Horizontal Safety Line Technical Specifications

[GENTLEMANJIM]

Hi there!

We have a Subcontractor who will be installing sheet profile cladding onto a new structural steel framed building at a height of 30-metres. I have rejected their planned Horizontal Safety Line for a number of reasons, their Engineers Calculations were based on static load. Also, they plan to run the safety line 40-metres without any intermediate supports.

What I need to do is have comprehensive technical specifications in order to fully assess the requirements.

Any help would be appreciated.

Many thanks & a Happy New Year for 2012.

Gentleman Jim

[sigilli]

Get a qualified engineer to look at this.

The system must do three things:
1) Resist the load imposed by the fall (you don't want it to break),
2) Limit the fall travel distance (you don't want the guy to crash on the floor below) and
3) Limit the force on the worker when he stop falling (you don't want to break his ribs when he reaches the end of the rope).

The actual admissible values depend on your jurisdiction. To give you an idea, in Canada, we try to limit the force on the worker to 8kN or 6kN for harnesses with shock absorbers. The province of Ontario limits the travel distance to 1.5m.

To check the strength, you code may require you to use an approach based on the forces in the horizontal safety line or based on the energy to be absorbed in the fall. It is basically the same, since Force x Distance = Energy. Limiting the forces and the travel distance, you also limit the energy.

To find out what the forces in the system are, you can use the maximum force imposed on the worker (item 3 above), multiplied by a factor for the maximum number of workers at a time on the safety line and use it as a concentrated vertical load at the middle of the horizontal safety line. You can then figure out the tension in the cable using the catenary equations. Don't forget to include the weight of the cable, as it is not unsignificant. It is not uncommon to end up with reactions at the end of the cable in the magnitude of 90 kN for a single worker.

Make sure that the system's set up instructions defines the deflection of the cable without any loads, as this will define the initial tension in the cable. Verify the travel distance by comparing the loaded and unloaded deflection of the system. Include any extra travel caused by the harness' shock absorber.

The IRSST (Health and Safety research institute in the province of Quebec) has some documentation on the subject, touching both the force and the energy side of the problem, but to my knowledge, the stuff is available only in print and in French.

This topic is not rocket science and can be mastered with simple logic but as I said before, get a QUALIFIED engineer, either yours or the contractor's, to look at this. Ask for the calculations, so you can learn and compare with my comments. Do not accept the usual "That's good enough" and "We always do it like that".

This is safety we are talking about.

[sigilli]

Another thing. Very important. Don't stop checking at the end of the safety line. Check that anchoring structure can resist the load from the safety line. How many times have I seen people attach well designed monorails, safety lines, etc. to flimsy pieces of structure.