J 843.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



361 



THE PENNSYLVANIA CANAL AQUEDUCT, 

 (With an Engraving, Plate XIII.) 



(FROM THE AMERICAN JOURNAL OF THE FRANKLIN INSTITUTE.) 



Description of the Jl'ooden J(jueihwt carrying the Pennsylvania Canal across 

 the rieer Alleghany, at Pittsburg ; from actual meamrement. With 

 remarlcs by JohnC.Trautwine, Civil Engineer, of Athens, Tennessee, U.S. 



This aqueduct was built in the year 1829, by Mr. Lotlirop, of Pitts- 

 burg, at an expense to the State of 101,000 dollars. It has two abut- 

 ments and six piers of stone, and seven arches of timber of 130 feet clear 

 span each ; the whole length of the aqueduct between the abutments 

 being 1092 feet. The masonry is of cut ashlar, in large courses, with 

 rubble filling. The material is a gray sandstone, rather too soft, in 

 my opinion, for such parts of a structure as are exposed to rapid cur- 

 rents, bringing down heavy fields of ice and drift wood, as is the case 

 with the Alleghany. The piers are 7 feet thick on top ; they batter 

 one inch to a foot on the sides, are semi-circular at their down-stream 

 ends, and are provided with break-water starlings. The average 

 height of the piers is about 40 feet. 



The timber of the aqueduct is white pine, with the exception of 

 the chords and the pier-posts, which are of white oak, as being better 

 calculated to resist the great strains that come upon them. I conceive, 

 however, that any precaution of this kind would apply with more force 

 to the pole plates, or cap- pieces, as it is well known that t!ie resist- 

 ance of timber to compression is much less than that to extension. 

 Therefore, if extra precautions are requisite for the chords, they are 

 doubly so for the poles. 



The width of the aqueduct, from out to out of weather boarding, is 

 about 34 feet. The height of the trusses, from bottom of chords to 

 top of poles, 16 feet. The canal trunk has a top width of 1(5 feel, a 

 bottom width of 15 feet, and a depth of 5 feet. The depth of water 

 is generally 4 ft., sometimes 4 ft. 3 in. There are four trusses to each 

 span. These particulars, however, with the other principal dimen- 

 sions, are so plainly indicated by the drawings as to require no farther 

 explanation. Throughout the drawings, the same letters refer to the 

 same parts. 



Scantlings of the Principal Timbers and Irons. 



Arch pieces, 7x 14 in., L, six to each truss, or composing each rib. 



Pole plate or cap, J, of inner trusses, 10 X 15 in. 



Ditto, F, of outer trusses, 10 x 12 in. 



Chords, C, of both inner and outer trusses, each in two pieces, each 

 piece 7 X 15 in. 



Queen posts, Q, in body, 9 x 10 in. 



Ditto, in heads and feet, 10 X 14 in. 



Main braces, S, 8 X 10 in. 



Counter braces, X, 8 X 4-1 in. 



Butting pieces, P, at heads of queen posts of outer trusses, 5x 7 in. 



Ditto, T, or straining pieces at heads of queen posts of inner 

 trusses, 5 x 7 in. 



Diagonal braces, U, in roof, and under canal truck, G X 7 in. 



Extra braces, M, on outer trusses only, X 12 in. 



Straining sills to ditto, it, 6 X 10 in. 



Straining pieces to ditto,y, X 12 in. 



Straining sills to pier posts,_;', G x 10 in. 



Fishing pieces to poles, b, over piers, G x 12 in. 



Roof tie beams, E, 7 X 12 in. 



Floor girders, G & H, 9 x 18 in. 



Additional pieces, A, under girders marked G only, 9x15 in. 



Plank, of footway, towpath, and canal trunk, thickness 3 in. 



Weather boarding, N, thickness 3 in. 



Iron suspending rods, R, IJ in. diam. 



Screw bolts, 1 in. square; spikes for planking of canal trunk, i incli 

 square ; spikes at heads and feet of braces, % in. square. 



Vol. v.— No. C2.— November, 1842. 



In Fig. 1 it will be seen that the tops of the lower girders are not 

 all in the same horizontal line, but that every other one, H, is raised 

 about one foot above the intermediate ones, G. When the aqueduct 

 was first built, only one of these sets of girders was employed, being 

 supposed sufficient to sustain the weight of water in the trunk. Their 

 distance apart was about ten feet in the clear. But when the water 

 was let into the trunk, the girders began to yield under a head of about 

 three feet, and it was found necessary to double their number, and 

 moreover to bolt, or rather stirrup, to every alternate one, viz. those 

 marked G, an additional under girder. A, Figs. 2, 3, and 4. 



The reason for thus alternately raising and depressing these lower 

 girders was, to permit the passage of the two courses of lower diago- 

 nal braces from one H to another, over G, (see Fig. 9, and also the 

 dotted lines a, a, a, in Fig. 3). Where the H's cross the inner chords, 

 (which are lower than the outer ones,) they are made to bear on them, 

 by means of the blocks /(, li, Fig. 3. These blocks are not shown in 

 the transverse section, it being supposed to be taken near one of the 

 girders, G. Over the girders, G, are seen (Fig. 3,) blocks O, O, 

 marked in dotted lines ; there are transverse timbers, resting on G, 

 and helping to support the flooring plank of the canal trunk, as there 

 are no longitudinal joists in the aqueduct. The pieces O, do not in- 

 terfere with the diagonal braces, merely running from each inner 

 chord to the braces. 



The floor of the trunk rests only on the upper course of diagonal 

 braces, on the transverse pieces, O, and on the raised girders H. 



Each girder, H and G, is sustained by the two inner chords, and by 

 four suspending rods of iron, li in. in diameter, of which two are 

 shown in Fig. 4, by R,R. In the outer trusses, the tops of these rods 

 bear on the tops of the arches, or curved ribs, (see Fig. 1,) by means 

 of wooden saddle pieces, D, Figs. 3 and 4. In the inner trusses, they 

 bear upon the tops of the poles, or caps, and are covered by a 

 longitudinal capping piece, of 3 in. thickness {m, Fig. 4); this 

 capping is rounded otTat top, so as to prevent the chafing of the tow- 

 rope. 



To bring the bearing of the suspending rods more equally on all 

 the arch-pieces, blocks are inserted between the arch-pieces, ver- 

 tically, at the place of each rod, as shown in Fig. 3. M, M, in Fig. 1, 

 are extra braces, employed only in the outer trusses; A, A, are their 

 straining sills. At their upper ends they abut against short pieces of 

 G X 12 (/,/,) firmly bolted to the poles. These extra braces are in 

 pairs, being on both sides of the poles. 



j,j, Fig. 1, are the straining sills for the pier braces, S, S, serving to 

 stiflTen the pier posts ; see Fig. G, which also shows the cast iron abut- 

 ment plates, g, g, for receiving the feet of the curved ribs. They are 

 merely flat plates, 3 ft. 8 in. deep, 2 ft. wide, 2 in. thick, and without 

 any flaunches. The recess in the pieces, for receiving the feet of the 

 ribs, and pier posts, is but eight inches deep. See also Fig. 2. 



P, Figs. 3 and 4 are the short butting-pieces for relieving the heads 

 of the queen posts from the action of the main braces. In the inner 

 trusses, instead of these short butting-pieces, long straining-pieces, 

 reaching from one post to another, are used, T, Figs. 3 and 4. T is 

 represented in Fig. 3, to save the trouble of another engraving. 



The suspending rods are about 5 feet apart; but it is seen in Fig. 1 

 that these rods stop short within some fifteen feet of the piers, because 

 the curve of the ribs in that interval would not allow of the passage of 

 girders past them. In this interval, therefore, the ends of the giiders 

 are supported by pieces, K, Fig. 7, bolted to the ribs. 



s, s. Figs. 2 and 4, are uprights, placed 4 ft. apart, for spiking the 

 sideplankingof the canal trunk to; at their heads they tenon into a 

 continuous cap-piece, r, and at their feet they tenon between the 

 strings e and n. v, and the blocks /, being supported by the queens, 

 prevent s from spreading outwards. 



Where the curve of the arches comes below the line of /, the latter 

 is dispensed with, and v rests against the arches themselves. 



r, r, are two pieces, of 3 X 6 in- spiked to the queens, for receiving 



3 £ 



