1840.] 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



125 



— we shall be at no loss to account for tlie present condition of Slate works 

 in general. The dctieieneies of this year in Pennsylvania alone, are estiniateil 

 at 11 millions of dollars, and except the Erie canal, there is not a govern- 

 ment work in the Union which has paiil the ordinary expenses, inclnding of 

 course, interest on cost. If the Erie canal lie placeil on the footing of the 

 canals of Pennsylvania, that is if its exclusive right to carry all the freight to 

 and from western New Voik, the western States ami Upper Canada he abo- 

 lished, the gross income of the canals of Xew York will bear a less propoition 

 to the exiienditnres, than does the revenue of the public works of Pennsyl- 

 vania to the annual outlays on the internal imiirovements of that common- 

 wealth. Nntwithstaniling their financial embarrassments, wc are happy to 

 say, that mi other State in the Union has resorted to this mode of giving a 

 " delusivi' |irosi>erity " to their public works, and there is some reason to 

 believe that the long reign of " exclusite privileges " in this State is about to 

 close. My making innnediate arrangements for retiring from the construc- 

 tion of canals, the State of .\ew York may yet escape with trilling loss, and 

 with this object in view, the people would readily submit to the present mo- 

 impoly of freight from the north and west for a few years longer. This ap- 

 [lears to us the most judicions course to be pursued in order to avoid a jier- 

 manent debt, and it certainly offers an honourable retreat from a position in 

 which it is daily becoming more dirticult to maintain ourselves. 



The great efforts which have been niaile by the inhabitants on (be line of 

 the New York and Erie railroad, under the most discouraging circmnstances, 

 to aiil in the constrnetion of that undertaking, show that private enterpri/.e 

 is not yet extinguished in this State, and we have to record the astonishing 

 and gratifying circumstance, that — notwithstanding the different State go- 

 vernments have made every exertion to absorb all the spare capital of this 

 country and of Europe for their own Utopian schemes, the year 18S9 has 

 seen more works eompleled by cumjianie.t than by .'■f/afes. Private energy 

 and entcr])rise have succeeded wliere the power of government has been 

 unequal to the task, ami while the star of " free trade " floats triumphantly 

 on the banners of the Bay State, and indeed thronghont New England, we 

 will not despair of seenig, in the Empire State, railways as judiciously pro- 

 jected, as well constructed, as profitable to the proprietors, and as useful to 

 the public, as those of .Massachusetts, when they shall he left equally free to 

 the people of the former as thev alwavs have been to those of the latter 

 State. 



CN TRELLl.S BRIDGES. 



(n',//i an Engraving, Plah: VIII. j 



We ;irp princi|)iilly iiulebtcil fur this paper to n communication of 

 Mr. Moncure Robinson in the Rcrue Gent rait di I' ..-Irchiteclare. 



Mr. Itliiel Town of Xew Haven, an architect at Xew York invented 

 a bridge of a pecnliar construction wliicli lias nnicli tlie appearance of 

 a bridge invented liere by the Kite Mr. Smart. The principle is one 

 wliidi has been adopted by Seppiiigs in naval arcliiteclnre Tlie 

 advantages attributed^ to it are tliat bridges willi openings of con- 

 siderable spaa may be erected with small pieces of wood. These 

 bridges are bnilt on piers far apart and formed of a truss, if it may 

 be so termed, of continuous trellis work, m.ide of planks, double or 

 treble, 10 or 12 inches wide, and 3 to 3i inches thick, placed parallel 

 to each other at an angle of about 45o to the horizon, crossing nearly 

 at right angles, and alternating from right to left. The angle at which 

 the trellises cross is not strictly a right angle, for the interstices form a 

 kind of lozenge, which if three feet long would be about 2 feet inches 

 broail, which are about the general ilimensions. At the crossings the 

 planks are secured with pins. The bolloin of tlie trellis work is 

 strengthened on eacli side by string pieces running from one end of 

 the bridge to the other, and made also of small pieces of timber 12 

 inches by 3, in lengths from 35 to 10 feet. The siring pieces on each 

 side of the trellis work are double, so tliat each trellis is secured by 

 four pieces of timber, six inches thick on each side of the trellis. The 

 joints being equally distributed throughout the length of the siring piece. 

 At the top of the trellis is a similar string piece running in the same 

 manner the whole length. On the lower strings are placed the trans- 

 verse beams which carry the timbers of the Hoor. The njiiier string 

 piece in the bridges wliicli were first constructed carried tlie roof, 



The trellis work is secured at the crossings bv pins of sound oak, 

 an inch and a quarter thick, carefully turned on a mandrel. These 

 pins lit neatly into holes previously liored. They are f.irther secured 

 by a weilgelike pin driven into their centres on each side. This latter 

 precaution is however only adopted on the more expensive bridges. 

 These pins are two in number at the crossings, and four at the string 

 pieces, they are the only means of securing the timbers to eacli 

 other, as they are too thin to admit of framing. The only iron work 

 in the whole bridge amounts to no more than a few nails and pins used 

 in some of the joints. 



Such was Mr. Town's original plan, and we shall now proceed to 

 describe the improvements which have been subsequently introduced. 

 It is evident that on this system the timbers of the floor may be laid 



either on the upper or lower part of the trellis work. By laying them 

 on the lower part, tlie sides and roof may be more readily completed, 

 but the other plan, which has been preferred for railways in the United 

 ■ States, admits of the carpentry being strengthened by horizontal and 

 vertical braces, and gives additional securitv to the bridge. The 

 ordinary wooden bridgi's, called in America Burr's bridges, after a 

 carpenter of that name who imiu'oved them, are so elastic that the 

 trains can only pass over them very slowly, while on good trellis bridges, 

 particularly those made by Mr. Robinson, locomotives can run at full 

 speed, a great advantage with regard lo railways. 



The height of the trellis depends on the strength required in the 

 bridge, and necessarily increases with the opening or span. For ex- 

 tensive works where 2lio feet span is required, the trellis is made 17 

 or IS feet higli. Mr. Town recommends that in most cases the height 

 of the trellis should be a tenth or twelfth of the span. When the 

 flooring rests on the string piece the height of the carriages will not 

 admit of (he trellis being less than 13 or 14 feet. Some, of these 

 bridges have been built of 220 feet span. 



Throughout the timbering the two lines which present the greatest 

 resistance are directed, one, following a horizontal right line lead by 

 the lower extremity of the timbering; the other, following a curved 

 ari'h, which rests by its two extremities on this riglit line. The trellis 

 bridge has great strength at its base on account of the siring piece 

 formed of four pieces secured two and two; but it is not so strong 

 along the upper curve described by the iileal line of the greatest 

 thrust. The more the trellis is raised, the more tlie upper string 

 piece, wdiicli strenglliens the timbering, dilTers from this ideal line, 

 it has therefore lieeii observed that trellis briilges of l.xige span are 

 apt to settle; and once bent, they lose mueli of their strength. Mr. 

 Town jiroposed several ways of reiuedyiiig this inconvenience. To 

 increase the resistance of the trellis, it may be doubled on each side of 

 (he bridge; (his .\lr. Town has tried, separ.iting the pieces of which 

 it is composed, so tll.it (he horizontal diagonal of the lozenge between 

 four adjacent trusses should be four feet six inches, iustearl of three 

 feet, 'i'liis increases the cost of (he wood of each side of the bridge 

 .'iO per cent., but on two-way bridges Mr. Town gets rid of the trellis 

 work which he used to place between the two-ways, the quantity of 

 wood remains (he same. The string piece may be slreulhened by 

 re]ieating it at the crossing immediately above the pieces of the trellis. 

 In the bridge at Richmond these two methods of strengthening the 

 timbering have both been used. By laying the flooring on the (op of 

 the timbering, and by having open bridges, as previously observed, a 

 means of ]n-eveu(ing the settlement is obtained, by interior braces. 

 Besides the weight of the roofing is got rid of, whicli is of little good 

 for railways, where it would be more likely to catch fire from sparks. 

 This danger is partiiailarly to be feared in America, where wood is 

 burned by the locomotives, and so more dangerous sparks are produced 

 than from coke. 



Trellis bridges are of the greatest use in the United States, because 

 being formed of thin planking, they can be built in a short time. Thus, 

 for instance, the viaduct by which (he Philadelphia and Norristown 

 Railway is carried over the Wissahiccon, Ts feet above tlie bed of tlie 

 stream, and -183 feet long in three spans, was built in (5S days. For 

 the same reason the wood required for the trellis work, being easily 

 conveved, costs less, in many cases, than that required by any other 

 kind of wooden bridge. On (he Pottsville and .Siinbiiry Railway, in 

 Pennsylvania, the wood for small truss bridges, for crossing roads is 

 12 dollars per lOUU feet super and inch thick, which is equivalent to 

 two loads of timber in England. That for trellis work costs only S-4 

 dollars. 



These bridges are formed of pieces all exactly on (he same model 

 and of (he simplest form, so (hat all the trusses for' (he trellis work may 

 be easily cut with exactness by ordinary nieidianical means, and the 

 holes Ijureil for receiving (he |)ins. The beams not being a( all arched, 

 but flat, it follows tli.it the jiiers are not subject ed to the later. il thrust, 

 to which they are exposed in other bridges; and they only require a 

 thickness necessary (o resist the vertical pressure represen(ed by the 

 weight of the bridge. 



Trellis briilges are very much increasing in use in the United States ; 

 a dozen yi'ars ago, one of 2200 feet length in ten spans was built over 

 the Susquehannah, at Clarke's Ferry, near Duncan's Island, just above 

 Harrisburg, (he capKal of Pennsylv.tnia. One of 1530 long was built 

 over (he Hudson, at Troy, in the State of Xew Yiu'k, for (be railway 

 from Troy to Ballston Spa. The cliief spans are 180 feet. It is divided 

 into two' ways, eacli 1.") feet wide, anil separated by an additional 

 (rellis. It is made with the flooring at the bottom, and double trellises 

 at eaidi side. In 1S35, others of gre.it span existed at Xeubury Port, 

 Northampton and Springfield, all in MassachusseUs, at Tusc.iloosa in 

 Alabama, at Providence, &c., and since then many more have been 

 built. 



