JOURNAL OF THE FRANKLIN INSTITUTE OF THE State of Pennsylvania, DEVOTED TO THE MECHANIO ARTS, MANUFACTURES, GENERAL SCIENCE, AND THE RECORDING OF AMERICAN AND OTHER PATENTED INVENTIONS. AUGUST, 1834. FOR THE JOURNAL OF THE FRANKLIN INSTITUTE. Description of the Viaduct near Peters' Island. By JOHN C. TRAUTWINE, Architect and Engineer, Philadelphia. THE SITE. This extensive structure, upon which the Columbia and Philadelphia rail-road crosses the river Schuylkill, is situated three miles above the latter city, at the property of the late Judge Peters, on the western, and that of Isaac C. Jones, Esq. on the eastern side. At the site of the bridge, and for some distance above it, the stream preserves an average width of 850 feet, but widens considerably immediately below it. The western shore rises very gradually from the water's edge for about 150 yards, until it arrives at the foot of the high elevation which forms that slope of the river valley, and which the railroad overcomes by means of an inclined plane, and a stationary engine of sixty horse power. On that side, the depth of water under the bridge is only four or five feet, but it increases gradually to within twenty yards of the opposite shore, where it is twenty-two feet. From this latter point the bottom rises very abruptly into the high and precipitous rocky bluff of the eastern slope of the valley. The bed of the river is a soft black mud, overlaying the solid rock to a depth of from four to ten feet. VOL. XIV.-No. 2.-AUGUST, 1834. 10 COFFER-DAMS AND MASONRY. The bridge consists of seven arches, six piers, and two abutments. Five of the arches span 138 feet each; and the remaining two 125 feet each, making with the piers, which are thirteen feet broad on top, a total of one thousand and eighteen feet in the clear between the abutments. All the masonry is founded on the solid rock, with the exception of the western abutment and the western pier, both of which stand on dry land, and rest on a firm natural gravel. The founding of the water piers, and of the eastern abutment, whose face extends a little distance into the stream, was effected by means of coffer-dams, of a very simple construction, suited to their situation in slack water. They were framed one at a time on Peters' Island, (which is a small spot in the middle of the river, about one-eighth of a mile above the bridge,) and after being launched, and towed to their proper position, were well moored, and finally sunk by placing large stones on a temporary platform made for the purpose. The strains caused by the uneven bearings on the rock were in no case sufficient either to rup P b b ture the frames, or even to throw them so far out of line as to create much difficulty in driving the piles. Their entire length, including an up-stream salient angle, was about eighty feet; their breadth thirty-four feet. They were composed of a single row of uprights u, one being placed at each of the five angles, and others at intervals of fifteen feet, along both sides, but none at the ends. Transverse beams, b, about thirty feet long, extending from side to side, were notched into the uprights, and secured to them by iron dogs. In order to resist the increased pressure of the water towards the bottom, the vertical distances between these beams decreased gradually from three feet near the surface, to eighteen inches at the rock; they were removed as the piers were built up, and their places supplied by short struts abutting against the masonry. Outside of the uprights, and in number corresponding to the transverse timbers, were spiked rows of horizontal beams, o o, &c., surrounding the whole dam: over these, the sheeting piles, p, of three inch pine and hemlock plank, were driven, until their feet rested on the rock, and their heads remained about eighteen inches above high water mark. This operation was performed with perfect ease, the softness of the mud rendering necessary no other instrument than a heavy wooden mallet worked by two men. The pile-planks were properly directed in their descent by a horizontal leading beam, g, at the top of the frame, and were kept close together at their feet by the usual method of sloping one end, thus With these precautions they drove kindly, and but little trouble was afterwards experienced from leakage. Gravel was thrown in around the dam, and permitted to form its natural slope, until it reached to within one or two feet of the surface of the water. Slight as the construction of these dams appeared, the result proved that they were all sufficiently strong for the purpose, except that of the eastern pier, which, being unable to resist the pressure of a twenty-two feet head, (or if we include the semi-fluid mud, one of a twentyeight feet,) failed in consequence, and was afterwards repaired with additional precautions. As each coffer-dam was sunk, the water was taken out by six or eight common pumps, worked by a steam engine of six horse power, floated on a scow; and the rock, after being cleared of the mud (which was raised in buckets, partly by windlass, and partly by handing it up successive stages,) was properly levelled and stepped off for receiving the foundations of the piers. The piers at high water mark are sixty feet long, exclusive of the triangular pier-heads, or starlings, and batter both below and above water at three-fourths of an inch to a foot, as high as the skew-back, from which spring the curved ribs of the superstructure. From the skew-back they rise vertically to the level of the chord pieces C, which rest on them. Their height above water is thirty-five feet, and their breadth at high water twenty feet. The abutments are thirteen feet thick at their base, and batter externally three-fourths of an inch to a foot, that their faces may accord with those of the piers; internally they are vertical. The western wings form circular segments to a radius of fifty feet, their chord being seventy-nine feet. They are nine feet thick at the face of the abutments, and six feet at their extremities; they finish off by two flights of steps twenty-four feet high; like the abutments they batter three-fourths of an inch to a foot outside, and are vertical inside; they are surmounted by a coped parapet wall two feet thick. The exterior masonry of both piers and abutments is a very neat hammer-dressed rangework, forming a system of alternate headers and stretchers, of which no course is less than twelve inches in thickness, nor any header less than three feet in length. The back-joints, both vertical and horizontal, are at least six inches in width, and are neatly pointed above high water, below which they are laid in Roman cement, extending eight inches back from the face. The material employed is a handsome, compact, gray, gneiss rock, from the extensive quarries at the Falls of Schuylkill, distant two miles above the bridge. The interior is of rough rubble, varying in size from eight or ten cubic feet, down to a few inches; laid in full mortar, and the interstices completely filled with liquid grout, which was run in as the work was raised, at every twelve or eighteen inches. The filling in is principally from quarries of a very compact black gneiss, opened for the purpose on the eastern edge of the river, at the immediate site of the bridge. No cramps or chains are inserted into any part of the masonry, it being considered that the large dimensions of the face stones, and the accuracy of their joints, rendered such precautions entirely superfluous. For handling the stones a long pole was planted in each pier near its foundation, and supported a sliding crane, which was moved upwards as the masonry proceeded, and finally was lifted over the top of the pole when the pier was completed, the pole itself being left enclosed in the stone work. The expense of removing the coffer-dams being considered greater than the value of the materials composing them, they were permitted to remain round the piers. The total amount of masonry in the bridge is 19200 perches. THE SUPERSTRUCTURE. The putting up of the timber work, which had been already prepared on the ground, was commenced at the western end of the bridge; the abutment and piers on that side having been finished some time in advance of the others. For this purpose a temporary scaffold was erected, to support the wooden superstructure; it consisted merely of slight trestles, varying from twenty-five to sixty feet in height; they were composed principally of ten by twelve timbers, strengthened by a very few transverse longitudinal braces, and placed twenty feet apart. Their legs were sunk into the mud of the river until their feet touched the rock. Notwithstanding the apparent lightness and insecurity of this scaffold, it not only sufficed to sustain the weight of the platform, but also resisted, effectually, the force of several considerable freshets, nor did any accident occur upon it from want of strength. On the upper or transverse beams of these trestles were laid longitudinal timbers, extending from one to another, throughout the entire length of the scaffold: they supported the adjusting or raising blocks, which were merely short rough pieces of timber placed transversely on top of each other to rectify any inequalities arising from the uneven bearings of the feet of the trestles. Resting on the blocks, and also on the piers, were laid the chord pieces C, to which were applied and attached, in order, the queen posts Q, poles P, braces R, and the curved ribs T T; after which the scaffold was removed. The settlement consequent on this latter operation, did not amount to half an inch in any arch. The straining beams V and the straining sills U, together with other secondary timbers, were not introduced until after the removal of the scaffold. The bridge was originally intended for the accommodation of the rail-road traffic alone; but in anticipation of a proposed turnpike to cross at the same place, it received an additional breadth sufficient for the purposes of both, at the same time admitting of a foot passage four feet in width between the two. The entire breadth, from out to out of the queen posts, is now forty-nine feet eight inches. |