer” of the scaffold. If it is an engineered scaffold – then the design engineer is responsible for the load calculations and ratings.
The vast majority of access scaffolds, however, are designed by the erector – not an engineer. Therefore, in these situations, the person responsible for load calculations is the erector.
What goes into a basic scaffold load calculation?
The best metaphor for understanding scaffold load calculations is that of a chain. Each of the four major scaffold component areas represents one link in the chain:
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- Link 1: Scaffold Foundation
- Link 2: Post/Standards/Uprights
- Link 3: Load Bearing Ledgers/Bearers/Transoms
- Link 4: Planks/Platforms
Each link must be separately evaluated and calculations performed to determine “the weakest link”. This is what determines the final scaffold load rating.
All scaffold load calculations must be based on accurate component ratings obtained from the scaffolding equipment manufacturer.
Always ensure that you are in possession of the applicable Technical Manual for the scaffolding equipment that you own/rent.
What does it mean when the scaffold is made from a mix of different manufacturers’ equipment?
If the equipment does fit together compatibly – there can be some substantial differences in the component load ratings from one manufacturer to another. (We have found variances of up to 40% in the same size, shape and otherwise interchangeable components!)
Furthermore, it is not sufficient to generalize that one manufacturer is typically stronger or weaker than another. Manufacturer “A” may have stronger single ledgers than Manufacturer “B” – but they may have substantially weaker truss/double ledgers.
If your organization is using a mix of different scaffolding manufacturers in the same structure, it is your responsibility to 1) ensure that the components are compatible and 2) base your scaffold load calculations on the lowest component ratings used to construct the scaffold.
What load ratings should we be using on our scaffolds?
Regulations across North America can vary, but the scaffolding industry has generally agreed on the following three load rating categories:
Light Duty: Up to 25 pounds per square foot (122 kg/m2) of platform area, evenly distributed.
Medium Duty: Up to 50 pounds per square foot (244 kg/m2) of platform area, evenly distributed.
Heavy Duty: Up to 75 pounds per square foot (367 kg/m2) of platform area, evenly distributed.
Distributed loads more than 75 pounds per square foot (367 kg/m2) often require the scaffold to be professionally engineered.
Is the scaffold platform going to be “point-loaded” with materials, tools, or equipment?
As noted above, most scaffold load ratings are based solely on distributed loads. Point loads, where heavy materials or equipment more than 250 lbs./114kg. are placed on the platform, will require additional preparation. They may even require an engineered design to distribute the point load. Simple solutions – such as pallets or planks – are often employed to resolve simple point loads such as motors or pumps.
Field Questions
- Who is responsible for calculating load ratings on our scaffolds?
- What exact process do they use? (Could you walk me through how you do a scaffold load calculation?)
- Do we ensure at least a four times load safety factor in all of our access scaffold load calculations?
- What are the typical scaffold duty ratings found on our scaffold tags?
- What manufacturers’ technical/engineering manual(s) do we use for our site’s calculations?
- If a mix of scaffolding equipment is on site – what ratings do we use for our load calculations?
- Do we have scaffolding components that are not compatible with each other? Why?
- How do we design our scaffolds to accommodate “point loads”?
- Do we need additional scaffold components to improve our load ratings? (e.g., truss ledgers, stronger platform equipment, double trusses, etc.)
- If a scaffold user anticipates to need a higher scaffold load rating than the existing tagged rated load – what do they do?
- Do we only allow Competent Persons to assess and change the purpose and load rating of the scaffold?
- Are scaffold users trained to interpret the duty ratings in light of their work?
- Are there any scaffolds observed with questionable loading or unplanned point loading?
- Do scaffold users understand the concept of “Distributed Loads” vs “Point Loads?”
