CONSTRUCTION OF TIMBER BRIDGES

Colonist, Volume IV, Issue 415, 15 October 1861, Page 3

CONSTRUCTION OF TIMBER BRIDGES

Mr. T. Wooue read the following paper on ■« a New Mode of constructing Timber Bridges' before the Royal Victorian Society.

The construction of bridges and viaducts in the formation of railways being one of the greatest difficulties to be overcome, and a material item of expense, it is of consequence the subject should be well considered before an undertaking of this nature is attempted to be carried out, and as the circumstances of different countries vary considerably in respect to their geological formation, climate, effect of climate, on that formation, material for construction of the works, and climate effect on that material, wealth, and other resources, it is absolutely necessary in securing success that each should be consideied separately in reference to their respective peculiarities, for nothing can be more absurd than to conclude that a system that has been found to succeed in one country must necessarity do so in another, when their circumstances are totally different.

The geological formation, climate, material, and other resources of New South Wales being entirely different from those European countiies that have adopted railways, or America, it is neccessary we should endeavour to-suit the construction of our roads to its peculiarities rather than blindly follow the detailed example of other places, if we mean to succeed.

New South Wales, with the exception of isolated patches, is formed by horizontal beds of sandstone, extensively disruptured by violent convulsions of nature, which has left the surface of the country divided into numerous horizontal plateaus and ridges intersected by deep ravines that have almost perpendicular sides. If it is possible to cross these ravines on the level of those natural plateaus, or pass from ridge to ridge by artificial means, preserving the original surface of the country, railways may be laid down in this colony at such moderate cost as would be remunerative; for, that case the earthworks would be light as the road would traverse the natural surface of this sandstone bed. Whereas, if it is necessary, from inability to cross these abrupt ravines, to take a direction that requires tunnelling-and extensive excavations through the sandstone ridges left by these convulsions, across the watershed of the country, the cost of construction must be enormous' from the difficulty and extent of the excavations that would be required. Hence the necessity in New South Wales of paying particular attention to the system of bridging to be adopted, ai?d of obtaining, if possible, a practicable mode of crossing the irregularities of its surface, which, if carried out judiciously, will so materially reduce the cost of constructing railways or other roadi.

But this peculiarity in the surface of the

country is not the only reason we should give strict attention to the bridging. New South Wales is subject more than any other country to sudden and copious rains, which inundate the rivers and low grounds in a remarkable degree. To crop these over the highest water level, and resist the floods is a matter of difficulty, and has hitherto been found impracticable, as requiring bridges of such elevation and extensive span as, under the present mode of construction, is not within the reach of these colonies.

Bridges for heavy traffic are constructed of either stone, bricks, iron or wood. Neither stone nor bricks are good materials for arches resting on lofty piers, from the great weight of those materials, and the tendency such arches have to thrust the piers out of their perpendicular position, which their great elevation gives them less power to resist, more particularly under the vibration of railway trains in motion. Since bridge? of that description have become necessary for railway purposes, iion, in the shape of girders or tubes, have been extensively used instead; these, although not diminishing the weight of the arches or beam to any great extent, have reduced their pressure on the piers to a perpendicular direction, instead of a lateral thrust, which it is obvious the piers are much more capable of resisting. Such iron bridges have been found to be efficient and beneficial in thickly populated countries, where great wealth abounds, and sufficient traffic exists to meet the necessarily large outlay they occasion, but they are totally inapplicable in a colony where there is neither sufficient money to erect them, nor population to support an intercourse that would make them remunerative if erected; therefore, if we cannot adopt a cheaper mode of crossing our ravines and rivers than by those means, we must be shut out from the benefit of railway communication for many years, as the cost oi such works is more than the colony under existing circumstances can afford.

Having these view* before me in 1847, when I endeavored to point out a* more economical mode of establishing railways, I designed the wooden bridge, or viaduct, that is the immediate subject of this communication with the object of meeting the difficulties here mentioned ; but, although the design has been freely offered for the public benefit, it has been left in oblivion, as unworthy of notice ; but, perhaps, now that the colonists are roused to the serious evils attending the existing imperfect means'of internal communication, and the small prospect there is of improvement, the subject will be taken into consideration.

From what I have said, it well be seen that the objects to be sought in constructing bridges or viaducts of magnitude are cheapness of construction, strength and rigidity in the arch or beam that extends from pier to pier, combined with lightness and the least possible strain on piers which are necessarily so elevated. Of all substances, timber, is, in my opinion, the only one that possesses the properties necessary to secure these qualities in a bridge; and it does so from its combination of great strength (when acted on in its longitudinal direction), lightness, elasticity, and adaptation to be placed in any form, and it only requires a true calculation of the strength required in its several parts, a proper adjustment of the materials in those parts, and correct mechanical skill in putting them together, to secure a most efficient machine.

The superiority of wood to iron and stone, as a material of construction, results from the extraordinary lightness which it unites with its strength. The following table shows the result of experiments that were made on our colonial timber at the universal Exhibition, held in Paris, in 1855, under the direction of Captain Fowke, the secretary to the British Commissioners, at the instance of Sir William Macarthur. The table exhibits the extraordinary fact that while our strongest timbers average but the weight of water, they require a weight of from one to two tons to break a stick only four feet long, and less than two inches square, and from four to six tons to crush one cubic inch against its fibre. With such timber as this, what may not be done ? •g "£ > g'£ S-a • g | s*^B si** i Ha. lbs. Black butt ~ 0897 3857 11020 5 Iron bark .. 1032 3416 9921 Blue gum .. 843 2655 8818 4 Illawarra box 1170 4518 9920 True box .. 0970 3086 8818 4 Stringy bark 0864 2755 8818 4 Bastard box.. 1115 3571 9700 Rough barked iron bark.. 1016 4519 13227 6 Greyguin .. 0927*. 3507 9920 4£ \ Mahogany .. 0953 2976 9921 4J This bridge is composed of stone piers, supporting a wooden superstructure. The latter is formed of a series of timber struts, ranged in three or more vertical arches, which are tied together by duplex perpendicular braces, and trussed by diagonals; the whole forming a trussed beam of great strength, incapable of longitudinal expansion. These beams are to be extended between each pair of piers, on which is to be placed the platform that supports the roadway. This combination of timber will be best understood by a reference to the drawings. It is evident, and the foregoing experiment prove, that timber resists the greatest strain lengthwise, or on its longitudinal fibre, and it is well understood in mechanics that the strongest form in which it can be jointed together is the triangle, simply because, when compressed in that shape, the strain comes on the longitudinal fibre : and it is also well known that the arch form is the strongest that can be given to any substance spanning from one pier or buttress to another, which is intended to sustain a downward pressure ; but the arch is attended with'a serious evil—-its disposition to horizontal thrust which acts injuriously on the

piers, and renders it useless, unless counteracted by a mass of masonry in the form of a buttress, which cannot be carried to any great elevation. Now it .will be seen by referring to the skeleton before you, that the arched beams of which this bridge is composed, are formed of a series of triangles, each of which, as long as the joints remain firm, is incapable of compression to the extent that the timber will bear on its longitudinal fibre, which has been shown is from four to six tons on each square inch of its section, according to the nature of the timber employed. These triangles are grouped in the form of an arched beam, and as each individually is incapable of compression, it follows that the old beam partakes of the same property, and is incapable of longitudinal expansion. Hence we have a beam possessing all the properties that it is most desirable to obtain —properties that no other bridge that I am aware of has; lightness, combined with great strength in both material and mechanism, and, above all, no tendency to thrust the masonry on which it rests out of its perpendicular position, which is the one great difficulty to be contended against in bridge building. In this the pressure on the piers will be directly downwards. As to durability, the nature of our timber approaches iron in that respect, when not attacked by white ants, and I never heard of an elevated bridge being so affected, for the animal cannot exist without moisture.

The principle involved in this bridge is, that the strain on every part that is affected in resisting the downward pressure of the platform acts on the longitudinal fibre of the timber, while the timber is so placed and trussed together that the whole forms one rigid beam, incapable of longitudinal expansion.

When this structure is placed upon its respective piers the whole pressure is directly downwards. There is no lateral tendency, consequently the action on the piers is directly on those points where they are capable of offering the greatest resistance. This, coupled with the lightness of the material, enables their being elevated to a degree that would be unsafe in any other description of bridge. We possess in this colony the finest timber arid stone in the world for such purposes, and in thus arranging them, I believe we should have a more efficient structure, than it is practicable to erect with any other material. As I have said, neither bricks nor stone are applicable to the construction of arches resting on such lofty piers as we require in this colony, from the great weight of these materials, and the lateral thrusts of such arches, and the utmost limit of iron bridges has already been arrived at, did not their great expense place them beyond our reach; but such timber as we possess is capable of almost unlimited powers, if handled with skill and judgment, and will enable us to cross most of our ravines and rivers without difficulty, for I see no reason against bridges of 150 to 200 feet span, and ot equal height, being efficiently carried out on this principle. As I consider this bridge will unlock the the southern country to railways, by enabling them to cross the intervening ravines and rivers, I have called it • The Key Bridge.'— Melbourne Paper.