Part 22 (1/2)

On the West Cornwall Railway a type of viaduct similar to that described above was adopted; but as the general height was not so great, the spans were 50 feet each, and the longitudinal beams were supported at three points in each span, instead of at four as on the Cornwall Railway. In consequence of the nature of the foundations, the piers of the nine viaducts on this line were for the most part formed of upright timbers well braced together, standing upon masonry footings. The viaduct at Angarrack, 98 feet high, with 16 spans, which was constructed in 1851, was remarkable for its light appearance, owing to the small number of timbers in the superstructure and piers.

Mr. Brunel paid great attention to the preservation of the material of the timber bridges and viaducts. As early as 1835 he had been in communication with Mr. Faraday as to the best method of testing the extent to which the Kyanising solution penetrated into wood. Mr. Brunel made a careful trial of all the different methods of preserving timber, and employed the more successful of them on a very considerable scale.

He was so impressed with the importance of the preserving processes being properly applied, that he on several occasions preferred to keep the operation of preserving the timber in the hands of the Company, in order that it might be done thoroughly, and under his own supervision.

He also minutely attended to the details by which timber structures may be protected from decaying influences.

_Cast-Iron Bridges_.

Mr. Brunel did not make an extensive use of cast iron for the superstructure of bridges. His views as to the employment of this material in girders are clearly expressed in the following extract from a letter to one of the Directors of the Great Western Railway:--

April 18, 1849.

Cast-iron girder bridges are always giving trouble--from such cases as the Chester Bridge, and our Great Western road bridge at Hanwell, which, since 1838, has always been under repair, and has cost its first cost three times over, down to petty little ones, which, either in frosty weather or from other causes, are frequently failing. I never use cast iron if I can help it; but, in some cases it is necessary, and to meet these I have had girders cast of a particular mixture of iron carefully attended to, and I have taught them at the Bridgewater foundry to cast them with the f.l.a.n.g.e downwards instead of sideways. By these means, and having somebody always there, I ensure better castings, and have much lighter girders than I should otherwise be obliged to have. The number I have is but few, because, as I before said, I dislike them, and I pay a price somewhat above ordinary castings, believing it to be economy to do so.

I won't trust a bridge of castings run in the ordinary way, and at foundries where I have not a person always watching; and, even if I did, the weight requisite in a beam of ordinary metal and mode of running would more than make up for the reduced price.

The bridge at Hanwell referred to in this letter was one on the main line of the Great Western Railway, over the Uxbridge road. In 1847 the planking caught fire, and the cast-iron girders were destroyed by the heat.

The researches of Mr. Eaton Hodgkinson had drawn attention to the importance of a proper proportionment of the top and bottom f.l.a.n.g.es of cast-iron girders, and Mr. Brunel now made some experiments on this point. As part of this investigation, eight girders, 30 feet long and 16 inches deep, were tested by weights until they gave way. The comparative areas of the top and bottom f.l.a.n.g.es were varied until a correct proportion between the two was arrived at. The general result of these large-scale experiments showed a lower breaking-weight than that deduced from Mr. Hodgkinson's formula.

When Mr. Brunel afterwards had occasion to use cast-iron girders, which was chiefly for road bridges over railways, they were made of the form which his experiments had shown to be the best;[94] but he repaired the Hanwell Bridge with wrought iron.

At about the same time the necessity for spanning wide openings had led to larger girders being required than could be manufactured in single castings, and Mr. Brunel had a large cast-iron girder made, 46 feet long and 4 feet deep, of five pieces bolted and keyed together. It was tested until it gave way with a load of 92 tons on the middle. The result showed that the several parts had been well connected, and that the strength of the beam was not much less than the calculated strength of a beam of the same size in a single piece. Mr. Brunel did not, however, use girders of this construction, as the rapid introduction of wrought iron rendered it unnecessary.

Cast iron was introduced, though not for girders, in many of the brick and stone bridges on the Great Western Railway. It was used in the form of troughs sunk into the crown of the arch in bridges where the headway was very limited. The rails were laid along the bottom of the trough within a few inches of the soffit or underside of the arch.

Although, after the careful experiments and investigations he had made, and the experience he had obtained, Mr. Brunel did not make use of cast iron for large girders, he looked forward to the possibility of such improvements being introduced into the manufacture as would enable sound castings of considerable size to be made of h.o.m.ogeneous material.

He expressed this opinion in a letter to the Secretary of the Commission on the Application of Iron to Railway Structures. This Commission (which Mr. Brunel called 'The Commission for stopping further improvements in bridge building') was appointed 'for the purpose of inquiring into the conditions to be observed by engineers in the application of iron in structures exposed to violent concussions and vibration.' Mr. Brunel, in common with most engineers, thought it would be very inexpedient that any _regles de l'art_ should be laid down, and took up the cudgels boldly on behalf of the liberty of the profession:--

March 13, 1848.

At present cast iron is looked upon, to a certain extent, as a friable, treacherous, and uncertain material; castings of a limited size only can be safely depended upon; wrought iron is considered comparatively trustworthy, and by riveting, or welding, there is no limit to the size of the parts to be used. Yet, who will venture to say, if the direction of improvement is left free, that means may not be found of ensuring sound castings of almost any form, and of twenty or thirty tons weight, and of a perfectly h.o.m.ogeneous mixture of the best metal? Who will say that beams of great size of such a material, either in single pieces or built, may not prove stronger, safer, less exposed to change of texture or to injury from vibration, than wrought-iron, which in large ma.s.ses cannot be so h.o.m.ogeneous as a fused ma.s.s may be made and which when welded is liable to sudden fracture at the welds?[95]

_Wrought-Iron Bridges._

Notwithstanding the cost of wrought iron, but a short time elapsed between its introduction into bridge building and its use in structures of great magnitude. Mr. Brunel had been long familiar with the application of riveted wrought-iron work, and he was the first to encourage its use on a large scale in s.h.i.+pbuilding by recommending its adoption in the 'Great Britain' steam-s.h.i.+p in 1838.

_Girder Bridges._

The strains on girders made of h.o.m.ogeneous material have been carefully and ably calculated by mathematicians; and the investigations thus made have directed inquiry into the right channels for determining the nature of the stresses on the several parts of the built-up structures now so much in use. Principles have by degrees been laid down, and lines of thought have been suggested and followed out which were unknown at the time when wrought-iron girders were first introduced in the construction of railway bridges.[96]

[Ill.u.s.tration: _Scale of feet_.

Fig. 6. Experimental Girder.

_Transverse Section_.]