|Name||Forth Railway Bridge|
|Design||Sir Benjamin Baker & Sir John Fowler|
|Contractor||Sir Thomas Tancred, Sir William Arrol, Travers Falkiner, Joseph Phillips, M. Coiseau|
|Where||Just north of Edinburgh, Scotland|
|Latitude||N 56 00' 02"|
|Longitude||W 03 23' 20"|
|Why||Takes the railway from Edinburgh to Dundee over the Forth of Firth|
|How to read the bridge||Read more.....|
|Overall type||Truss with cantilevers and suspended spans|
|Width||Towers are 36.6 m. wide at bottom and 10 m. wide at top.|
Two approach viaducts and the cantilever bridge.
The 2 main spans are 521.3 m. - made up of a 106.7 m. long suspended truss supported at each end by a 207.3 m. cantilever. The outer arms of the cantilver are each 210.3 m. The columns of the two outer towers are 44.2 m. apart along the length of the bridge and the columns of the central tower are 79.3 m. apart.
From the south there are 4 granite masonry arches and then 10 truss girder approach spans of 51.2 m.
From the north there 3 masonry arches and then 5 truss girder approach spans of 51.2 m.
|Height of Towers||100.6 m. above high water level.|
|Height of rail track||47.85 m. above high water level.||Navigational height clearance||46.03 m.||Materials||Riveted steel|
|How to read the bridge||Read more about the book metaphor...|
The diagram below shows the chapters of the bridge.
The lighter blue illustrates a panel as a paragraph. The sentences of struts and ties are in a darker blue.
The photograph on the right shows a closer view of one tower with cantilevers but this time the sentences are shown in a brighter colour.
The picture on the right shows the bridge during construction.
The photograph on the left was also taken during construction and you can see the lattice of the tension members quite clearly.
The diagram on the right shows the struts and ties of the cantilever. All the lattice members are ties. The others are tubes as struts - either as bottom chords of the truss or lattice bracing.
|Chapter 2||The bridge deck|
The deck girder is a truss girder as shown below running through the entire bridge.
There are two trusses 4.88 m. apart which vary in depth across the bridge.
Both top and bottom members are trough shaped. The struts are vertical and the ties diagonal.
The girders are continuous from one end of the cantilever tower to the other and are supported
by plate girders across the brdige and by vertical struts.
The sentences are the individual steel structural struts and ties making up the girder both along the length of the bridge.
The central crossed bars brace the two longitudinal girders together.
The words are the steel plate, rolled steel sections (largely angles) and rivets used to build the struts and ties shown on the right.
The four columns sit on plates on top of the 18 ft tall (above high water level) circular granite piers.
Under the piers are the caissons or large watertight chambers in which men excavated the soil.
The picture on the right shows the caisson.
The upper part of the caisson was temporary. The air-tight floor at the bottom was supported by four lattice girders.
The lower part became the actual foundation when it was eventually filled with concrete and rubble.
Bearings: At the base of the towers where five tubular and five latticed girders meet is a flat plate called the upper bedplate.
This is so arranged that it rests on, and, in some cases, can slide or move on another bedplate, the lower bedplate, which is fixed to the masonry pier.
Only one of the main junctions (called skewbacks) out of the four at each pier was fixed. The other three move a limited amount through temperature changes and lateral wind pressure.
Temperature changes are catered for by sliding bedplates at each pier, by rocking posts at one end of each of the suspended spans and by roller bearings in the two cantilever end piers.
Lateral movements are catered for by play in the bedplates of the fixed cantilevers and by vertical pins in the joints between the ends of cantilevers and the suspended central girders.
|Grammar||Technically the bridge is a way of taking forces from up in the air down to the ground.
So imagine the flow of those forces through the structure.
Think of a truck standing on the bridge and how its weight is transmitted through the bridge to the ground.
Read about the principle of the cantilever and suspended span for trusses.....
Benjamin Baker's very famous illustration of the principle of the cantilever and suspended span is shown below.
Notice the heavy weights hanging at each end.
In order to keep the balance in the actual bridge the equivalent of those weights are provided in a large box at the ends of the outer cantilever girders.
The central suspended span (where the man in the middle of the photograh is sitting) is balanced. However if two trains were to cross the bridge at the same time and meet in the central suspended girder at either side of the central tower then the tower would become unbalanced.
That is why, looking along the length of the bridge, the columns of the central tower are further apart (79.3 m.) than the columns on the other two towers (44.2 m.). Simply to avoid the central tower tipping under that loading.
It is impossible to do justice to the immensely complex operation of building tis bridge. At one time over 4000 men were working on it.
In brief the first job was to build and sink the caissons. The ground was excavated and concrete poured in. In the picture to the right you can see the cutting edge at the bottom where the men were working. Imagine what the working conditions were like. Electric lighting was used but wasn't reliable and would often fail.
The granite piers were built on top of the caissons.
Bedding plates were fixed on top of the piers. The positioning was crucial and everything was measured using theodolites and surveying equipment. Some of the pieces of lattice girder were rivetted before being lifted and rivetted into place. Piece by piece the tubes and lattice girder were created and the lifting cranes crept slowly along the members.
You can get a feel for this process from the diagram below and left. There are some more contemporary photographs and diagrams in the slide show above.
For a really detailed description click on the Reference below by W. Westhoven.
Westhofen W, The Forth Bridge, Reprinted from Engineering, Feb 28 1890
Visit the web site of the Forth Bridges Visitor Centre Trust