Bridges The 100ft Line technique

BRIDGES A list of bridges on the canal.   LOW BRIDGES The lowest bridge over the canal is at Reading Road, Fleet, with Pondtail Bridge, Fleet, a close second.

This method of bridge design dates from the 18th century, and many of the brick accomodation bridges on the canal were built to it. The principle was re-discovered when the Canal Society persuaded Hampshire County Council (HCC) not to demolish the very dilapidated Broad Oak Bridge near Odiham, and instead to rebuild it. A survey of the remaining wing wall was carried out, and this suggested that the alignment of the wing walls was elliptical on plan with a vertical curve that approximated to a circular arch.     In addition, the brickwork courses sloped down from the centre of the bridge towards the abutments at a gradient of about 1:15.     The practical difficulties of providing templates or guide lines to set out the new brickwork, with each course out of plumb in two directions, whilst still allowing erection of staging for a working platform, were overcome when it was realised that the shape of bridge was generated by an ellipse with foci 60 feet apart at a distance of 40 feet from the crown of the arch, and it merely needed a 100 foot long piece of string to set out the horizontal curve.     Two datum posts to locate the foci of the ellipse are set with their anchorages at the level of the crown of the arch (10ft 4.5ins).     Vertical alignment     Each end of the 100ft line is fixed to one of the posts and the ellipsoid which is produced gives both vertical and horizontal alignment.

Horizontal alignment     [Place mouse over picture] The photo is of Guildford Road Bridge. Although still constructed using the 100ft line, it is one of two on the canal built with Sarsen stone as well as bricks.     On the brick built bridges the result of the 100ft line technique produces a structure such as Broad Oak Bridge, rebuilt in 1981 by the Canal Society.   BACK

Last updated December 2001