Horizontal lifeline modelling

Hi everyone,
I'd like to model an horizontal life line system to see if it can keep the worker safe in case of falling. It consists of a steel flexible cable/wire with a fiber core (16mm; 6x19 class 1770) that is atached to 2 steel columns (distanced of 22,5 meters); at the center of the lifeline a mass of 100 kg will be dropped to submit the lifeline to the impact load (usually called maximum arresting load); the 100kg mass is attached to the lifeline trough a rope with 2 meters of lenght and it will free fall for 2 meters lenght. The system must resist to the maximum arresting load. For simplicity, the cable can be considered as an arch of 2 straight lines with an initial vertical "deflection" at the center of the steel cable (equal to 2% of the distance between the columns = 0,02 x 22,5m) and the 2 steel columns can be considered as rigid (see the attached figure). I need to obtain the maximum tension force in the steel cable, the maximum horizontal load in each of the steel columns (anchorage) and the maximum vertical deflection at the center of the steel cable. So my questions are:

1 - What's the best aproach for modelling the cable in Mecway?
2 - To model the impact from the 100kg fall can I do a static analisys applying a maximum load in the center of the cable F = m x h x g = 100kg x 2m x 9,8m.s-2. Is this correct?
3 - Do you have any suggestions of how to model the dinamic load and deflection in time (in order to compare the difference between the 2 models - dinamic and static)?

Thanks a lot.
Best regards
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Comments

  • edited October 6
    I have feeling this might be easier with a hand calculation but I don't know how.

    1. The easiest way is thin beam elements so that their bending stiffness is very small. You might need to artificially increase the stiffness to keep the diameter small enough. You could also use truss elements but then you have to preload them while they're straight (apply a tension to the horizontal cable) otherwise the nonlinear solution probably won't be able to start since they have zero lateral stiffness before they're loaded.

    I'm not sure about the effect of the two materials. Perhaps use two coincident elements - one for each material. Is there any non-linear stiffening? For example, the fibers being loosely coiled at first, then straightening out under load?

    2. No, because you don't know the maximum load. m*g*h is the kinetic energy it would have after falling through a height of h which doesn't tell you the force needed to stop it. The stiffer the cable, the higher the force even with the same m,g,h. You might be able to estimate a maximum load by assuming a constant stiffness for the cable so the problem reduces to a 1 DOF linear spring-mass system. But that still leaves the problem of choosing the stiffness.

    3. Use the CCX solver with the nonlinear dynamic analysis type. Nonlinear because the stiffness of the horizontal cable will increase with deflection, and dynamic because part of the load will be due to decelerating the person as their fall slows.
  • Thanks a lot for your reply Victor.
    The hand calculation is based in the formulation of the attached file (for an arrest force of 6kN, an initial sag of 2% of the span lenght and considering the anchorage rigid).
    I see the modelling will be harder than I've thought, specially for a beginner like me.
    If I can model it I'll post it here.
    calculations.jpg
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