Life of Robert Stephenson by William Pole: Chapter IV (Volume 2)

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CHAPTER IV. (Volume 2). The High Level Bridge at Newcastle-on-Tyne

THIS bridge (High Level Bridge, Newcastle), although of less magnitude than either of the two other large iron bridges selected for illustration, is one of the most celebrated of Mr. Stephenson’s works. It has for its object to form a double communication, by railway and by common road, at a high level, between Newcastle on the north, and Gateshead on the south bank of the river Tyne.

The river runs through a deep valley or ravine, the average level of the land on each side being about 100 feet above the water of the river. A bridge of considerable antiquity, crossing at the bottom of the valley, formed the only passage, and as the streets leading down from the level part of the town on each side were exceedingly steep, the passage from one elevated shore to the other was fraught not only with much difficulty but with positive danger.

This, however, was endured for hundreds of years as a necessary evil; it was only about the commencement of the present century that the idea of avoiding the difficulty by a bridge at a higher level began to be seriously entertained. The matter was first mooted by [R. B. Dodd| Mr. R. B. Dodd]], a local engineer, and a public meeting was held in furtherance of the plan, which, however, did not meet with any adequate support.

About 1825 Mr, Telford proposed a bridge on the present site, and other engineers are said to have renewed the scheme from time to time; but the project best known was proposed, some years after Mr, Dodd’s, by the late Mr. John Green, an architect of Newcastle, and an attempt was made to form a company to carry it out, the chairman being Mr, John Hodgson Hinde, an influential inhabitant of the town. The late Lord Grey and several other gentlemen of local influence exerted themselves to promote the measure, but the requisite capital, £30,000, was not forthcoming from the public; while the corporation of the town felt they were not warranted in incurring an enormous outlay, which could not, they conceived, be reimbursed by any tolls they could impose upon the traffic across it.

Mr. Hinde, however, still persevered. In 1843 he induced Mr. George Hudson to aid in the undertaking, and to become vice-chairman of the company, and it was resolved that Mr. George Stephenson should be consulted on the matter. Accordingly a new prospectus was drawn up of the ‘High Level Bridge Company,’ in which his name appeared on the direction, while that of his son was appended as consulting engineer. George Stephenson had a design prepared of a bridge with bowstring girders and a double road, similar in its essential features to that ultimately adopted; but it was determined that the application to Parliament should be made on Mr. Green’s plans and estimates, and that they should be re- modelled by Robert Stephenson after the Act was obtained.

On June 18, 1844, the Newcastle and Darlington line, laid out by George Stephenson as a portion of the great trunk line by the east coast to Scotland, was opened to public traffic, and completed the communication from London to the Tyne. The terminus of this railway was at Gateshead, and consequently the traffic was cut off by the deep chasm of the river from the town of Newcastle, and the railway isolated from the hues already made on the north side, as well as from the proposed continuation of the great trunk to Scotland.

The inconvenience of this was felt most severely. It was found that nearly half a million local passengers frequented the station in the course of a year; and as it was situated on the high ground, they had to descend the steep road, on a declivity of 1 in 8 or 9, to cross the river, and ascend again by a similarly steep hill to the plateau of Newcastle on the other side. The cost of conveyance of passengers and merchandise by coaches and omnibuses across the river reached the enormous amount of £1000 per week.

At this time a considerable agitation took place respecting the prolongation of the Scotch trunk line from Newcastle to Berwick, for which the route had been surveyed previously by George Stephenson; and this gave a renewed impetus to the question of the bridge. It was seen that as the railway must, if possible, be made to cross the Tyne, it would be highly advantageous to combine both railway and common road crossings in one bridge, the expense of which, though too great to be warranted by the road traffic only, might be very well justified by the addition of that of the railway: and the promoters of the new line wisely saw the advantage that would accrue to their interests if, in conjunction with their works, they could offer to the town the boon of the high-level carriage road crossing, which had been so long desired. They accordingly took up the project of the ‘ High Level Bridge Company,’ and by making it a combined road and railway bridge, incorporated it into their railway survey as an integral part of the scheme.

A rival line to Mr. Stephenson’s, promoted by Lord Howick, with Mr. Brunel as the engineer, was, however, started: this was to be worked on the atmospheric plan,‡ and was intended to cross the Tyne by a bridge considerably to the westward of the town, and at a low level, having gradients of 1 in 50 on each side.

The rival schemes came before Parliament in May 1845, and the High Level Bridge formed one of the most prominent features of the Stephenson line. Mr. Robert Stephenson was examined at some length, and gave a full description of the proposed bridge. Many objections were raised by the opposite party, on the grounds of the great height of the structure and of the viaducts connecting it with the town—of the risk of the trains running off—of the danger of fire to the houses from the projection of live coke from the engines— of the chance of frightening horses on the carriage road of the bridge by the noise of trains passing overhead ;—and the obstruction the bridge would cause to the ventilation of the town.

These objections were all answered. Mr. Stephenson gave his strong opinion that the plan of the bridge proposed was the only one by which the objects aimed at could be properly combined; and after a hard and prolonged contest, the bill for Mr. Stephenson’s line passed, including in it the sanction for the erection of the High Level Bridge across the Tyne.

Mr. Stephenson at once put the design in hand, and the dravrings were prepared, under his immediate direction, by his assistant, Mr. Thomas E. Harrison, who afterwards became resident engineer on the line, and is still consulting engineer to the larger ‘North Eastern ’ system in which it is now incorporated.

The roadway across the bridge consists of two platforms ; the upper one carries three lines of railway, while the lower forms the common public road. The approaches of the railway are curves in contrary directions, but those of the public road are in a straight continuation of the line of the bridge. The height of the rails on the upper platform above the low water of the river is 120 feet. The level of the carriage road is about 23 feet lower than that of the railway.

The bridge stands about 90 yards to the west of the old bridge. The river at this spot is 515 feet wide at high water; but as the bridge has to cover also the sloping shores on each side, its whole length 181,372 feet. There are six large openings, each of 125 feet span in the clear, stretching over the river and the flat portions of the banks, the slopes being covered by smaller abutment arches up to the high level on either side.

The piers, some of which, from foundation to summit, are as much as 146 feet high, are built of a hard and durable sandstone, obtained from quarries in the neighbouring coal formation. They are founded on piles, the spaces between which are filled up with concrete. Many of the piles are 40 feet long, and all are driven through the hard sand and gravel forming the bed of the river, till they reach the solid rock below. The piles are 13 inches square, and are placed 4 feet apart from centre to centre. The greatest weight that can come upon each of them is 70 tons, supposing none to be carried by the intervening spaces of concrete. This is a very heavy load, which could only be warranted by the goodness of the strata into which the piles are driven. In moderately compact clay it is usual to consider the maximum bearing power of a pile to be about 12 tons; in hard clay about 25 tons ; but in gravel, of which the bed of the Tyne consists, 70 or 80 tons are often allowed; indeed, many engineers consider the bearing power in such strata only limited by the resistance of the fibre of the timber. In this case also, the feet of the piles rest on the solid rock, which puts all doubt at rest as to their stability. Mr. Stephenson, however, with his characteristic desire to satisfy his mind thoroughly on the point, tested one of the piles by laying on it a load of 150 tons, which was allowed to remain for several days; but on its removal no settlement whatever had taken place.

The masonry of the piers commences about 2 feet below low-water level. The lower portions in the stream are provided with cutwaters. The foundation surface of each pier is about 76| feet by 22|; the section of the tall shaft of the pier is about 46 feet by 14 feet, lightened by an arched opening 12 feet wide.

But the most important part of the work is the iron superstructure, and it will be interesting to consider the motives which led Mr. Stephenson to a decision as to the nature of the structure by which the openings were to be spanned. It will be seen by reference to the historical notice in Chapter II., that in 1845, when this work was designed, the science of iron bridge construction was only partially developed. The experiments for the Britannia Bridge, which had ultimately the effect of bringing wrought-iron girders into use, had but just commenced, and the only kinds of iron bridges then adopted for large spans were—the suspension bridge— the compound trussed girder—the cast-iron arch—and the bowstring girder. We may therefore conceive Mr. Stephenson considering the applicability of each of these systems in turn. The suspension bridge would have answered well enough for the common roadway, but it was inapplicable to the railway from its want of rigidity. Mr. Stephenson had indeed, just at this time, investigated carefully the possibility of its application to the Britannia Bridge, and decided against its fitness for railway purposes. The compound girder of cast-iron, trussed with wrought-iron bars, had been used by Mr. Stephenson somewhat extensively, and was at this time being ap- phed by him to the Chester Bridge of 100 feet span; but he probably shrunk from extending a construction yet scarcely tried to dimensions so much larger, and to a situation so much more perilous than anything previously encountered.

The cast iron arch must at first sight have recommended itself strongly for adoption. Its principles were thoroughly known; its strength and stability were indubitable ; and it would have made by far the handsomest structure in an architectural point of view. But to Mr. Stephenson’s far-seeing and eminently cautious professional judgment, objections revealed themselves which he did not feel himself able satisfactorily to overcome. Arches involve outward thrust at their extremities, and to resist this thrust, so as to keep the whole structure in perfect equilibrium, great stability in the piers and abutments is absolutely essential. Now this quality Mr. Stephenson did not see his way to insuring. The piers were of great height, and economy demanded all possible saving in their bulk ; so that they would stand up from the depth below as long slender legs, on the top of which it would be highly injudicious to allow any considerable oblique strain to fall; and though the arches on each side of any pier might theoretically be supposed to counteract each other’s thrust, and to throw the resultant strain vertically down the body of the pier, Mr. Stephenson’s experience told him this could not in practice be relied on. But more than this, he anticipated difficulty with the foundations of the piers; he knew by his borings he should meet with treacherous strata; and by calculation of the weight each pier would have to support, he found that his bearing piles must sustain a very heavy and unusual load. Under these circumstances he considered a slight settlement of the piers as a contingency quite possible, and which he could not with certainty avoid by the utmost skill and care. And as such a settlement would have endangered in a serious degree the equilibrium of any arches resting upon the piers, he deemed it prudent to give up the idea of using the arch system of construction, to which, it is well known, in suitable cases he had a strong leaning.

There only remained therefore the bowstring girder, a form which fortunately combined all the requisite conditions. It was simple in principle, strong and stable, well understood, and entirely free from the objections to the arch, inasmuch as, like all other girders, it was self-equilibrated, gave nothing but vertical weight upon the piers, and would allow of a slight settlement in them without serious danger. Moreover, the form of this girder was eminently adapted for the double roadway, the top of the arch being at a suitable level for the stiff platform of the rails, while the horizontal tie, or string of the bow, defined the lighter carriage road. Thus Mr. Robert Stephenson’s comprehensive and acute practical reasoning enabled him to justify, and indeed to reproduce, the design which had originally occurred to his father.

He had already built, on the London and Birmingham Railway some years before, handsome bridges of the same construction, 50 feet span, which, although they had only one roadway to carry, had a double horizontal bar, above and below, as if they carried two; and it is not improbable that this feature (introduced there chiefly for stiffness) may have suggested the peculiar applicability of the form of girder to the purpose of carrying a double road.

Whether, if the High Level Bridge had been designed ten years later, Mr. Stephenson would have adopted the bowstring arch in preference to wrought-iron girders, it is difficult to say : the latter would certainly have been lighter and cheaper, but it would be difficult to find any form of girder, even with all our modern knowledge, that would make so appropriate, so substantial, and at the same time so handsome a bridge as that actually built. It was a mistake in the architectural design to put a pier in the centre instead of a space, but probably this was determined by engineering considerations.

Each span or bay is crossed by four main girders, the chief feature of each girder consisting of a cast-iron arch or bow, the ends of which are connected together, and the thrust taken, by a wrought-iron tension rod or tie.

The rise of the cast-iron arch is 17 feet 6 inches, or a little less than one-seventh of the span. It is made in five segments, strongly bolted and accurately fitted together ; the depth is 3 feet 6 inches at the crown ; the section is that of a double-flanged girder, having 133 square inches area of metal in the two outer girders, and 189 inches in the inner ones, which have more weight to support. The tension ties, or strings of the bow, consist of flat wrought-iron bars, 7 inches by 1 inch, each outer girder being tied by four of these, and each internal girder by eight. The girders are ah strongly braced together with diagonal frames.

The railway platform is placed above the top of the arches, a series of pillars being carried up throughout the spandrels, so as to support entablature beams lying horizontally above them the whole length of the bridge. Cast-iron cross-bearers rest on these, extending in one length over the four main girders; these again support longitudinal timber joists, on which a flooring of double diagonal planking, jointed and tongued with hoop iron, and well caulked with pitch, is laid; the three lines of rail are fastened down to the planking in the ordinary way.

The lower roadway for common traffic is hung from the arches by wrought-iron suspending rods, bearing longitudinal beams similar to those of the top platform, and upon these cross-bearers rest, carrying a double planked roadway in like manner. This is paved with wood-blocks set in pitch, and Covered with sand and gravel.

The four girders are so placed as to leave space for a carriage-way between the two inner ones, with footways between these and the outer girders on each side. The carriage-way is a little over 20 feet wide, and the two footways are 6 feet each. The total width of the bridge from outside to outside is about 46 feet.

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Fig. 7 shows an elevation, and fig. 8 a transverse section of one of the spans of the bridge, from which a general idea of its structure may be obtained. The general elevation is given, on a small scale, in plate at p. 71 of this volume.

Provision is made for the expansion and contraction of the iron superstructure by fixing the girders firmly to the first, middle, and fifth piers, and making them free to move on the second and fourth, as well as on both the abutments. There are no rollers, but the bearings have surfaces fitted for sliding on each other. The motion caused by a variation of temperature of 32 degrees was found by experiment to be 0T53 of an inch for each span.

The land abutments are founded upon a bed of strong clay, which underlies the sand and gravel, no piles being used. They are built of stone similar to the piers, and are of handsome design, carrying the roadways over the slopes on each side by masonry arches of solid construction. Designs were made for ornamental entrances to the bridge, in keeping with the decorative features of the structure; but as the authorities of the railway and the town grudged the few hundreds of pounds necessary for their erection, the work is left incomplete.

The contract for the bridge was let on August 17, 1846, and the work was commenced in October; but many difficulties occurred in driving the piles which considerably retarded the progress of the work; and among others the peculiar effect of ebb and flow during this operation was thought by Mr. Stephenson worthy of special notice in an engineering point of view, as being not generally observed. During flood tide the sand became so hard as to resist almost entirely the utmost efforts of driving, while at ebb it was quite loose, and in no way hindered the operation. It was found necessary to abandon the driving on many occasions during the entrance of the tide.

Another difficulty arose from the quick-sands beneath the foundations. Although the piles were driven to the rock the water found its way up, baffling the attempts to fill in between them; this, however, was ultimately remedied by using a concrete made of broken stone and Roman cement, which was continually thrown in till the bottom was found to be secure.

The piles were driven by Nasmyth's steam pile-driver, this being one of the first cases in which it was used; and by its quick action the driving was effected in much less time than by the ordinary means. The ram of the engine weighed a ton and a half, and had a fall of 2 feet 9 inches. It was worked incessantly night and day, driving at the rate of sixty or seventy strokes per minute; and in several instances the pile-heads burst into flame, and burnt fiercely under this rapid action of the ram. It was found by an experiment that after a pile had been driven with the ordinary machine as far as it would go, the application of the steam driver would force it down 15 feet farther.

The coffer-dams for the piers were formed of double rows of piles, filled in with clay puddle ; when they were removed, the piles were not drawn, but were cut off level with the bed of the river by a circular saw, the lower parts being left to protect the foundations as well as to avoid disturbing the ground by their extraction.

The ironwork of the superstructure was manufactured by Messrs. Hawkes, Crawshay and Co., of Newcastle; but before any of it was made, Mr. Stephenson instituted a large series of experiments upon different kinds and different mixtures of cast iron, with the view of ascertaining what description of metal would be most advantageous for the purposes of the bridge. These experiments were made with great care, and Mr. Stephenson considered them the most extensive as well as the most accurate series then existing. They are printed in full detail in the Report of the Royal Commission on the Application of Iron to Railway Structures, 1849. They led Mr. Stephenson to the following conclusions :—

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The first heavy casting was made in February 1847. Each arch was temporarily erected at the manufacturers’ works and tested before removal, and all the detached parts received a separate test previously to their final trial.

The total quantity of masonry in the bridge is 686,000 cubic feet; the weight of ironwork is 5,050 tons. The cost of the entire work, including that of the temporary bridge, was £243,000.

The iron superstructure was erected on centres supported by scaffolding from below, which likewise answered the purpose of carrying the temporary roadways at different stages of the work. Each bay was divided by a timber pier, leaving a clear opening on each side of about 53 feet; the parts on which each segment of the cast-iron arch rested being strutted from the sides. The segments were lowered to their places by a large traverser or ‘ Goliah,’ running on a tramway about 3 feet below the rail level.

So much importance was attached to getting the railway traffic across the ravine as soon as possible, that it was thought worth while to erect alongside the scaffolding a temporary bridge of timber, which anticipated by a year the opening of the main structure.

The bridge was examined and passed by the Government Railway Inspector on August 13, 1849, and was formally opened by the Queen in the month following.

W. P.

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