Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/367

Rh COMPOUND STKUCTURES.J B 11 I D G E S the boom is uniformly strained over its whole length. This truss would not be so light as a Warren girder if both were made of the same depth, and if the end struts in the Warren girder did not require to be much stiffened in con sequence of their great length. The Fink truss is, however, generally made in practice more cheaply than the Warren girder, because the depth of a girder practically depends on the greatest length of strut which is admissible, and for equal lengths of strut the Fink truss gives a deeper beam than the Warren girder. Fig. 86. GO. Bolivian Truss (fig. 8G). This truss is the result of superposing seven simple frames consisting of a top member, a strut, and two ties. The stresses are easily computed. It is one form of the old false suspension bridge already alluded to ( oi), with the difference that the top member replaces the horizontal resistance at the points of support. The defect of this truss is that two ties supporting any strut except the central one are of unequal length; expansion or extension, consequently, affects these unequally. It is inferior to the Fink truss. Fig. 87. 61. Schaffhausen Truss. The famous wooden bridge of Schaffhausen (fig. 87, see also 76) is in its main parts a compound bridge, composed by superposing a series of simple frames of the type shown in 1, 2, 3, and 4. Nos. 1 and 2 are imperfect frames, i.e., if the joints were flexible they would collapse in consequence of the want of the diagonals across the centre parallelogram. The stiff ness of the joints supplies this want. G2. Dredge s Suspension Bridge. This bridge differs from the usual suspension bridge in having the suspending rods inclined, and in the use of a lower member, which may be a compression member transmitting a thrust piers, as in 88, or a tension member, as in 886, 323 to the with a Fig. 88a. Fig. 88i. Fig. 88d. maximum tension at the centre. Fig. 88c and fig. 88d show the reciprocal figures- corresponding to those two cases. This bridge is somewhat stiffer than the ordinary suspension bridge, but is far inferior to the complete framed bridge. G3. Arch or Suspension Bridge, hinged at Abutment* and Centre. Figs. 89 and 90 show two designs of con- Fig. 89a. siderable merit, consisting of two frames (the shape of which might vary considerably) hinged together at A, and supported on hinges at B and C. The direction of the sustaining forces at A and B is to bo determined as for a simple pair of beams hinged at A, B, and C. Find w l and w/, the proportions of the load on AC borne by C and A re-