sleeve, close by the drum, revolves a steel-faced ring having two arms, one of which carries a counterweight, and the other a roller. On the steel face is a groove lying in the tangent line, drawn through this roller to the circumference of the cable. The wire (which is No. 10 gauge galvanised) is led from the drum over the roller, and through this groove to the cable. The drum is then turned one way and the ring the other, which has the effect of winding the wire off the drum and on to the cable, the counterweight keeping it all the time under strong tension. The wire as it coils on the cable squeezes itself between the last coil and the sleeve, and so pushes the latter along the cable for just the distance required. As soon as the cable is wrapped it receives several coats of oil paint, and is then ready to take the superstructure. This is put on symmetrically to the towers, and simultaneously in the middle and side spans. After the greater part is attached the holdfasts of the saddles are removed, and the cables thus left free to assume their natural position of balance. W. R. B. Viaduct over the Douro. (Revista de Obras Publicas e Minas, Lisboa, vol vii., pp. 1-34, 57-89, 97–136.) The memoir presented to the Royal Portuguese Railway Company by MM. Eiffel et Cie., the designers and constructors of the viaduct across the river Douro, in the neighbourhood of Oporto, contains a minute description of the work then proposed to be carried out, and which has since been completed and opened for public traffic. The leading dimensions of the viaduct are, a total length of 1,158 feet from end to end of the abutments, and a height of 201 feet from low-water level to the upper surface of the rails. The central opening is 525 feet wide, and clears the river, which at the point of crossing was found to be deep, and the bottom of such a character as to make it advisable not to construct any piers upon it. An arched form was selected as the most suitable for the purpose, notwithstanding the unusually large span, which called for novel treatment and a special design. It became necessary, in the first place, to abandon the use of any spandril filling, the reasons given in the Paper being that the expansion of the various parts would tend to disturb the equilibrium, and would require a large quantity of metal, otherwise unnecessary, to be used to counteract its effects and insure the stability of the structure. The plan which suggested itself was to make the arch with sufficient stiffness to resist unaided any uneven distribution of load coming upon it, and this was secured by giving it the form of a crescent, having a depth of 32 81 feet at the crown and tapering gradually down towards the abutments, where it terminates upon pivots in the usual manner. The ribs of the arch are two in number and have the intrados and the extrados joined together by vertical pieces cross-braced; they are placed 49.21 feet apart from centre to centre at the springing line, and are inclined upwards towards each other, so that the width between them at the top becomes reduced to 12.96 feet; the whole is substantially tied together, by a system of cross-braced framing. The spreading out of the base is for the purpose of enabling the structure to resist the violence of the storms to which it may be exposed. On either side of the central, or river arch, the viaduct consists of continuous girders supported by piers composed of wrought-iron columns in clusters.1 On the south side there are two spans of 122.62 feet each between the centres of the bearings, and one of 120 16 feet from the centre of the bearing to the face of the abutment; on the north side there are two spans of 122 62 feet and 120 16 feet respectively. In the original design it was proposed that the longitudinal girders should be carried on the top of the arch, but this was afterwards modified, as it appeared advisable to reduce, as much as possible, the area exposed to the action of the wind. They were therefore lowered and placed so that the level of their upper surface should correspond with the extrados of the arch at the centre; in fact, the central portion of the girders, for a length of 170.21 feet, forms part of the arch. As the remaining distance from this to the piers on either side was too great to span conveniently, a support was placed upon the haunch of the arch to divide it into two equal spans of 94.33 feet. The continuous girders are of the single intersection lattice type, with a uniform depth of 11.48 feet; the total length of those on the south side is 557 34 feet, and on the north 434 72 feet. They are in couples, placed 10.17 feet apart, and carry the permanent way for a single line of railway, by cross girders on the top. The clear width between the parapets is 14.76 feet and the total length of the metallic superstructure 1,163 feet. The Paper gives the dimensions of the various plates and angle irons employed in the viaduct, and their general arrangement. The weight of the ironwork in the arch is stated to be 504 tons, and of the girders, supports, and permanent way over the arch 223 tons, making the total weight of the central opening 727 tons. The longitudinal girders and superstructure of the remaining portions of the viaduct weigh 26 cwt. per lineal yard. The rolling load was calculated not to exceed 72 cwt. per yard. The Paper concludes with an interesting series of tables, nineteen in number, showing the strains to which the arch will be subjected under varying conditions of loads, temperature, and action of wind, the calculations of which can only be properly appreciated by referring to the Paper, where they are given in great detail, G. KN. The piers as actually executed have been made of masonry. New York and Long Island Bridge. (Report of Board of Consulting Engineers, New York, 1877.) On May 1st, 1876, competitive designs were invited for a bridge for road and rail, upwards of 10,000 feet in total length, and including two large spans of 734 feet and 618 feet clear opening, and 135 feet high. In order to place the several designs on the same basis a full specification of the loads, strains, and general conditions accompanied the invitations to tender. Rolling Load. For the single line of railway a load of 5,000 lbs. per lineal foot for 15-feet spans and under, decreasing to 3,000 lbs. for a 60-feet span, and to 1,620 lbs. for an 800-feet span. For the roadway 75 lbs. per square foot for ordinary spans, and 50 lbs. for the two large spans. Wind and Temperature.-A wind pressure of 24 lbs. per square foot acting at right angles to the bridge and upon a train of cars 10 feet high; or a pressure of 40 lbs. per foot upon the unloaded bridge. Changes of temperature to the amount of 150° to be provided for. Tension Members.-A factor of safety of 3 for dead load, wind and changes of temperature, and of 8 for rolling loads, which for "double-rolled refined iron" is equivalent to a working strain of 8 tons per square inch when the load is wholly dead, 5 tons when half dead and half rolling, and 3 tons when wholly rolling. Compression Members. For wrought-iron square-ended pillars a strain of 8,000 lbs. per square inch up to a length of twenty-four times the least radius of gyration, and above that length the factor of safety to be as for tension members, and the ultimate resistance per square inch to be calculated by the formula in which I is the length and p the least radius of gyration. Eight sets of plans and estimates were submitted to the consulting engineers. The estimated cost of the 734-feet span ranged from 558,000 to 1,160,000 dollars, and the types of construction included the ordinary truss, hinged arches, suspension cables with stiffening girders, and the cantilever and central girder type sometimes known as the "continuous girder of varying depth." Of these it will be necessary to describe only the three premiated designs. (1) Messrs. Clarke, Reeves, and Co. submitted a design and tender for the bridge upon the hinged arch system of Captain J. B. Eads, M. Inst. C.E. The arch proper consists of two lenticular struts or girders resting against each other in the centre where they are hinged, and also hinged at the top of the piers. From the top of the pier to the rock, some hundred feet below, similar struts extend in continuation of the arch form. The main arches, from which the roadway is suspended, are of "Phoenix" or hollow columns of wrought iron 30 inches in diameter, and the lower members of plates and angles. The piers also are composed of "Phoenix" columns, and as they are designed to take only such strains as may come upon them from unequal loading of the arches and a portion of the strain of wind pressure, they are subject to both tensile and compressive strains, and are proposed to be anchored to the foundations. Messrs. Clarke and Co. would erect the arches by building them out panel by panel from the shore, suspending the same by inclined stays fastened to temporary towers at the piers and anchored back. For the single-line bridge the amount of their tender was 1,767,274 dollars, of which sum the 734-feet span constituted 601,910 dollars, and the 618-feet span 437,107 dollars. (2) The Delaware Bridge Company tendered for the work and submitted a design of considerable merit, in which the main spans were of the cantilever and central girder type. Each cantilever or bracket is divided by two intermediate chords, parallel to the inclined top chord, into three subsidiary brackets superimposed to each other, and 36 feet deep. By thus avoiding the necessity of carrying all the weight to the top of the central tower over the pier, the designer claims that there results great economy in the towers and in the long compression members of the web. In erecting the bridge the two cantilevers would be built out from the piers, and the 200-feet central girder would be built on a wooden truss 300 feet long, counterbalanced and rolled across the space intervening between the ends of the cantilevers. There was little difference between the tenders for this and the preceding design, the total amount being 1,778,315 dollars, of which 561,868 dollars were for the 734-feet span, and 421,550 dollars for the 618-feet span. (3) Messrs. Henry Flad and Co. furnished a design and estimate for a work somewhat similar in general principles to the last, but the cantilevers in this instance consisted of a number of long straight suspension links extending from the top of the towers to equidistant points along the roadway. Sagging of the links is prevented by a system of circular arc bracings connecting the several parts. No tender was submitted for a single-line bridge, but for a double line the estimated cost was 851,949 dollars for the 734-feet span, and 691,289 dollars for the 618-feet span, amounts about 8 per cent. in excess of the Delaware Company's tender for the same work. B. B. [1877-78. N.S.] X On Surveying for Railways in Sumatra. By J. L. CLUYSENAER. (Tijdschrift van het Koninklijk Instituut van Ingenieurs, 1876-77, pp. 73–86). The Author considers two methods specially worthy of consideration. The one commences with a study of existing maps, from which a certain direction is chosen and levelled. With the other method, after a careful examination of the ground, a direction is assumed for the general survey, from which a strip of country can be defined for detail survey. The former method was followed by the French engineers in prospecting for the line from Constantinople to Adrianople, of which country scarcely any, even the roughest, maps existed. Two observers with aneroids, set out on horseback for Adrianople, the one direct, the other exploring the cross valleys. At distances of 100 yards altitudes were taken, while the standard barometer at Pera was read from hour to hour, and the route was fixed by cross bearings to important points such as mountain-tops, minarets, &c. In this way 25 to 30 miles per day could be accomplished, whilst the altitudes showed a maximum error of 6 feet when compared with subsequent spirit-levelling. On the basis of this survey the direction was chiefly decided upon. After these observers followed a person with an aneroid, who marked out a practicable route by small flags. After him followed a theodolite party; 2 miles behind them a measuring party drove stakes at every 30 yards, sketched the country, the breadth of the roads, streams, &c., and were followed closely by a spirit-leveller, who took cross sections in addition to a longitudinal section. In this way a progress of from 3 to 6 miles per day was made. From the resulting plan the final direction was determined, and a survey made in the usual way, with repeating circle, level compass, and aneroid, curves having been set out. A measuring party drove stakes at every 10 yards, whilst another took cross sections, and the rear was brought up by a spirit-leveller. The average day's work was from 1,600 to 2,200 yards, and is said to have cost only £32 per mile (500 francs per kilomètre). After describing the methods given by Oppermann,1 Hoffmann,2 and Bösch, the Author gives a geographical description of the island of Sumatra, and proceeds to detail the method followed in that island, the object of the survey being to determine the direction of railways and roads for the conveyance of coal from the coalfields of the Padanj Highlands to the West Coast. Each party consisted of a chief engineer, a second engineer, and 1 Oppermann, C. A., "Traité complet des Chemins de Fer économiques.” Paris: Dunod, 1873; pp. 12-34. 2Ueber Tracirung von Eisenbahnen in offenem und coupirtem Terrain," by F. Hoffmann, in Allgemeine Bauzeitung,' Wien, 1870, p. 59. 3 L. Bosch, "Topographische Aufnahme für Vorarbeiten in Eisenbahnen," in "Zeitschrift des Architekten- und Ingenieur-Vereins zu Hannover,' 1872. |