This is equivalent to 8 tons on each panel. There will, therefore, be seven loads of 8 tons resting on the apices of the top The girder, and fig. 135 the stress diagram for the left half. The stresses are given in Table LVIII. All these stresses may be analytically checked by Ritter's method of moments. For example, by drawing the dotted sectional line a b, we can find the stresses on the bays K C, BO, and the diagonal B C. It will be seen that in girders of this description, loaded uniformly along the straight flanges, the stresses in the diagonals are all compressive, whereas in the bowstring girder they are all tensile. This fact of itself renders the lattice fishbellied girder less economical than the bowstring type. Second Case. With this distribution of load, the dead weight is just the same as in the first case, and is equivalent to loads of 3.5 tons resting on each apex of the girder. The live load puts an additional weight of 4.5 tons on the first three apices on the left, and a weight of 2.25 tons on the central apex. These weights produce an upward reaction of 23.5 tons at the left, and 16.75 tons at the right abutment. 1 To construct the stress diagram, on a vertical line (fig. 136) take OJ = 23.5 tons, and OJ1 = 16.75 tons, the reactions of the abutments. Set off J K = K L=LM = 8 tons, M M1 = 6·75 tons, and M1 L1 =L1 K1 = K1 J1 =3·5 tons, the loads at the apices taken in succession from the left. The diagram is then constructed in a similar manner to the last, and the stresses are given in Table LIX. 1 1 B ل N M L K H H. F D B E G. C C Fig. 137 represents another form of braced curved girder which is frequently employed, not only in bridges, but also in public buildings, where it is used for supporting the roof principals. It will be seen that the top flange is straight and the bottom curved upwards in the arch form. Fig. 137. This gives it a graceful appearance, which makes it a favourite design for exhibition buildings, &c. E Example 6.-A lattice girder of the type shown in fig. 137 is 80 feet span, 15 feet deep at the ends, and 7 feet 6 inches at the centre. The top flange is divided into eight bays of 10 feet each. Determine the stresses (1) when the girder is loaded with a weight of 20 tons resting at the centre of the top flange; (2) when loaded with 40 tons distributed. the girder for the central load. The vertical line A J is taken equal to the abutment reaction of 10 tons. There is no stress on the two end bays, A O and A, O1, of the bottom flange. The stresses are given in Tables LX. and LXI. 201. Definition.-A crane is a structure used for lifting weights, and, in addition to the framework, includes the mechanism, such as the gearing, &c. It is only with the former we are here concerned. There are many varieties of cranes, as regards the structural character of their framework; we will refer to some of the principal, and show how the stresses on their different parts may be determined. 202. Jib Cranes.-A simple form of jib crane, sometimes known as the wharf crane, is that which is shown in skeleton outline in fig. 139. It consists of three main members, viz. :— The vertical post, A D, The inclined jib, B C, The crane post is bedded into the ground, its extremity or toe, D, usually resting in a socket, so that the crane may be turned round, A D, as a vertical axis, in a horizontal direction. The post acts as a cantilever, the maximum stress on which occurs at B. The jib, BC, is always exposed to a direct compressive stress, while the stay, A C, is always sub jected to a direct tensile stress. Both the post and jib may be made of iron, steel, or wood; while the stay is usually made of wrought iron or steel. The weight to be lifted is suspended at C, the point of intersection of the jib and stay; the chain from which it is hung passing over a pulley at C and then round the drum at E, which is fixed to the crane post. Usually there is an arrangement by means of which the jib may be raised or lowered-turn Fig. 139. ing round its foot, B-by shortening or lengthening the stay. With a single pulley at C, the tension on the chain is always |