Span set (round sling)

A span set is an endless loop of load-bearing polyester yarn inside a woven cover. It is soft, light, lies flat around an awkward shape and does not mark what it is wrapped around, which is why it is the standard way to attach to a building beam or to pick a truss.

Its rating depends on how it is rigged. Used as a basket, with both legs vertical and the load in the bottom of the loop, it carries substantially more than when it is used in a choke, where the sling passes through itself and grips. Choking is convenient and secure, and it costs capacity. A sling rated at a given load in a basket carries noticeably less choked, and spreading the basket legs apart reduces it again by the same geometry that governs a bridle.

The jacket is a wear indicator rather than decoration. Its job is to be damaged before the yarn inside is. A cut, abraded or heat-marked cover means the sling is withdrawn and not inspected hopefully, because once the core is compromised there is nothing visible to assess.

Edges are the other hazard. Polyester cuts on a sharp steel corner under load, so softeners go between the sling and the edge as a matter of course, not when somebody remembers.

Staging and rigging — owned inventory, our own crew, and a site survey before anything is specified.

• Shackle The steel link that connects a sling to a point or a motor — rated, stamped, and ruined by being loaded from the side or closed with the wrong pin.
• Working load limit (WLL) The most a piece of rigging hardware may be asked to carry — typically a fifth of what it would take to break it, and halved again by convention for anything over people.
• Rigging point One place where a load is attached to the building — counted and rated individually, because the building cares where the weight is, not how much is in the room.