Southdown Road Skew Bridge

The easiest way to visualise Boucher's concept for the ribbed skew arch is to consider a regular arch bridge that carries the railway at right angles across the road and then to slice it vertically at regular intervals along the axis of its barrel, the planes all being parallel with the faces of the bridge, rather like the way a loaf of bread is sliced. The individual slices are then slid laterally with respect to one another in order to achieve the required oblique alignment. While the intrados of a "true" skew arch is smooth and cylindrical, the intrados of this type of "false" skew arch has a stepped appearance. Thus, the need to lay helical courses of brick at such an extreme angle to the horizontal is avoided as the multitude of conjoined regular arches approximate the desired structure.^[5] Because of the extreme skew angle the span of each "slice" (known as the span on the skew) is much greater than the perpendicular distance between the abutments (known as the span on the square), the latter being the usable span for road traffic passing under the bridge. Thus, despite the impressive looking span when viewed face on, the usable span along the axis of the barrel is less so, to the extent that the road is visibly narrower where it passes under the bridge.^[6] For a "true" skew arch the ratio of the span on the square to the span on the skew is equal to the cosine of the skew angle,^[6] but for a ribbed skew arch this ratio is made even less favourable due to the steps in the barrel wall.^[5]

• Hereford Road Skew Bridge – a ribbed skew arch made from stone and brick.
• Skew arch – a discussion of ribbed and other forms of skew bridges.