Where does sag collide?

In structural mechanics sag is the downward deflection of a horizontal member under load. A sagging bending moment is the convention for a moment that bends a beam concave upward, putting its lower fibres in tension and its upper fibres in compression, the opposite of a hogging moment. The deflection depends on the load, the span, and the flexural rigidity of the section. The referent is the elastic response of a loaded member, a mechanics quantity distinct from a tectonic basin or a transmission conductor's droop.[1]

Unit

The vertical deflection, in millimetres, of a loaded member

Recovery means

Not applicable

Measured by

Computed from load, span, and flexural rigidity, or gauged directly as mid-span deflection

On an overhead transmission line, sag is the vertical distance an energised conductor droops below the straight line joining its two supports. It is set by the conductor's weight, span length, tension, temperature, and any ice or wind loading, and the hanging conductor follows a catenary. Engineers choose a sag large enough to keep tension within limits yet small enough to preserve legal ground clearance as the conductor heats and expands. The referent is the geometry of a suspended cable, not a bending beam or a subsiding basin.[2]

Unit

The vertical drop, in metres, of a conductor below its supports

Recovery means

Not applicable

Measured by

Determined from the catenary of span, tension, weight, and conductor temperature, and verified against clearance