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Last edited 27 Mar 2019
In structural engineering and architecture, ‘span’ is the term given to the length of a structural component – eg beam, floor, roof or floor truss – that extends (or ‘spans’) between two supports. Thus, a beam may be supported at either end, in which case it is said to span between the two points, and a floor may span between two (or three or even four) continuous supports.
 Span-to-depth (STD) ratio
Span-to-depth ratio (or span/depth ratio, also known as slenderness ratio L/h) is the ratio of the span length divided by the depth (or vertical height) of a component. It is an important parameter as it can affect structural behaviour, construction costs and aesthetics.
A 250mm-deep beam that spans 4m has a span/depth ratio of 16. If the span/depth ratio is less than two, the beam is considered to be ‘deep’. Eurocode 2 gives span/depth rules for designing reinforced concrete beams and slabs.
Span/depth ratios are useful in limiting the deflection of a member under service loads. Damage may result if limits are exceeded, eg cracking of plaster, partitions and supporting brickwork. If limits are exceeded wildly, catastrophic structural failure accompanied by potential loss of life may result.
In bridge design, span/depth ratio is an important parameter. The ratio relates the length of the bridge’s span to its girder depth. To ensure that design does not deviate substantially from past successful practice, the ratio is commonly chosen on the basis of experience and the typical values used on past bridges. A typical starting point for estimating bridge construction depths is to take an STD ratio of 20.
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