span

In construction and rigging, a span is the horizontal or near-horizontal distance between two consecutive points of support for a cable, wire, rope, or chain. These supports are typically posts, towers, anchor points, or structural members. The span determines how much the cable will sag under its own weight and any load it carries, and it directly affects the tension and stress in the cable itself.

Span length is critical in calculating cable tension using catenary formulas or simplified parabolic approximations. A longer span creates greater sag for the same cable weight and applied load. Tension increases nonlinearly with span length, particularly at the support points where failure is most likely. Engineers must account for this when selecting cable diameter, material grade, and support spacing to stay within safe working limits.

Variations by application

In transmission lines and power distribution, spans may run from 100 to 400 meters between poles, with sag carefully managed to maintain ground clearance and avoid conductor burnout. Suspension bridges use main cable spans measured in kilometers. In temporary construction rigging, span distances are often much shorter, typically 10 to 50 meters, with more frequent attachment points to keep tension manageable. Overhead crane systems use relatively short spans of 5 to 20 meters between rail supports.

The practical span is limited by several factors: maximum allowable sag (often set at 2 to 5 percent of span length), the tensile strength of the cable material, wind and ice loading conditions in the region, and the structural capacity of support points. Increasing span requires progressively larger or higher-strength cables, which becomes expensive and uneconomical beyond certain distances.

When cable fails or shows damage between supports, it typically occurs near the midpoint of the span where tensile stress and bending stress are highest. This is why inspectors pay particular attention to the deepest point of sag, where corrosion, fatigue, and abrasion concentrate over time. The term span itself comes from the original meaning, to reach or extend across, and is used consistently across disciplines wherever cables bridge gaps.