On the first English railways cast iron girder bridges for spans of to 66 ft.
Each girder is 1511 ft.
(d) Iron and Steel Girder Bridges.
A main girder consists of an upper and lower flange, boom or chord and a vertical web.
- Let a girder of span 1, fig.
A fundamental difference in girder bridges arises from the mode of support.
In addition, the hand railing on each side forms a girder 4 ft.
Now let w1', w 2 ' be the girder weights per ft.
In the case of braced girder bridges, the wind pressure is taken as acting on a continuous surface extending from the rails to the top of the carriages, plus the vertical projected area of so much of one girder as is exposed above the train or below the rails.
In addition, an allowance is made for pressure on the leeward girder according to a scale.
- It is convenient to consider beam girder or truss bridges, and it is the stresses in the main girders which primarily require to be determined.
- In the case of braced structures the following method is convenient: When a section of a girder can be taken cutting only three bars, the stresses in the bars can be found by taking moments.
In this connexion his most remarkable achievements were his railway bridges, especially those of the tubular girder type.
The great girder bridges over the Menai Strait and at Saltash near Plymouth, erected in the middle of the i 9th century, were entirely of wrought iron, and subsequently wrought iron girder bridges were extensively used on railways.
After various repairs and strengthenings, including the replacement of the timber girder by an iron one in 1880, this bridge in 1896-1897 was taken down and a steel arch built _ _ __ _ I ?
Rankine proved (Applied Mechanics, p. 370) that the necessary strength of a stiffening girder would be only one-seventh part of that of an independent girder of the same span as the bridge, suited to carry the same moving load (not including the dead weight of the girder which is supported by the chain).
There are two high-level footways for use when the bascules are raised, the main girders of which are of the cantilever and suspended girder type.
For spans over 75 ft., expansion due to change of temperature is provided for by carrying one end of each chain girder on rollers placed between the bearing-plate on the girder and the bed-plate on the pier or abutment.
14 shows the roller bed of a girder of the Kuilenburg bridge of 490 ft.
It will be seen that the girder directly rests on a cylindrical pin or rocker so placed as to distribute the load uniformly to all the rollers.
In the girders of bridges the horizontal girder is almost exclusively subjected to vertical loading forces.
In some girder -1 o bridges the members are connected entirely by riveting, in others the principal members are con nected by pin joints.
There was an idea of using suspension chains combined with a girder, and in fact the tower piers were built so as to accommodate chains.
But the theory of such a combined structure could not be formulated at that time, and it was proved, partly by experiment, that a simple tubular girder of wrought iron was strong enough to carry the railway.
Though each girder has been made continuous over the four spans it has not quite the proportions over the piers which a continuous girder should have, FIG.
The credit for the success of the Conway and Britannia bridges must be divided between the engineers, Robert Stephenson and William Fairbairn, and used for railway bridges in England after the construction of the Conway and Menai bridges, and it was in the discussions arising during their design that the proper function of the vertical web between the top and bottom flanges of a girder first came to be understood.
It would be economical, therefore, to make the girder very deep. This, however, involves a much heavier web, and therefore for any type of girder there must be a ratio of depth to span which is most economical.
At the cal culated position of one of the points of contrary flexure all the rivets of the top boom were cut out, and by lowering the end of the girder over the side span one inch, the joint was opened - -- Section of Newark Dyke Bridge.
By curving the top boom of a girder to form an arch and the bottom boom to form a suspension chain, the need of web except for non-uniform loading is obviated.
The top boom of each girder is an elliptical wrought iron tube 17 ft.
It was a pin-jointed Warren girder bridge erected from designs by C. M.
It is formed by a crescent-shaped arch, continued on one side by four, on the other side by two lattice girder spans, on iron piers.
In such rolling operations the girder is subjected to straining actions different from those which it is intended to resist, and parts intended for tension may be in compression; hence it may need to be stiffened by timber during rolling.
If the bridge is erected when the river is nearly dry a travelling stage may be constructed to carry the projecting end of the girder while it is hauled across, the other end resting on one abutment.
Sometimes a girder is rolled out about one-third of its length, and then supported on a floating pontoon.
Dwarf towers, built on the arch ring at the fifth panel from either side, helped to support the girder above, in erecting the centre part of the arch (Seyrig, Proc. Inst.
In the earlier girder bridges the live load was taken to be equivalent to a uniform load of 1 ton per foot run for each line of way.
An hour is likely to reach 40 to 50% for girder spans of less than 50 ft.
Run of girder; w 2 the weight of platform per ft.
On gross section of metal; = depth of girder at centre in ft.; = ratio of span to depth of girder so that r=l/d.
For a plate girder bridge of less height than the train, the wind is to be taken to act on a surface equal to the projected area of one girder and the exposed part of a train covering the bridge.
On twice the vertical projected area of one girder, treating the pressure on the train as a travelling load.