A RIVETED PLATE-CHORD.

CXVII. May be formed of flat plates as long as may be conveniently managed, connected by splicing plates of a little more than half the thickness of the chord plates, one upon each side, riveted or bolted with such a distribution of rivets, &c., as may not weaken the plates by more than the width of one rivet hole.

The area of rivet section should be at least to as great as the net section of the chord plate, on each side of the joint; and, go, Fig. 341 denoting the splicing plate, the distance cd, from the joint to the centre of the first rivet hole, should be at least twice the diameter of the rivet (depending somewhat upon the size of rivet and thickness of plate, as well as the soundness of grain in the iron). The succeeding rivets, a, e, f, &c., should be placed alternately on opposite sides of the centre, so that the oblique distance ac (=O), may equal the transverse distance (=T), + the diameter of whole (H). Then, representing the longitudinal distance bc, by L, we have T+HO, and (T+H)2 = 02, = T2+L2 = T2+2TH+H2; whence L = √2T.H+ H2.

=

If the plates be 6" wide, and T 31" (which is regarded as in good proportion, the above formula gives L21" very nearly, for a " hole. Then, 5′′ being allowed for the space ce, and 2" each for cd and eg, the splice plates would have a length of 201′′, and 3 of the whole section of chord plates would be available for tension; since an oblique section through two holes, would quite equal a direct transverse section through one hole.

The amount of rivet section above given is estimated upon the assumption that each rivet must be sheared

off in two places; and that it will resist, those shearings, each, with about of the force required to pull the rivet asunder by direct longitudinal strain.

It is obvious that the two rivets e and f, Fig. 34, sustaining a portion of the stress of the chord plate, relieve in the same degree the stress upon the portion between those rivets and the joint, or end of the plate; whence it is not necessary to preserve the same section in the portion thus relieved, as in other portions of the plate. Trefore the rivets a and c, nearer to the joint, may be larger than e and ƒ, when the section of plates requires more rivet section; provided always, that the least net section of splice plates, have as great an area as the chord plate has through only one of the smallest rivets. For instance, four " rivets are sufficient for plates 6" x 1". But plates 6" xğ" require more rivet section say " for e and f, and " for a and e; while, the same for the former and 1" rivets for the latter, give about the required section for plates 6′′ × ′′. This leaves in each case, the same proportion of net available section of plates.

Moreover, if rivets a and c be placed opposite to each other, and ƒ be removed to a, the rivets being 3′′ and 1" respectively. Then, the smaller rivets sustaining over of the stress, while the others sustain less than

, the latter may cut off of the net section (which is, in this case " less than the whole width of plate), and still leave enough to sustain more than their own legitimate share of the stress.

This may be done by one rivet or two, placed opposite ; and thus the length of splice plates may be shortened to 15 inches, instead of 203, as represented in the diagram. But, as in this case, the long plate has a net width of 51" and the splice plates, only 4"

the latter require 31 per C. more thickness than the former, so as to nearly or quite balance the saving in length.

As to the proportions of parts, in this kind of work, I would suggest that the thickness of plates be from th toth of their width, and the diameter of rivets, from 1 to 1 times the thickness of plates. If plates be very wide and thin, they may be liable to be strained unevenly, and if very narrow, an unnecessary proportion of section is lost in rivet holes.

CXVIII. The end connections of plate chords of this kind, may be effected by riveting on side plates at the ends, as seen at E, Fig. 341, so as to give a thickness that will allow about of the width of plate to be cut away by a hole for the connecting pin P, either round or oblong with squire ends for adjusting keys or wedges.

Or, the side plates may be omitted, and two keyholes made in the middle of the plate, one for a key having a thickness equal to the diameter of the smaller rivets, and far enough from the end to admit of another hole nigher to the end, with about 2" between the holes. This may, if necessary, have twice the width of the other hole, and should leave at least twice the width of hole, between hole and end.

The width of the wider hole,+twice that of the other, should equal about half the width of the plate; and the keys should be driven to an equal bearing before the work be subjected to use.

The connecting blocks used with this chord, sustaining only the horizontal action of diagonals, may be considerably lighter than those used with the links, especially in arch trusses. In order to transfer the horizontal action of diagonals to the chords, mortises may be made in the plates, as seen at m Fig. 34, not wider than the smallest rivets used in splicing, to receive tenons of wrought iron cast in the block.

As to the merits of the riveted plate, as compared with the link chord, it may be assumed that two splices are sufficient for any truss not exceeding 100' long, and that the weight of splicing plates and rivets will equal 4 or 5 feet extra length of plates, say 6 per cent upon a chord 80' long. To this we have to add about 14 per cent for extra section to compensate for rivet holes, making 20 per cent of iron lost in forming connections.

Links require about half as much extra material, to be taken up in bends, lappings, and enlargement of section at the ends; showing about 10 per cent less iron for the link, than for the plate chord. This would amount to about 400lbs. for two trusses of 80', with links of 1" round iron. But this may be nearly or quite balanced by 500 or 600lbs. of castings, which may be saved in weight of connecting blocks.

The economy of material being so nearly equal in the two chords, their relative merits must depend mostly upon the comparative cost of manufacture, and the relative efficiency of the chords in use. It is deemed far from improbable that the riveted plate chord might be found, on fair and thorough trial, to be worthy of extensive use in arch trusses, in place of the link chord. The fact that in the plate chord, the iron is used in its original condition, as it comes from the rollers, is certaiuly favorable.

BRIDGES WITH PARALLEL CHORDS.

CXIX. These may be constructed with or without vertical members, and in form, either rectangular, with vertical end posts, or trapezoidal, having inclined end members, or king braces, as exhibited in Figs. 12, 13, 18 and 19.

TRAPEZOIDAL TRUSS BRIDGE, WITH TENSION DIAGONALS AND COMPRESSION VERTICALS.

For short spans, less than 70 or 80 feet long, the simple cancel, as in Fig. 12, will generally be used, with trusses too low to admit of connection between upper chords, except in case of deck bridges.

The same plan of lower chords composed of links and cast iron connecting blocks, may be used, as already described for the arch truss. The connecting blocks are shorter, and may be cast in connection with the upright, or the latter may be in a separate piece. In the latter case, the block should have a suitable seat to receive the upright, and keep it in place.

As the upper chord depends upon the stiffness of the beam and upright for lateral support to keep it in line, the upright should be firmly attached to the beam, and at right angles therewith.

There is no means of estimating exactly the transverse force which the chord may exert upon the upright. But if the ends of chord segments be properly squared and fitted, the lateral tendency will be quite small. It is recommended, that each upright have a transverse strength sufficient to withstand a force of 1,000lbs. acting at the upper chord; that it have a web and flange form of section, with a width of web at the