Page:Forth Bridge (1890).djvu/14

Rh Bridges" (, vol. iii.) which went through three editions in this country, were republished at Philadelphia, and translated into German and Dutch, and published in the Transactions of the Austrian and Dutch engineers respectively.

In 1871 Mr. Fowler and Mr. Baker made designs and estimates for a bridge across the Severn, comprising two girder spans of 800 ft. each, and in 1873 Mr. Baker, at the request of the Corporation of Middlesbrough, designed the superstructure for a proposed ferry bridge across the Tees, which included a 650 ft. span on the same system (, vol. xvi., page 60).

In 1876 nine competitive designs were submitted for the proposed New York and Long Island Bridge, comprising one span of 734 ft. and one of 618 ft., and of these three designs, two were on the aforesaid system (Figs. 13 and 14). The first was that of the Delaware Bridge Company, and the second of Colonel Flad, the very able engineer who, under Captain Eads, carried out the great St. Louis steel arch bridge.

In the same year was built the first, and, so far as we know, the only railway bridge of the type under discussion (Fig. 15.) This is a bridge of 148 ft. span across the Warthe, near Posen. It might appear strange at first that the application to railway purposes of so well-known a system should have been deferred until 1876, but the explanation is that there are thousands of bridges in existence on the continuous girder, or in other words cantilever and central girder principle, but engineers as a rule have elected not to sever the bridge at the point of contrary flexure, or to make the girders of varying depth.

A glance at the annexed illustrations and description will satisfy our readers that there is nothing novel or untried in the principle of the structure designed for the Forth crossing. The reasons dictating the design in the case of the Forth Bridge are those which probably influenced the Red Indians in making the structure illustrated by Fig. 8—economy of material and facility of erection. It must be conceded, however, that except as regards principle the design is essentially novel, but the novelties are dictated by the unexampled size of the structure, and are due simply to the perfect adaptation of the principle of the continuous girder and the general laws regarding the strength of materials to the special conditions of the case. Thus it will be observed that the structure is a continuous girder of varying depth on plan as well as elevation, the central girder portion being of the ordinary width required for a double line of rails, and the cantilevers spreading out to an extreme width of 120 ft. at the piers. By this means the stresses on the horizontal bracing from wind pressure are much reduced, and lightness and compactness are attained. To further the same ends the whole of the vertical members are made of two struts inclined towards each other from base to summit and braced together. To reduce the extreme height of the structure, and bring the centre of gravity as low down as possible, the bottom members of the continuous girder are curved, springing from solid masonry piers at a height of 18 ft. only above high water, whereas in the design for the suspension bridge the main chains, carrying of course all the weight, were supported at a height of 550 ft. above the same point! The main compression members are steel tubes ranging up to 12 ft. in diameter, the tubular form being adopted for two reasons, firstly, because experiments have shown that inch for inch the tubular form is stronger than any other, and, secondly, because the amount of stiffening and secondary bracing is thereby reduced to the lowest percentage. It might be thought that columns 350 ft. in length were an untried novelty, but this is not so, as we have the precedent of the Saltash Bridge oval tubes 16 ft. 9 in. by 12 ft. 3 in. in diameter and 460 ft. in length, the strain upon which under the test load was higher per square inch than will be that on the steel columns of the Forth Bridge. The central girder portion is simply an ordinary double-line railway bridge of 350 ft. span with girders of a type intermediate between the girder of parallel depth and the bowstring. This is an economical type, and many Continental bridges have been so constructed.

When lecturing some years ago, Mr. Baker, with a view of presenting in a form easily understood and popularly remembered, a simple diagram of the manner in which the principal stresses of a cantilever bridge are distributed, devised the following arrangement of a human cantilever, or a living model of the Forth Bridge. (See Fig. 15.)

Two men sitting on chairs extend their arms, and support the same by grasping sticks which are butted against the chairs. There are thus two complete piers, as represented in the outline drawing above their heads. The central girder is represented by a stick suspended or slung from the two inner hands of the men, while the anchorage provided by the counterpoise in the cantilever end piers is represented here by a pile of bricks at each end. When a load is put on the central girder by a person sitting on it, the men's arms and the anchorage ropes come into tension, and the men's bodies from the shoulders downwards and the sticks come into compression. The chairs are representative of the circular granite piers. Imagine the chairs one-third of a mile apart and the men's heads as high as the cross of St. Paul's, their arms represented by huge lattice steel girders and sticks by tubes 12 ft. in diameter at the base, and a very good notion of the structure is obtained.

The chief desiderata in the Forth Bridge, which is the largest railway bridge ever yet built, were clearly as follow:

1. The maximum attainable amount of rigidity, both vertically under the rolling load and laterally under wind pressure, so that the work when completed may by its freedom from vibration gain the confidence of the public, and enjoy the reputation of being not only the biggest and strongest, but also the stiffest bridge in the world.

2. Facility and security of erection, so that at any stage of erection the incomplete structure may be as secure against a hurricane as the finished bridge.

3. That no untried material be used in its construction, or in other words that no steel be employed which would not comply with the requirement of the Admiralty, Lloyd's, and the Underwriters' Registry, as determined by the experience gained in the use of many thousands of tons of steel plates, bars, and angles for shipbuilding purposes.

4. That the maximum economy be attained consistent with the fulfilment of the preceding conditions. We think it will be apparent to most engineers and bridge builders that the original suspension-bridge design complied with none of these conditions, whilst the girder design complies with all. Of the present design it may be truly said that all the anticipations have been most brilliantly realised, and its merits can now, in the light of practical experience, and of actual facts, be more easily pointed out. In the first instance the distribution of weight not only offers the advantage of having the greatest proportion, nearly one-fourth of it, immediately over the main supports, where it is most easily erected, but it offers in those places where the wind pressure would act with the greatest amount of leverage, the least amount of surface to act upon. Thus, while in the central tower of the Inchgarvie pier, the most exposed to storms, the weight per foot run is 23 tons, and in the first bay of cantilevers 21 tons, in the central girders of 350 ft. length it is only a little over 2 tons per foot. In a similar manner the structure decreases rapidly in height and breadth of girders, as it extends from the massive central towers towards the extremities of the cantilevers. Again, for purposes of erection every portion of the structure, as put in position, offered itself as a staging for carrying operations further