Span (architecture)

Span (architecture)

Discussion

Encyclopedia
Span is the distance between two intermediate supports for a structure, e.g. a beam
Beam (structure)
A beam is a horizontal structural element that is capable of withstanding load primarily by resisting bending. The bending force induced into the material of the beam as a result of the external loads, own weight, span and external reactions to these loads is called a bending moment.- Overview...

or a bridge
Bridge
A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle...

.
A span can be closed by a solid beam or of a rope. The first kind is used for bridges, the second one for power line
Electric power transmission
Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers...

s, overhead telecommunication lines, some type of antennas
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...

or for aerial tramway
Aerial tramway
An aerial tramway , cable car , ropeway or aerial tram is a type of aerial lift which uses one or two stationary ropes for support while a third moving rope provides propulsion...

s.
The span is a significant factor in finding the strength and size of a beam as it determines the maximum bending moment
Bending Moment
A bending moment exists in a structural element when a moment is applied to the element so that the element bends. Moments and torques are measured as a force multiplied by a distance so they have as unit newton-metres , or pound-foot or foot-pound...

and deflection
Deflection (engineering)
In engineering, deflection is the degree to which a structural element is displaced under a load. It may refer to an angle or a distance.The deflection distance of a member under a load is directly related to the slope of the deflected shape of the member under that load and can be calculated by...

. The maximum bending moment and deflection in the pictured beam is found using:
moment:   Mmax = 1/8 · q · L2
deflection: fmax = 5/48 · Mmax· L2 / (E · I)

From this it follows that if the span is doubled, the maximum moment (and with it the stress) will increase four times, and deflection will increase sixteen times.

For long-distance rope spans, used as power line, antenna or for aerial tramways, see list of spans.