square bar upon a leverage equal to its width of side. Hence, a representing the area of section, D, the diameter, and L, the length, the safe stress = 4,500 acting AD L AD L on a projecting end, and 18,000 acting in the middle between two outside bearings; that is, a 1" pin with centres of outside bearings 6" apart, will bear in the middle, 18,000 × 785 = 2,355lbs. If the pin connect an 6 eye 1" thick, between one of " thick on each side, the length (L) between centres of outside pieces, will be 11"; whence, a 1" pin will bear 4 times as much as in the preceding case; or 2,355 x 4 = 9,420lbs. The × tensile strength of a 1" round rod, being 11,775lbs. being equal to the cross-section (0.785), x 15,000lbs. it shows that the strength of the rod is greater than that of the pin, in the condition here assumed, in the proportion of 11,775 to 9,420. Therefore, the stiffness of the pin being as the cube of the diameter, in order to find the diameter (x), of a pin for connecting 1" bars by eyes and connecting straps, we have this proportion 9,420 11,775 13:23, whence x 1.077 = to about :: = larger than the diameter of the rod to be connected, and in this proportion for any size of round rods connected by straps and pins or bolts. But if the eyes and straps be drilled, so as to fit the pin through the whole thickness, the action approaches the shear strain, and the pin should have about the area of section of the bars to be connected. The author would recommend, however, for general practice, that connecting pins be considered as acting by transverse stiffness, upon the lever principle, as above discussed. ARCH TRUSS BRIDGES. XCIX. The general form in outline of the Arch Truss, may be seen in Figs. 8 and 11. The forms of the different members, and the modes of connecting them to form the complete structure, are many, and a minute description of each possible variety, in this respect, even if such a thing can be regarded as practicable, will not be undertaken on this occasion. The arch may be of cast or wrought iron in various. forms of section. The following form of cast iron arch has been extensively used in the state of New York, with uniform success and satisfaction. The arch is composed of cast iron sections, equal in number to the number of panels in the truss; an odd number being deemed preferable. In Fig. 27, a on m presents a top view of the arch, and D, a top view of the chord, from end to centre; and A and B, enlarged cross-sections at p and q, adjacent to the cross-bars to be described below, and which also appear in the figure. Each piece consists of two side portions of an formed section, connected at the ends, and at 2 or 3 intermediate points, by cross-bars of a T formed section for the intermediates, and at the ends, with sections as seen at C, where a view of the arch connection is shown, as it would appear if cut vertically and longitudinally through the centre, and the near half removed. The width of the side plates of arch castings (from the top), should be about of the length of pieces, with an average thickness of from to of the width. The top plate, about the same thickness (or a trifle less, to prevent a tendency in the piece to become hollow backed in cooling), and a width, a little over one-half that of the side plate. The resisting power may be estimated as in the table of negative resistances under the head of square, &c., pieces, calling the width of side plates the diameter, and using the column under, for trusses supporting 12 feet or more width of flooring, and the column headed, in case of trusses supporting a width of 10 feet or less, to each truss. The intermediate cross bars should have about the same thickness of plate as the side portions, a depth, about that of side plates, and top plate not less than Bo as wide as the top plate of side portions. End cross-bars should have a top width of about the width of side plates, and cross-section sufficient to sustain a whole gross panel load for the truss, by transverse resistance. If it have a depth equal to of its length, and a form of section as strong as a rectangular bar, it will safely sustain 1,000 to 1,200lbs. to the inch; and it is recommended to allow one inch of section in each end cross-bar to every 1,000lbs, sustained at the joint. Then, there being two cross bars together, the point will be doubly secure. Semicircular notches in the ends of contiguous arch pieces, form a vertical circular hole at the joint, for the passage of the vertical member. FIG. 28. When the side plates are thin, the thickness should be increased for a few inches from the end, to afford a suitable bearing surface at the joint; and the ends of arch pieces should be fitted (usually by planing), to a proper bevel to form a fair joint. The joints, however, are sometimes formed by cutting tapcr key seats (as seen in Fig. 28), in one of the contiguous ends, to admit wrought iron wedges about an inch wide, and in sufficient number to give a bearing upon wedges, equal to at least onehalf the section of iron in the chord. This method has answered well in a large number of bridges, and is convenient for adjusting the arch in line; but the planed ends form much the more workmanlike joint. The centre arch piece has usually a full top plate over the whole width of the piece. FIG. 29. The endmost section, or foot piece of the arch, connects with the chord by means of horizontal holes in the feet to receive the ends of an oɔen end ink of the chord, which is secured by screws and nuts as shown in Fig. 29, representing an inside view of the foot of one branch of the arch. C. The chord is composed of two long links of round or square iron to each panel, connected by cast iron connecting blocks at points vertically under the arch joints. The form of these blocks is represented in Fig. 30. They diminish in length from the endmost to the centremost, the former being long enough to re ceive the links running parallel from the connection with the arch, and the next block, being shorter by twice the diameter of the link iron; the ends of links toward the centre of the truss, going next the end of |