1842.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



36.3 



•finest ill existence, the same defect exists ; also in the viaduct of the 

 Columbia and Philadelphia Railroad, near Philadelphia, and many- 

 others, built by the most talented and experienced bridge builders in 

 the country. 



-But in all these bridges, as well as in this aqueduct, the inner ends 

 of none of these long straining pieces are in contact with the heads of 

 the adjoining posts, against which they were intended to exert a pow- 

 erful compression. Consequently, they are not only useless, but posi- 

 tively injurious, as they add unnecessarily to the weight of the truss, 

 and thus absolutely increase that tendency to settle, which they are 

 intended to prevent. 



I noticed a splintering, or spalling off", of the stones supporting the 

 feet of some of the arches. The stones in this part of a bridge, as 

 well as those forming the facing of the starlings, should always be of 

 good quality ; and, in the former case especially, attention should be 

 paid to their toughness; soft sandstone should never be admitted. 



In an extreme case like this, of such an immense weight and so 

 soft a stone, or indeed with stone of the best quality, I should cer- 

 tainly prefer to have the recesses in the top of the pier for receiving 

 the feet of the curved ribs and pier posts, somewhat deeper than in 

 this instance, where they are but 8 in. deep, figs. 1, 2, and 6. The 

 pressure on the piers and abutments, so long as the bridge maintains 

 itself, is almost altogether vertical, and as it sometimes happens that 

 the bearing is not very fair, every precaution should be used to pre- 

 vent spalling. One of the spans, at the time I saw the aqueduct, 

 ■was (in my opinion,) in imminent danger of falling, in consequence of 

 this. 



I am confident that 2 trusses, instead of 4, would have been suffi- 

 cient to support the trunk of this aqueduct, especially if another 

 arch-piece had been added to the depth of the curved ribs, and the 

 height of the trusses made a little greater. This additional height 

 would, moreover, have added to the convenience of passengers on the 

 roofs of the boats, who are now obliged to stoop in passing through 

 the aqueduct. As the use of but two trusses would, of course, in- 

 crease the clear width between them, to admit the tow-path, the gir- 

 ders might be trussed by a heavy arch-piece, which would insure 

 abundant strength. A considerable diminution of expense would at- 

 tend such an arrangement. 



As to the piers of this aqueduct, they are, as before stated, but 7 

 ft. thick on top, or -rrr'Ta P^f' °^ the span ; or not quite i as great as 

 the proportion of those used in the Trenton bridge, which is the 

 boldest one cited by Tredgold, in his table of stone piers for wooden 

 bridges. 



The foundations of the piers of the aqueduct do not appear to have 

 settled in the least, or to have undergone any derangement ; still, even 

 in the face of this precedent, I should certainly prefer the foundation 

 to be laid at the depth of a few feet below the bottom, in structures 

 so important as this. There are, however, several bridges across the 

 Alleghany, in the vicinity of the aqueduct, and all their piers are 

 founded in this manner, (some of them, indeed, having but one course 

 of timbers in their platforms,) which is strong evidence of the suffi- 

 ciency of the plan, on a gravel bottom, even when exposed to tremen- 

 dous freshets. 



Viewed as a whole, this aqueduct, for which, I am under the im- 

 pression, there was no precedent, certainly reflects the highest credit 

 on Mr. Lothrop, for boldness and mechanical skill.* There are others, 



* We think it proper to remark here, that in the 6th volume of the London 

 Repertory of Arts, p. 220, is detailed at length, the sijecification of a patent 

 taken out in England, in 1796, by Mr. James Jordan, for constriicliiig aque- 

 ducts and bridges, with curved ribs of timber, or iron, and suspending there- 

 frum, by iron rods, the floor or trunk, as the case may be. The colebrated 

 bridge over the Delavvaic, at Trenton, commenced in 1804, is, in its general 

 outline, almost a precise copy of the plate illustrating Jordan's patent, in 

 the Repertory of Arts, and is, in principle, unquestionably an infringement 

 of that patent. Mr. Jordan also gives a plate of a projected aqueduct, pre- 

 cisely the same in its essential principles as that which forms the subject of 

 the'abovc paper. 



of much the same kind, on the canal ; all, I believe, designed by him. 

 The general arrangement of the timbers is not original with him, 

 having been long before practised, in very many instances, in common 

 bridges; but the application of it to the purposes of so extensive an 

 aqueduct, was certainly a very bold step ; aud its entire success is 

 proof of an intimate knowledge of what he undertook. 



I look upon this arrangement of timbers as the best yet devised for 

 large spans. It certainly admits of many improvements in its details, 

 and frotn some experiments of my own on the subject, I am under 

 the impression that considerable modifications in some of the more 

 important parts, might be made with advantage. I may take occa- 

 sion, in some future paper, to allude to them. 



The curved rib, it is well known, is stronger in the centre than at 

 any other point; or, in other words, a load, which, applied between 

 the centre and one of the piers, would destroy the rib, might be sup- 

 ported, with perfect safety, in its centre. But the truss which is con- 

 nected with the ribs, is weaker in the centre than at any other point ; 

 its resistance to a load acting at any point, being, as in the case of a 

 single piece of timber, proportional to the rectangle of the distances 

 from the point at which the load is applied to the points of support. 

 Therefore, the combination of the curved rib with the truss, secures a 

 more uniform degree of strength throughout the whole span, than 

 could be attained by either one, used separately. 



But besides this, another very important consideration attends the 

 combination of the curved rib with the truss, viz : that each not only 

 contributes its own share to the support of the load, but actually in- 

 creases the power of resistance of the other — that is, the two com- 

 bined will support a greater load than they could separately, were the 

 load divided between them. A curved rib, when employed by itself, 

 is very weak at the haunches, and readily yields to a load applied 

 there ; but if proper means be adopted for preventing the rib from 

 changing its form, its strength is wonderfully increased, indeed to such 

 an extent that actual crushing of the timber must take place before 

 the rib will yield to its load. Without this precaution, its flexibility 

 will permit it to bend, and fall through between its abutments, under 

 a load many times less than that necessary to crush it. Such a change 

 of form, or bending, is prevented by the truss, and thereby so great an 

 accession of strength is imparted to the rib, that if we could con- 

 ceive the truss acting only in this capacity of a stiffener to the 

 rib, without itself sustaining any portion of the load, still the strength 

 of the bridge would be increased many fold. I have seen curved 

 ribs of 200 ft. span, bend, and fall into the river, between their abut- 

 ments. 



The highest known freshet of the Alleghany, rose to about the floor 

 line of the canal trunk; the weather boarding of the outside formed 

 a kind of dam, against which, trees, barns, houses, &c., accumulated, 

 until they formed a wide field of drift on its upper side. A large 

 concourse of people stood on the banks of the river, expecting to see 

 the whole structure lifted oflT from its piers, and floated away; but it 

 stood perfectly firm, and I believe sustained no injury whatever. 



Engineermg Science. — The tunnel on the line of the Slieflield and Man- 

 chester Railway will be 3 miles in length, upwards of 600 ft. below the sur- 

 face or summit of the hill at its greatest height, and in rock formation 

 throughout its entire length. The works were projected and commr-nced 

 upwards of 2 years ago, under the direction of Charles Vignoles, Esq. Five 

 shafts were opened, at about half a mile distant from each other, for the pur- 

 pose of proving the formation, of facilitating the driving of the drift-ways, 

 and ultimately, of ventilating the tunnel. Whilst these were in progress, 

 the drift-w ays were carried on from each side, or face, of the mountain : the 

 distance, or length, driven, on the eastern side, extending to nearly 1,000 

 yards, and from the next shaft 180 yards. The junction between these two 

 portions of the drift-way was etfectcd on the 17th Sept., and the levels, w hen 

 checked, on a tie-bench, at the point of meeting, had varied but i) decimals, 

 or 1 in. nearly, and the range was within less than 2 in. of being geometri- 

 cally true*:- IVeekhj Papers. 



3 E 2 



