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Last edited 05 Feb 2025

Span

[edit] Introduction

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.

Approved document A: Structure, 2004 edition incorporating 2004, 2010 and 2013 amendments, defines span as: ‘The distance measured along the centre line of a member between the centres of any two adjacent bearings or supports.’

A general engineering principle is the longer the span, the deeper the structural component will have to be to safely support its self-weight and whatever it must carry, eg a floor.

[edit] 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.

More dramatic span/depth ratios can be provided by space frames: a rectangular space frame may have a span/depth ratio of up to 40, while this can be as high as 60 for a skew space frame.

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.