russes consist of simple trusses used as primary, secondary and
tertiary trusses, the secondary supported on the primary, and the tertiary
on the secondary. Thus, the Fink truss consists of king-post trusses; the
Pratt truss (fig. 63) and the Whipple truss (fig. 64) of queen-post trusses
alternately upright and inverted.

[Illustration: FIG. 63.]

[Illustration: FIG. 64.]

A combination bridge is built partly of timber, partly of steel, the
compression members being generally of timber and the tension members of
steel. On the Pacific coast, where excellent timber is obtainable and steel
works are distant, combination bridges are still largely used (Ottewell,
_Trans. Am. Soc. C.E._ xxvii. p. 467). The combination bridge at Roseburgh,
Oregon, is a cantilever bridge, The shore arms are 147 ft. span, the river
arms 105 ft., and the suspended girder 80 ft., the total distance between
anchor piers being 584 ft. The floor beams, floor and railing are of
timber. The compression members are of timber, except the struts and bottom
chord panels next the river piers, which are of steel. The tension members
are of iron and the pins of steel. The chord blocks and post shoes are of
cast-iron.

[Illustration: FIG. 65.]

33. _Graphic Method of finding the Stresses in Braced Structures._--Fig. 65
shows a common form of bridge truss known as a _Warren girder_, with lines
indicating external forces applied to the joints; half the load carried
between the two lower joints next the piers on either side is directly
carried by the abutments. The sum of the two upward vertical reactions must
clearly be equal to the sum of the loads. The lines in the diagram
represent the directions of a series of forces which must all be in
equilibrium; these lines may, for an object to be explained in the next
paragraph, be conveniently named by the letters in the spaces which they
separate instead of by the method usually employed in geometry. Thus we
shall call the first inclined line on the left hand the line AG, the line
representing the first force on the top left-hand joint AB, the first
horizontal member at the top left hand the line BH, &c; similarly each
point requires at least three letters to denote it; the top first left-hand
joint may be called ABHG, being the point where these four spaces meet. In
this method of lettering, every enclosed space must be designated by a
letter; all external forces must be represented by lines _outside_ the
fram