Roof Safety Lifeline FEA: Fall-Arrest and Static Strength Check on Metal Sheet Roofs

A rooftop solar EPC company needed proof that its horizontal safety lifeline would hold a worker who slips on a metal sheet roof. The system is a stainless steel wire rope running between end, corner and intermediate posts, each post bolted to an anchor plate that is riveted to the roof sheet. The same kit had to work on trapezoidal, Kliplock, standing seam and aluminium sheet roofs, and the client wanted a longer span between posts than its first design.

Shirsh Technosolutions carried out roof safety lifeline FEA in ANSYS Mechanical for each roof type. The rope, the posts and the roof sheet were checked under the user's static load and under a fall-arrest load, and later under the static strength test load of IS 3521 (Part 8) / ISO 16024, with a separate check of the wire rope grips at the rope ends.

Two linked models. The work was split into a rope assembly model and a roof assembly model. The rope assembly (wire rope, posts, eye bolts, shock absorber, tensioner and brackets) was solved first, and its support reactions at each anchor plate were carried into the roof assembly model of anchor plate, rivets and roof sheet. Gravity was applied at the roof tilt so that self-weight acted the way it does on site.

Static and fall-arrest load cases. The static case covered self-weight plus the user's working load. For the fall-arrest case the user's load was multiplied by a dynamic amplification factor taken from published work on multi-degree-of-freedom systems, so the posts and roof saw the peak pull of a fall. Second-order solid and shell elements were used, mesh error was kept within normal FEA limits, and von Mises stress was compared with the yield strength of each stainless steel, structural steel and aluminium part.

Roof sheet and span study. The roof assembly was re-run for different sheet thicknesses to find the minimum sheet the lifeline anchor can be fixed to, and the rope layout was re-analysed for the longer post spacing the client asked for. The reaction forces at each anchor point were tabulated for the client's own connection design.

Static strength check to the code. In a later round each post type (end post with and without shock absorber, corner post and intermediate post) was loaded with the static strength test load of IS 3521 (Part 8) / ISO 16024 in a nonlinear static analysis. An appendix modelled the wire rope grips (designed to IS 2361) at the rope termination, worked out the clamping force from the bolt tightening, and showed that the friction it develops is more than the rope pull, so the rope will not slip.

Local peaks, read with care. Peak stress above yield showed up only in very small areas at bolt threads, cotter pins and the rope junction, the kind of local peak a bonded FEA model gives at contact points. These were reported separately from the general stress level and the components carrying them were flagged to the client. The reports also state that the lifeline is for one-time use and must be replaced after it has arrested a fall.

Common questions

How is a roof safety lifeline checked with FEA? The rope and posts are modelled together and loaded with the user's weight, then with an amplified fall-arrest load and the static strength test load from the lifeline code. The reactions at each anchor are then applied to a model of the anchor plate and roof sheet, so the check covers everything from the rope down to the roof.

Why does a lifeline FEA include the roof sheet? On a metal roof the weakest link is often the thin sheet and the rivets that hold the anchor plate, not the steel post. Modelling the sheet shows how the load spreads, and it tells the installer the minimum sheet the anchor can be fixed to.