Span is the distance between two intermediate supports for a structure, e.g. a beam or a

bridge A bridge is a structure built to span a physical obstacle (such as a body of water, valley, road, or rail) without blocking the way underneath. It is constructed for the purpose of providing passage over the obstacle, which is usually someth ...

. A span can be closed by a solid beam or by a rope. The first kind is used for bridges, the second one for power lines, overhead telecommunication lines, some type of antennas or for

aerial tramway An aerial tramway, sky tram, cable car, ropeway, aerial tram, telepherique, or seilbahn is a type of aerial lift which uses one or two stationary ropes for support while a third moving rope provides propulsion. With this form of lift, the grip ...

s. The span is a significant factor in finding the strength and size of a beam as it determines the maximum bending moment and

deflection Deflection or deflexion may refer to: Board games * Deflection (chess), a tactic that forces an opposing chess piece to leave a square * Khet (game), formerly ''Deflexion'', an Egyptian-themed chess-like game using lasers Mechanics * Deflection ...

. The maximum bending moment

M_

and deflection

\delta_

in the pictured beam is found using: :

M_ = \frac

\delta_ = \frac   = \frac

where :

q

= Uniformly distributed load :

L

= Length of the beam between two supports (span) :

E

= Modulus of elasticity :

I

Area moment of inertia The second moment of area, or second area moment, or quadratic moment of area and also known as the area moment of inertia, is a geometrical property of an area which reflects how its points are distributed with regard to an arbitrary axis. The ...

Note that the maximum bending moment and deflection occur midway between the two supports. From this it follows that if the span is doubled, the maximum moment (and with it the stress) will quadruple, and deflection will increase by a factor of sixteen. For long-distance rope spans, used as power line, antenna or for aerial tramways, see list of spans.

References

{{DEFAULTSORT:Span (Architecture) Architectural elements Structural engineering Bridge components