For a 16 panel truss, as arranged in Figures 18 and 19. Suppose w = 12m (m representing 1,000lbs.) ; w' = 4m, and W 16m, w+w'; - diagonals (except the steep ones), inclining 45°.

=

=

The end brace, then, sustaining 71W = 120m, [LVI], produces tension equal to 60m, upon the first and second section of chord, in Fig. 18, the proportions for which will be here considered. Allowing then, 10m to the square inch, each half chord requires a plate of about 8" by", up to the second node from the end. This plate may extend say within 8" of the centre of the connecting pin at the 2d node, where it may be connected with a g" plate, by two splice-plates about 27" long (see A. Fig. 45), with a net section equal to the plate, or, say" thick. Fig. 45, exhibits a disposition of rivet and pin holes, at A, so arranged as to preserve the full section of plates, less the diameter of a single 1" rivet hole.

Or, the splice-plates may be 7" shorter, and thicker, and the two rivets next the joint (j), on either side, opposite one another, as at BB, Fig. 45; thus giving the same section (of splice-plates), through two opposite rivets in the thicker, as through one rivet in thinner and longer splice plates. In this case, the joint should be 41" from centre of connecting pin (p), and a little more, when the rivets exceed 1" in diameter.

At the third node, an increase of section is required, and a " plate may be added on the inside, lapping 9 or 10 inches back of the pin, with a " splice plate of the B pattern to balance the extra inch in width required for opposite rivet holes, and a 2′′ pin hole.

The inside plate continuing past the next, or 4th node, the "outside plate may be met by, and spliced to a " plate, in either of the modes indicated by A and

B, Fig. 45. be at least

5

On plan B, the outside splice plate should thick, and the inside one, ". In this, as in other cases where a thinner plate meets a thicker one, the former is to be furred out to the thickness of the latter.

At the 5th node, the outside plate may continue, while the inside one is succeeded by a ğ" plate, with a " splice-plate inside, and one of " thickness upon 16 the outside; splice-plates in all cases being intended to be upon the outside, and not between the two courses of plates forming the half chord.

The same general process being continued, each course being spliced at alternate nodes, and breaking joints with one another, we introduce in the outside course, a 1" plate from the 6th node to the centre of the chord, and a " plate from the 7th node, past the centre to the 9th node, and so on, with a reversed order of succession to the other end of the chord.

The two 1" plates in the outside course, should meet at the centre connecting pin, and all other joints should De a few inches from the pin, on the side toward the end of the chord, as in diagram, Fig. 45.

Each pair of splice plates should have a minimum net section, together with the net section of the continued plate, at least equal to the sections of the continued, and the thinner spliced plate, through one of the smaller rivets used in the splice; and the relative thickness of the two splice plates should, as nearly

as practicable, be inversely as the respective distances of their centres from the centre of the spliced plate.

For illustration; at the 6th node, the continuous plate is ", and the thinner spliced plate ′′, making in the two, a thickness of 1", by 7" for the net width; giving a section of 103 square inches. This splice requiring 11" rivets next the joint, to give the necessary rivet section, the net width of splice plates and continuous plate through two opposite 14′′ rivets, is only 51". Consequently, the aggregate thickness required to give 10 square inches, is about 1.91"; and, deducting 0.625" for the continuous plate, we have 1.285" for thickness of the two splice-plates.

Then, representing thickness of spliced plate by a (disregarding the furring plate, or including it in the quantity a), that of the continuous plate by b, that of the two splice-plates by c, and that of the thicker one by x; we form the following equation, as will be obvious on reference to Fig. 46, which is an edge view of splice at node 6.

xx (a+x) = (c—x) × (b+1 (a+c-x); whence, the formula xc × (a+2b+c) ÷ 2 (a+b+c).

This formula applied to the case represented in Fig 46, gives x 0.7804", and c-x=0.5046".

=

FIG. 46

58

трі

The letter a in the diagram shows the splicing of a 1" with a " plate, the thickness being equalized by a furring plate.

Figure 46 gives also, a general idea of the splices proposed for this kind of chord, in case of the adoption of

th sport splice plates and opposite rivets, as seen at Br, Fig. 45. p indicates the connecting pin (which, in the concrete plan of construction should be replaced by two opposite rivets, as seen in Fig. 44), having a cross-section in the parts passing through the chord plates, about equal to that of one of the two main diagonals connecting with each pin respectively, at the several nodes.

The body of the pin between chord plates, should have lateral stiffness enough to withstand the strese produced by diagonals horizontally, estimated upon the principles of the lever, which will be greater as the distance of diagonals from chord plates is greater, and the contrary. If the bearing of the upright upon the pin be between the diagonals and the chord plates, as by a bi-furcation like that at the upper chord (see a Fig. 38) the body of the pin will usually require a section about equal to that of the two main diagonals connected with it. But this is no certain rule.

The ends of the connecting pin should extend through the chord plates so as to receive a thin nut upon each end, and also the eyes of sway rods upon the inside end, in case that mode of connection be adopted for those parts.

In the case of trusses without verticals constructed in rivet work, the best balanced action will be secured by connecting diagonals between the splice plates, by means of rivets through both, thus bringing each diagonal bar directly over each half chord, and producing uniform stress, as nearly as is practicable. When diagonal bars do not fill the space between splice-plates, the deficiency may be made up by furring plates, or thimble rings.

Tension diagonals will usually require from 25 to 33 per cent of extra section to make up the loss in rivet holes. In thrust diagonals, no allowance need generally be made for rivet holes, as rivets properly distributed, will not impair the efficiency of the member in withstanding compression.

With regard to the relative merits of this kind of lower chord, it requires, in the proportions above assumed, namely, 8" width of plates and 1" diameter of the smaller rivets, about 14 per cent of extra section on account of rivet holes, through the whole length, For splice plates and rivets, at least an equal amount should be allowed, making 28 per cent for waste material, over and above the net available length and cross-section. The corresponding waste in the link chord, and in the eye-plate chord [CXIV], can scarcely exceed 10 per cent, when the connections are made with wrought iron pins.

Hence, the advantage as to economy of material, seems decidedly in favor of the latter plans; and the cost of manufacture can hardly be estimated in favor of the former. If the riveted chord, then, have any claim to favor and preference, it is mostly owing to the fact, that being manufactured cold, it escapes the deteriorating effects frequently resulting to iron in the process of forging and welding, and the risk of flaws, and imperfect cohesion of the welded surfaces.

How far this consideration should be regarded as an offset, or an overbalance to 15 or 20 per cent, of material lost in rivet holes and splices, further experience and observation alone can probably determine.