Summary
- Yes the load is significant structurally; as it is a product of the mans’ weight and the speed he is falling at; if we get it wrong the man will fall to the ground and the structure may fail posing a significant risk to others.
- The speed is determined by gravity and the possible free fall distance.
- The potential free fall distance is dependant on the building layout, PPE used and the location of the safety system.
- With a wire system the distance between supports (or post centres) has a significant effect- The greater the post centres, the greater the wire deflection, the greater the fall distance and potential speed.
- Due to the number of variables the design load should be taken as the limiting load for the PPE, 6kN per man/user.
- Finally the shock absorbing qualities of all components must be taken seriously and be quantifiable, the legislation stipulates that selection of the equipment must best limit injury to the user.
The “British Standard” man weighs 15 stone 8 pounds, 100kg on metric measurements. This does not seem a significant load in construction terms.
No one would consider a ball bearing heavy, however very few people would willingly drop one on their toe. Because we all instinctively know that when objects fall they gather speed and momentum, so the further something falls the more force is required to stop it.
Given that basic principle of physics let’s put ourselves in the position of a man standing on a roof.
To introduce the basic concept of the loads associated with a fall, I will assume that I have carefully put on my harness and attached this to a basic anchor point using a standard 1.75m lanyard, which will extend by an additional 1.25m once deployed. So the worst case is that I can fall 3m.
- At 1 m due to gravity I am travelling at 9.81m/s or 22 mph
- At 1.75m when my lanyard should start to deploy I am travelling at 39mph – the impact load to my body of coming to a sudden halt is already a very uncomfortable thought.
- Just prior to my fall being arrested after 3m I could be travelling at 66mph.
Now let’s consider I have attached myself to a wire or cable system, I now have the potential to fall a greater distance due to the deflection of the wire. If I assume that the wire supports are no greater than 10m apart I now have a maximum possible free fall height of between 5 and 6 m, with a potential impact speed of 100mph.
Clearly, if the wire is fixed at greater centres the deflection of the wire and free-fall distance will be greater.
Simplistically
Impact force is a function of speed and mass
So after falling 5-6 m a 100kg man will exert an impact force of around 5kN equivalent to 500kg or a large motor bike.
Anyone with experience of the construction and maintenance industries would accept that whilst 15 stone 8lb is a reasonable ‘median’ weight, a significant proportion of users are likely to weigh over 16 stone.
Most systems are designed for 2 users and the possibility of a rescue situation must be accounted for.
Lanyards generally start to deploy at just over 4kN and exert no more that 6kN at the point of their connection.
For this reason the fall load must always be taken as min 6kN per user.