Steel truss bridges - Computational Analysis and Design of Bridge Structures - page 300

(a) Variable depth cantilever (b) Deck truss

(c) Warren truss without verticals (d) Camelback truss

(e) Half-through truss

Figure 10.4 (a-e) Various truss bridge types.

and more contemporary appearance, thus minimizing the use of half-
through trusses.
There are several geometric guidelines for determining truss configura-
tions. AASHTO requires minimum truss depths of one-tenth (1/10) of
the span length for simple spans. For continuous trusses, the distance
between inflection points can be used as the equivalent simple-span length
to determine the minimum truss depth. It is generally desirable to propor-
tion the truss panel lengths so that the diagonals are oriented between
40° and 60° from horizontal. This keeps the members steep enough to be
efficient in carrying shear between the chords. This angular range also
allows the designer to maintain a joint geometry that is relatively compact
and efficient.
Floor systems can use a series of simple-span stringers framing into the
floor beam webs (Figure 10.5a) or continuous stringers sitting on the top of
the floor beams (Figure 10.5b). Whenever there is depth restriction require-
ment, framed systems serve to reduce the overall depth of the floor system
by the depth of the stringers.
Traditionally, when modeling and analyzing, truss can be idealized
assuming that the members are pinned at the joints (free to rotate inde-
pendent of other members at the joint) so that secondary stresses ordi-
narily need not be considered in the design, except certain cases like the
half-through bridges built in China (as illustrated and discussed more in
Section 10.7), which look like a truss type, but more like a frame-type
bridge where joints are taking bending moments. To exclude the bending
effect, joints are typically detailed so that the working lines for the diago-
nals, verticals, and chords intersect at a single point. However, bending
stresses resulting from the self-weight of the members should be considered
in the design. This idealization of a truss bridge simplifies the modeling and
lowers the analyzing effort.