Chapter ToC – Next >
Brickwork and Masonry Bridges—Hanwell Viaduct—Maidenhead Bridge—Flying Bridges—Letter from Mr. Brunel on Bridge Construction (December 30, 1854)
IN Chapter IV a general history has been given of the railways of which Mr. Brunel was the engineer; but the bridges and viaducts designed by him are so numerous and important that it has been thought advisable to devote a separate chapter to their consideration.
The bridges selected for mention have been grouped according to the nature of the material used in their superstructure. This arrangement is the most convenient one for giving a concise description of the most remarkable of Mr. Brunel’s bridges, and for stating the circumstances which guided him in the determination of the particular form of construction used in each case.
The works are therefore divided into four groups, namely, brickwork and masonry, timber, cast iron, and wrought iron. [1]
Brickwork and Masonry Bridges. [2]
The viaduct which carries the Great Western Railway over the valley of the river Brent near Hanwell is the first of Mr. Brunel’s important railway works. [3] It is a handsome brickwork structure, 65 feet high, with eight semi-elliptical arches, each 70 feet span and 17 feet 6 inches rise. The spandrils of the arches are lightened by longitudinal spandril-walls; the piers are also hollow, and the structure is throughout made as light as possible. It is on this account interesting, as showing the care taken by Mr. Brunel from the commencement of his practice to distribute the material in the simplest and most effective manner. [4]
The great bridge over the Thames at Maidenhead contains two of the flattest, and probably the largest arches that have yet been constructed in brickwork. The river, which is about 290 feet wide, flows between low banks; in the middle of the stream there is a small shoal, of which Mr. Brunel took advantage in building the centre pier.
It was originally intended that the foundation of the bridge should be on the chalk, which was at a short distance below the surface; but it was found to be very soft, and Mr. Brunel therefore decided to place the foundations of the bridge on a hard gravel conglomerate overlying the chalk. The main arches are semi-elliptical, each of 128 feet span and 24 feet 3 inches rise. They are flanked at each end by four semicircular arches, one of 21 feet span, and three of 28 feet span, intended to give additional water-way during floods. The radius of curvature at the crown of the large arches is 165 feet, and the horizontal thrust on the brickwork at that point is about 10 tons per square foot.
In the interior of the structure immediately landward of the large arches, Mr. Brunel constructed flat arches loaded with concrete. The centerings of these were struck, and an active thrust opposed to the main arches before their centerings were eased. [5] The line of pressure of each main arch was diverted downwards by the thrust of the flat arch adjoining it without the necessity of employing a great mass of brickwork in the abutment.
The woodcut (fig. 1) shows the form of the main arches and the flat arch referred to. [6]
The Maidenhead bridge is remarkable not only for the boldness and ingenuity of its design, but also for the gracefulness of its appearance. If Mr. Brunel had erected this bridge at a later period, he would probably have employed timber or iron; but it cannot be a matter of regret that this part of the Thames, although subjected to the dreaded invasion of a railway, has been crossed by a structure which enhances the beauty of the scenery.
There are two other large brick bridges over the Thames, one at Gathampton and another at Moulsford, that at Moulsford crossing the river obliquely at an angle of 45°. In each of these bridges there are four arches, of 62 feet span on the square.
Other good examples of brick bridges are the turnpike road bridge, 60 feet high, with three arches, across the deep cutting at Sonning Hill, and the bridge, with one opening of 60 feet and four side arches of 18 feet span, over the river Kennet at Reading.
The bridge over the Avon at Bathford, of 87 feet span, and the bridge crossing the same river at Bath, with an arch of 88 feet span, are handsome Bath-stone structures with semi-elliptical arches. Near Bristol there is an ornamental bridge of masonry with three Gothic arches, the centre arch having a span of 100 feet. [7] Another bridge of Gothic design, with two arches of 56 feet span, carries the railway over the Floating Harbour. [8]
The bridges which have hitherto been noticed are all on the Great Western Railway. On the Bristol and Exeter Railway there is a large stone bridge over the New Cut at Bristol, built in 1840, which has a single segmental arch of 120 feet span, and 20 feet rise. Owing to some imperfect workmanship in the interior masonry of the arch, and possibly to some unequal yielding of the abutments, the crown sunk much more than had been expected.
On his later railways Mr. Brunel did not build large arches of brickwork or masonry, though he constructed several lofty and extensive viaducts of these materials with spans varying from 40 to 60 feet.
Mr. Brunel seldom employed artificially piled foundations to support masonry. When the ground was soft, he preferred to rely on a large extent of bearing surface, and ensured uniformity of settlement by an accurate distribution of the load. Several of his large viaducts and bridges, standing on ground of a soft and spongy nature, were constructed on this principle.
A class of bridge of striking outline was used in the cuttings on the Bristol and Exeter Railway, and on the other railways subsequently made. Bridges of this class were called flying bridges. Instead of arches resting on piers and abutments, the bridge has a single arch, reaching from one side of the cutting to the other, and springing from the slopes, which it helps in some measure to support. A flying bridge of large dimensions near Weston-super-Mare carries a road across the cutting at a height of 60 feet above the line of rails, with a clear span of 110 feet.
The quantity of masonry in these bridges is much less than in those of the ordinary construction; and lofty and expensive centering is not required, as the bridge can be built before the cutting is excavated to its full dimensions.
This class of bridge, by the avoidance of abutments and counterforts, simplifies the construction of skew arches, while on sharp curves it presents but little obstruction to the view along the line.
A curious use of arches of this construction, as applied by Mr. Brunel, may be seen on the South Wales Railway near Llansamlet, between Neath and Swansea. A deep cutting through the coal measures showed a tendency to slip, and a large amount of excavation would have been required to flatten the slope, as a hill rose immediately above the side of the cutting. Four of these flying arches were thrown across the cutting at short intervals, and weighted with heavy copper slag, so that the sides of the cutting are kept apart by the thrust of the loaded arches.
Among the skew bridges on Mr. Brunel’s railways, there are a few of extreme obliquity. Of these may be mentioned two large road bridges near Berkeley, over the Bristol and Gloucester Railway, one being 48° and the other 53° off the square. Both the bridges are of brickwork, and in the arch of the first one, which was set in Roman cement, hoop iron was introduced in the manner successfully employed by Sir Isambard Brunel. On the South Devon Railway, near Plympton, there is a skew bridge 63° off the square.
On the Great Western Railway, in the neighbourhood of Bath and Bristol, there are skew bridges of ashlar masonry built on the mechanically correct principle of spiral tapering courses, the bed-joints in every part of the arch being made at right angles to the lines of pressure. By this method the arch does not depend for its stability on the friction and cohesion of the materials, as it does to a great extent in very skew bridges, built in the usual way with spiral parallel courses, especially when the arches are semi-circular or semi-elliptical.
Mr. Brunel’s bridges of masonry and brickwork were well known for the comparatively small quantity of material used in them; and, though it was requisite that the materials and workmanship should be of superior quality, their cost was comparatively small.
The specifications he prepared for all his works, and on which the contracts were based, were noted for the completeness with which they were drawn up, and for their not requiring a standard of perfection higher than that which was actually to be carried out. The confidence with which Mr. Brunel was regarded enabled him to insist with effect on the work being executed according to his interpretation of the contract.
In connection with the design of engineering works, and especially of brickwork and masonry bridges, the following letter from Mr. Brunel to one of his assistants, who was abroad, will be found interesting:—
December 30, 1854.
Let me give you one general piece of advice—that while in all works you endeavour to employ the materials used in the most economical manner, and to avoid waste, yet always put rather an excess of material in quantity. You cannot take too much pains in making everything in equilibrio; that is to say, that all forces should pass exactly through the points of greater resistance, or through the centres of any surfaces of resistance. Thus, in anything resembling a column or strut, whether of iron, wood, or masonry, take care that the surface of the base should be proportioned that the strain should pass through the centre of it. Consider all structures, and all bodies, and all materials of foundations to be made of very elastic india-rubber, and proportion them so that they will stand and keep their shape: you will by those means diminish greatly the required thickness: then add 50 per cent. So in trussed framework of wood or iron, experience shows that you cannot refine too much upon the perfection of the designing of every little detail by which all strains are carried exactly through the centres of the rods or struts and the centres of the bearing surfaces. And remember, always in retaining walls to give plenty of batter; never build an upright wing-wall, or retaining wall. To a man who has an instinctively mechanical mind—and no other can be an engineer—the advice I have given you above is all I need say; but this advice is the result of a good deal of experience, purchased by failures of my own, and by looking at those of others, and is, I assure you, valuable advice, to be followed literally and strictly, and not to be considered as a mere theoretical refinement, to be neglected in practice. Practically too much attention cannot be paid to these precautions. I have found that there is not a single substance we have to deal with, from cast-iron to clay, which should not practically be treated strictly as a yielding elastic substance, and that the amount of the compression or tension, as the case may be, is by no means to be neglected in practice any more than in theory. Bear in mind also that which is too often neglected and involves serious consequences, that masonry or brickwork has not half the strength which is generally calculated upon until the mortar is hard, and that you cannot keep centres or shores up too long.
[1] It would of course be impossible here to give a description of all Mr. Brunel’s bridges, or even to refer to the most important of them with that minuteness which would be required if this were a book written for professional use. The following publications may be consulted:—Bourne’s History and Description of the Great Western Railway, 1846; Brees’ Railway Practice, 1837; Simms’ Public Works of Great Britain, 1838; Proceedings of Institution of Civil Engineers, vols. 14, 25, and generally; Molinos et Pronnier, Construction des Ponts Métalliques, 1857; Humber’s Cast and Wrought-Iron Bridge Construction, 1861; and Humber’s Record of Engineering for 1866. At the end of the description of many of the bridges in this chapter a note has been given of publications in which the bridge has been referred to.
[2] In the early days of the Great Western Railway special designs were made for every one of the ordinary bridges over and under the railway; but when, in consequence of the rapid extension of the Great Western system, the number of bridges to be designed became very large, Mr. Brunel had a set of ‘standard drawings’ prepared and engraved, which embodied the experience gained, and contained designs suitable for various situations. The contract drawings were made by adapting to the particular circumstances of each case the standard drawing which was most applicable to it. This system, besides securing uniformity of construction, introduced a considerable amount of economy; since, the standard drawings being based upon the results arrived at in an extensive practice, the proper structural arrangements and dimensions were indicated with far greater accuracy than could be attained in a reasonable time by an independent calculation in each individual case. [3] It was called the ‘Wharncliffe Viaduct,’ in acknowledgment of the services rendered to the Company by the late Lord Wharncliffe as Chairman of the Committee in the House of Lords. Drawings of this bridge are given in Simms’ Public Works of Great Britain, 1838, pl. 54, 55, and 56; and in Bourne’s History and Description of the Great Western Railway, 1846.Chapter ToC – Next >