In engineering, span is the distance between two adjacent structural supports (e.g., two piers) of a structural member (e.g., a beam). Span is measured in the horizontal direction either between the faces of the supports (clear span) or between the centers of the bearing surfaces (effective span):^[1]

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 tramways.^{[citation needed]}

Span is a significant factor in finding the strength and size of a beam as it determines the maximum bending moment and deflection. The maximum bending moment ${\displaystyle M_{max))$ and deflection ${\displaystyle \delta _{max))$in the pictured beam is found using:^[2]

${\displaystyle M_{max}={\frac {qL^{2)){8))}$

${\displaystyle \delta _{max}={\frac {5M_{max}L^{2)){48EI))={\frac {5qL^{4)){384EI))}$

where

${\displaystyle q}$ = Uniformly distributed load

${\displaystyle L}$ = Length of the beam between two supports (span)

${\displaystyle E}$ = Modulus of elasticity

${\displaystyle I}$ = Area moment of inertia

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.