Moving

In some locations, especially flat areas around rivers, provision for navigation can mean building a high bridge. You then have to find the right compromise between the length and the steepness of the approaches. These approaches can be expensive.

A possible solution is to build a low bridge, and move some part of it when water craft need to pass. The main methods are lifting the bridge on one or two horizontal axes (bascule bridge), and rotating it about a vertical axis (swing bridge). Against this is the interruption to road traffic when the bridge is moved.

A famous example is the Tower bridge in London, opened in 1894. This very strange construction includes two 100-foot lifting trussed cantilevers between the towers, and an asymmetric suspension bridge at each end.

Over the central span, a pair of trusses carries the chains that take the tension between the two suspension bridges, so that the towers are not pulled sideways. The footways across the top are no longer in use.

The suspension spans are held up by unusual chains, which cross over, and are linked by diagonal bracing, making them rather truss-like

This bridge in Amsterdam has two lifting leaves, counterbalanced by weights overhead. This is a design that has been used many times in the Netherlands.

A similar bridge, but with only one lifting section, is located in Gloucester, where the Sharpness to Gloucester canal reaches the docks. A massive counterweight sits over the road.

Numerous swing bridges are found along the canal, which traverses the flood-plain of the river Severn. These bridges are turned about a vertical axis by means of a geared windlass. This one, at Fretherne, is fully open, and a boat is passing through. Barriers warn people on the road to stop.

Some other types of bridge use a rolling bearing, instead of rotating on a horizontal axle. Still others have a vertically lifted section. Transporter bridges have a high gantry from which a movable platform is suspended.

An earlier method is to sling a rope across the river, and to pull a boat across by hand. An ingenious variation is to provide a rudder, with which the boat can be steered at an angle. Energy from the flow carries the boat across the river: the ferryman doesn't have to do anything except steer. The boat is linked to the transverse rope by a rope with a pulley at the end, which runs along the transverse rope.

Bascule bridges

Tower bridge - extensive notes - photograph -

http://www.hartwell.demon.co.uk/tbpic.htm

http://www.bardaglea.org.uk/bridges/bridge-types/bridge-types-swing.html

Seattle photographs - http://www.staff.ncl.ac.uk/john.knapton/sebasc.htm

Milwaukee photograph -

http://www.library.wisc.edu/etext/WIReader/Images/WER1443.html

Florida bridge - Data - photograph -

http://www.saj.usace.army.mil/digitalproject/dpn/sajn_850.htm

Pomeranian bridge - photograph - http://www.all-in-all.com/english/3501.htm

Explanation - diagrams - http://www.rensselaer.edu/~brawi/frame_ft_jbt/bforces11.htm

Erasmus bridge - Extensive notes - photographs -