1948.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



33 



ARCHED TIMBER VIADUCTS. 



fWith Engravings, Plates III. and IV. J 

 (From papers read at the Institution of Civil Engineers.) 



The Arched Timber Viaducts on the Newcastle and North Shields 

 Railway, erected by Messrs. John and Benjamin Green, of New- 

 castle-upon-Tyne. 



In the formation of the numerous railways which have been com- 

 pleted within the last few years, perhaps that which has demanded 

 the greatest exertion of skill, judgment, and varied ingenuity, is 

 the construction of the bridges and viaducts, whether of stone, 

 brick, iron, or timber. The excavation of a quantity of earth, or 

 cutting through a hill to fill up an adjacent hollow to the required 

 level, IS in most cases a work of little more than manual labour, 

 unless some unforeseen extraordinary difficulties occur in the strata, 

 which may require energy and promptitude in adopting such mea- 

 sures> as will overcome them in the most eifectual and least expen- 

 sive manner. But when rivers are to be spanned and ravines to be 

 crossed, where there exist uncertain and variable beds, and, in 

 many instances, in the vicinity of towns and populous districts, 

 where houses, manufactories, and other buildings are on the imme- 

 diate spot, the space required for filling up such ravines with a 

 mound, (the base extending far on each side, beyond what is re- 

 quired for the width of the railway) would, by the consequent de- 

 struction of property, often involve such ruinous expenses, as to 

 render the adoption of that method impracticable. Recourse must 

 then be had to other and more scientific means, in the erection of 

 bridges or viaducts, with piers occupying a small superficial area, 

 built up to carry the necessary superstructure, and adapted to the 

 locality in which they are placed. Various considerations are in- 

 volved in fixing on a certain plan : yet the cost is that of the utmost 

 importance, and invariably presents itself first in all great works of 

 the kind. It will not be denied, therefore, that the great desidera- 

 tum for the engineer, is the adoption of such means as will fully 

 answer the purposes and the ends at which he aims, and to effect 

 this without a waste of any kind of material; for every thing that 

 is not fairly and usefully employed in adding to the staliility and 

 strength of an erection, can only be considered as superfluous and 

 injurious matter; and fitness and a skilful disposition of parts, com- 

 bined with correctness of design, may be said to form the great 

 merit of all structures. 



The cost of the construction of viaducts and bridges for railways 

 generally forms so important an item in the gross amount of the 

 cost of a railway, that the engineer is led to devise new means of 

 completing his works in such a manner, as to possess stability and 

 durability, without plunging his employers into unnecessary ex- 

 penses. 



Stone has been generally applied as the best material for bridges ; 

 in many cases, however, it cannot be used throughout, and in large 

 arches, where the heights are too low for the spans, cast-iron is 

 frequently adopted, and more particularly in forming trusses of 

 various kinds, when the under side as well as the upper side of the 

 platform is required to be horizontal, or nearly so, as in the case of 

 a railway and turn])ike-road crossing each other, and only leaving 

 space enough between the surface of each to allow of the free pas- 

 sage of carriages ; but the cost and weight of these bridges is gene- 

 rally equal to that of stone. A wood superstructure, however, 

 effects much in this respect; provided a durable mode of construc- 

 tion is adopted ; for the cheapness and strength of the material 

 itself being so great, in proportion to its bulk and weight, the piers 

 of a bridge or viaduct can be considerably lightened, and much less 

 material be used in their formation than when the superstructure 

 is to be of stone or iron. 



Almost all the wooden bridges that have heretofore been exe- 

 cuted in this country are constructed with straight timbers, trussed 

 and framed like the ordinary forms of roofing. On account of the 

 shrinking, from tlie number of joggles, and the weight of the work 

 itself, the roadway sinks, and the framing generally becomes bent 

 or crippled, often to an alarming extent ; besides, such a system 

 could never be carried beyond a certain extent, as the spans of such 

 framing must be limited to what is usually practised in roofing. 



A new s)'stem of building timber bridges, composed of layers of 

 deals 3 inches in thickness, turned over a centre, into the form of 

 arched ribs, has been introduced and applied extensively, in North- 

 umtierlaud and Durham, and in Scotland, by Messrs. John and 

 Benjamin Green, of Newcastle-upon-Tyne. 



This mode of constructing the laminated deal arch suggested 



No. 125— Vol. XI.— February, 1S48. 



itself to Mr. Green in 1827-8, when he was engaged in designing the 

 bridge for crossing the river Tyne, at Scotswood : ivliere the depth 

 of water, its rapidity during floods, and the uncertainty of th« 

 foundations, would have rendered the construction of a number of 

 piers, in the current, a very expensive operation, and Mr. Green 

 was therefore induced to recommend to the company the chain 

 bridge which is now tlirown across the Tyne at that place, as being 

 the cheapest durisble structure, and possessing advantages over 

 every otlier kind in such a situation. 



The subject of wooden arches continued to engage Jlr. Green's 

 attention, and for his own satisfaction, he bad a model made of an 

 arch 120 feet span, at a scale of one-twelfth of tlie real size, which 

 so satisfied him as to the advantages and safety of that mode of 

 construction, that in 1834, when the Newcastle and Carlisle Rail- 

 way Company oft'ered a premium for the best plan of a railway 

 bridge, for crossing the ri\er Tyne, above Scotswood, Mr. Gre«n 

 submitted his model and design in competition, when they were 

 approved of and selected by the directors, and obtained tlie pre- 

 mium. 



This bridge was to consist of five segmental wood arches, each 

 having two ribs of 120 feet span, which were to be erected upon 

 timber piers of piles and framings, with stone abutments. Tlie line 

 of railway could not allow a greater elevation tlian 21 feet above 

 high-water level, and the platform was in consequence suspended 

 with iron rods between the springing of the wood arch and the 

 crown; tlie roadway was therefore partly suspended from and partly 

 supported by the ribs. 



In 1833 Messrs. Green were concerned in projecting a railway 

 from Newcastle to North Shields ; and afterwards being employed 

 by the Comjiany for the bridges on that line, where, from the mag- 

 nitude of two of them and the number that occur, the cost was a 

 very important consideration, they were induced to recommend 

 this plan of the laminated timber arch. Having made designs and 

 carefully studied the details, these bridges were commenced in 

 1837 ; one at the Ouse Burn in the eastern suburb of Newcastle, 

 and the other at 'Willington, about four miles further on the rail- 

 way. 



The OusE BrRN Viadvct is 918 feet in length, and 108 feet in 

 height from the bed of the burn ; it has five timber arches of tlire« 

 ribs each; three of the arches are 116 feet span, and two IH feet 

 span ; there are two stone arches of -10 feet span at botli ends of 

 the bridge, which were introduced to give length to the abutments, 

 so as to prevent the mounds endangering them, by coming too close 

 upon the steep banks of the ravine. There are five piers built of 

 drafted and broached ashlar masonry, from the foundations to the 

 full height, with spaces in the middle, leaving an average thickness 

 of 5 feet of ashlar work ; all the spaces are filled in with rubble 

 masonry, made solid by grouting. On the sides of each pier are 

 buttresses projecting 2 teet 11 inches, and diminishing with oft'-sets 

 up to the roadway. 



The greatest thickness of the piers at the springing is 15 feet ; 

 that of the highest pier at the foundation is 20 feet 3 inches, and 

 at the top, immediately underneath the platfoim, it is 6 feet 6 inches 

 thick; its width, including the buttresses, is 33 feet 10 inches 

 above the footings, and 20 feet across the last or highest ofl^-set 

 underneath the roadway. 



The springing for the arched ribs is 40 feet down the piers, 

 where large off-sets are formed with the inner splays or slopes, 

 radiating from the centre. t)n these springing stones, cast-iron 

 flanged plates or sockets, each weighing 15 cwt. for each rib, are 

 bedded with oakum, into spaces which are cut 2 inches deep in the 

 masonry, and secured w itli wrought-iron bolts run with lead, fas- 

 tened down with nuts and screws on the outer surface ; the bolts 

 are U inch diameter, and 1 foot 9 inches long. The ends of the 

 rilis are inserted into these iron sockets as a springing and are well 

 caulked. 



The two middle piers are built upon piles driven into the clay, 

 to an average depth of 35 feet below the surface, and the founda- 

 tions generally required great attention, for it was found that 

 considerable excavations of old pit workings had been made around 

 and immediately under the line of the bridge. From the extreme 

 eastern pier, a coal seam had been worked out, extending beyond 

 the east abutment; and in digging for the west pier, a pit shaft was 

 discovered in the centre of the area of the foundation. It was 

 fortunate that it was not so near as to endanger the stability ot the 

 pier, and that the construction had not proceeded without its being 

 observed. This shaft had been worked to a depth of 70 feet, and 

 in order to render the structure secure, both it and the seam on 

 the other side of the ravine were built up with weU grouted rubble 

 masonry. All the timber used in the carpentry was of the best 



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