The life of Isambard Kingdom Brunel, Civil Engineer Part 21

In 1841 Mr. Brunel constructed a timber bridge of five spans to carry a public road over the Sonning cutting of the Great Western Railway, a short distance east of Reading. The total width of the s.p.a.ce across which the road had to be carried was 240 feet. The superstructure rests on four tall frameworks or trestles of timber forming the piers. Two of these piers are on either side of the railway, and the others are about halfway up each slope.

The road rests on a platform of timber planking, carried on three longitudinal beams, which are supported at nearly equal distances by timber struts radiating from points on the piers about 12 feet below the level of the carriage road. The system of arrangement of these struts will be best understood by a reference to the woodcut given below (fig.

5, p. 187) of one of the Cornwall viaducts, of which the Sonning bridge may be regarded as in some measure the prototype.[85]

The skew timber bridge on the Great Western Railway near the Bath Station, carrying the line over the river Avon, was constructed about the same time as the Sonning bridge. It has two spans of 36 feet each on the square, but the obliquity is so great that the span on the skew is 89 feet. Each opening has six laminated arched ribs parallel to the line of the railway. These support the platform of the bridge, and are built up in five layers of curved Memel timber, six inches thick, bolted together. The thrust is counteracted by iron ties connecting the ends of the ribs. The inner spandrils are filled in by cross-ties and braces, and those of the outer ribs by ornamental cast ironwork.

The two bridges already described are almost the only timber bridges of importance on the main line of the Great Western Railway from London to Bristol. Shortly after the completion of this railway Mr. Brunel began to make an extensive use of timber in his designs, and in so doing took full advantage of the largeness of the material, in order to avoid intricacy of construction.

A well-known arrangement for forming beams of greater strength than could be obtained by single pieces of timber was adopted by Mr. Brunel after a careful investigation of its merits. This arrangement consists in joining together two beams of timber placed one above the other, by means of bolts and joggles, so as to form a beam nearly equivalent in strength to a single piece of timber of the same depth as the two pieces united.[86] By this plan, the length which could be spanned by simple beams, without the introduction of trussed framework, was nearly doubled.

The distance between the piers of railway bridges is generally too great to allow of the superstructure being constructed of simple beams, and in such cases Mr. Brunel adopted forms of framing similar in the arrangement of their parts to the common designs of king and queen trusses employed in roofs.

One of Mr. Brunel's early timber viaducts was that erected in 1842 at Stonehouse, on the Bristol and Gloucester Railway. It consisted of a series of five openings of queen trusses 50 feet span, resting on piers formed of timber trestles.

[Ill.u.s.tration: QUEEN TRUSS.]

[Ill.u.s.tration: KING TRUSS.]

In the Bourne viaduct, across the Stroudwater Ca.n.a.l, on the Cheltenham and Great Western Union Railway, there was a span of 66 feet, with three timber trusses, for the two lines of way. Each of these trusses may be described as a king truss with an internal queen truss. The inclined timbers or princ.i.p.als rested in iron shoes upon the piers, and were connected together by bolts and joggles.

The upper horizontal or collar beam of the queen truss carried the roadway planking, which was continued upon beams supported by the princ.i.p.als. The timbers carrying the roadway received support from struts radiating from the feet of the queen posts, which were connected with the apex of the king truss by iron ties. The horizontal tie bars were of wrought iron. The arrangement of the truss is shown in the woodcut (fig. 2).

[Ill.u.s.tration: Fig. 2. Bourne Viaduct.

_Scale of feet._]

The side openings consisted of four spans of 30 feet, with trusses of the Stonehouse viaduct type, of one span of 25 feet and ten spans of 20 feet, with double beams.

The St. Mary's viaduct, across the ca.n.a.l in the Stroud Valley, was constructed with one span of 74 feet, with trusses similar to those at the Bourne viaduct.[87]

In the year 1846 Mr. Brunel made an elaborate series of experiments on the strength of large timber. Some account of these is given in the note to this chapter.

Fortified by the information thus obtained, he was able to proceed with confidence to an extensive use of timber in the viaducts of the South Devon, the Cornwall, and other railways.

Between Totness and Plympton, the South Devon Railway, running along the skirts of Dartmoor, crosses four deep valleys, by lofty viaducts, all of the same design.

Three of them can be seen at one time, and they form striking and elegant features in the landscape.

The viaduct at Ivybridge is the highest of these. It is on a curve, and has eleven openings of 61 feet each; the extreme height is 104 feet.

[Ill.u.s.tration: Fig. 3. Ivybridge Viaduct.

_Scale of feet._]

The piers are of masonry, each consisting of two slender and slightly tapered shafts about 7 feet square, rising to the level of the rails.

The superstructure was originally designed for a railway on the Atmospheric System, and was therefore only intended to bear the load of a train of carriages. The framework was placed below the level of the rails, and, as will be seen in the woodcut (fig. 3), it consists of a polygonal frame, with a few subsidiary struts, the feet of the main timbers being tied together by wrought-iron rods. There are two of these frames, one at each side of the bridge, to support the planking of the roadway. Before the construction of the viaducts was proceeded with, a complete span of the superstructure, consisting of a pair of the frames with the planking, was erected at Bristol, and tested to ascertain the efficiency of every part.

When it became necessary to strengthen the superstructure to enable it to carry the weight of locomotives, a strongly trussed parapet was added above the trusses, as shown in the woodcut. After the lapse of twenty years, the timber having begun to decay, wrought-iron girders have been inserted, which rest on the stone piers; the framing, however, has not been removed.

Shortly after the completion of the viaducts on the South Devon Railway, those on the South Wales Railway were constructed. The most important on this line are those at Landore and Newport.

The viaduct at Landore, near Swansea, is 1,760 feet long, as the railway here crosses a wide valley. It has 37 openings, and there are a variety of spans, one of 100 feet, two of 73 feet, two of 64 feet, two of 50 feet, and the rest of about 40 feet each. Most of these consist of a superstructure of queen trusses. The piers are of different materials, some being almost entirely of masonry, some partly of masonry and partly of timber, and others entirely of timber, according to the nature of the foundation.[88] The chief feature is the centre span, with an opening of 100 feet, the superstructure of which is a very fine piece of timber-work.[89] It has four trusses, one on either side of the two lines of rails, of the form shown in the woodcut (fig. 4). The truss consists of a four-sided frame placed within a five-sided frame, the angles of each polygon being connected by bolts and struts with the middle of the sides of the other polygon.

[Ill.u.s.tration: Fig. 4. Landore Viaduct.

_Scale of feet._]

The planking of the roadway rests on double beams, supported at several points in the manner shown in the woodcut, each point having suspension-rods to connect it with the nearest angles of the frames. The arrangement of the double polygonal frame and of the tie-rods enables the transverse strength of the timbers to exercise considerable resistance to any distortion of the shape of the truss by a rolling load. To prevent any tendency of the top of the frame to yield sideways under the compressive strain, the tops of the trusses are connected by transverse struts or braces, the two outside trusses being steadied by raking ties attached to outriggers projecting from below the flooring of the bridge. The thrust of the polygonal frames is resisted by wrought-iron tie-bars at the level of the roadway beams. All the tie-rods in this bridge are double, with one bar on each side of the timbers, to avoid the necessity of making large bolt-holes.[90]

The viaduct at Newport consists of eleven spans with queen trusses, resting on piles. The main span, over the river Usk, is 100 feet, and was constructed with timber trusses very similar to those at Landore.

Shortly before it was finished, the viaduct was burnt down. In rebuilding it, wrought-iron trusses were employed for the main span.

The works of the Cornwall Railway were commenced in the year 1852. The district through which the line pa.s.ses is very deficient in the materials requisite for the construction of a railway. The granite of the country is for the most part only applicable for ashlar; and the slate, which is flat-bedded and so far fit for rubble masonry, is frequently inferior in quality.

In consequence of the number of valleys that the railway had to cross, the aggregate length of the viaducts, thirty-four in number, exclusive of the Saltash bridge, is upwards of four miles on a line of sixty miles. By the use of timber, a great saving was effected in the first cost of the works; and though it is a material which in time requires renewal, its use on the Cornwall Railway enabled the line to be made with the capital at the command of the Company; while, allowing for the cost of subsequent repairs, the total expenditure did not differ much from what it would have been had the superstructure of the viaducts been of more durable materials. The comparatively small cost of these structures enables them to be, in certain places, economically subst.i.tuted for embankments, as was done on the Cornwall Railway.

The viaducts are to be found over the whole length of the line, but they are most frequent between the Liskeard and Bodmin Road stations, where the railway crosses numerous branches of the Glynn valley.

Most of these viaducts are of one type of construction.

The piers are formed of plain walls, built up to thirty-five feet below the level of the rails, those of the more lofty viaducts being strengthened by b.u.t.tresses. In the woodcut (fig. 5) is shown a portion of the St. Pinnock viaduct, from which the form of these piers will be understood.

This viaduct is the loftiest on the Cornwall Railway, the rails being at a height of 153 feet above the ground. A description of the superstructure will serve to explain the design of the princ.i.p.al viaducts on the line.

The roadway planking rests on three beams, which run longitudinally throughout the whole length of the viaduct. Each of these beams consists of two pieces of timber, one above the other, fastened together by bolts and joggles. The piers are 66 feet apart, centre to centre, and the longitudinal beams are supported, at four nearly equidistant points in this s.p.a.ce, by straight single timbers radiating from the tops of the piers. The feet of the timbers, which rest on the masonry in cast-iron shoes, are connected together by wrought-iron tie-bars; and the framework is made rigid by iron diagonals.

[Ill.u.s.tration: _Scale of Feet_ _Transverse section_

_Plan of base of pier._ _Plan of top of pier._

Fig. 5. St. Pinnock Viaduct.]

It will be observed in the transverse section (fig. 5) that the whole weight of the superstructure is concentrated immediately over those points in the piers where the three b.u.t.tresses meet. The diagonal braces which are attached to each set of the main timbers give transverse stability to the superstructure.[91]

It was desirable, both in first construction and in subsequent repairs, to have a uniform dimension for the spans, and the subdivision of 66 feet was determined on as being suitable for the economic construction of the greater part of the work. The subdivision of this length was such as to allow of single whole timbers being sufficient for the direct supports of the longitudinal beams; and as these beams were supported at intervals of 15 to 20 feet, no intermediate trussing was required. As the inclined timbers met the tops of the piers at a moderate inclination, the outward thrust caused by unequal loading of the spans of the viaduct was inconsiderable, and was easily counteracted by light iron ties.

The stone for the piers was for the most part procured in the neighbourhood, the design of the masonry being such as to enable stone of the country to be used; and, as the timber superstructure was built in pieces of moderate size,[92] and easily obtained, the expenditure was probably not far from the minimum under the existing conditions.[93]

On the South Devon and Tavistock Railway, the viaducts are six in number, and from 62 to 132 feet in height. In these the piers were made of a somewhat simpler form than those just described. At the lofty viaducts, the b.u.t.tresses were made with a uniform batter throughout their height. The Walkham viaduct, near Tavistock, 132 feet high, with fifteen openings of 66 feet span each, may be considered to exhibit the most matured design of Mr. Brunel's timber viaducts.