文档介绍：470-2 1
where minimal costs are required, or where
skilled labor is unavailable .
Flat-span and arch-span types of bridges
are available in the United States for spans
ranging from 3 000 to 48 000 (10 to 160
ft) . Lengths over 19 500 (65 ft) are fabri-
cated in sections to be bolted together in
the field. Carbon-steel or self-weathering-
steel bridge structures finished with wood-
en decking and handrailings are also avail-
able . Loadings are designed in accordance
with AASHTO specifications . Optional
handrailings 1 350 (54 in) high are available
when use by bicyclists is anticipated .
The same careful consideration of foun-
dation and abutment design is required for
prefabricated bridges as for site-built
bridges because manufacturers provide
only bridge superstructures .
Table 470-12 gives the sizes of prefabri-
cated bridges currently available in the
United States .
, PRO13LEMS OF ERECTING
BRIDGES
Every site will present its own distinctive
problems in the erection of a bridge . Four
of the mon areas of difficulty are :
1 . Access to the site: where access is
particularly difficult, it may be neces-
sary to design main structural mem-
bers with small pieces that can be
assembled at the bridge site.
2 . Amount of clear, level area for
assembly of bridge elements: if struc-
tural elements have to be assembled
at the site before being placed across
a gap, a suitable level area is
required .
3 . Lifting main structural members into
position : the method used to place
these members will depend on their
length, weight, stability, and the
nature of the equipment which can e
be brought to the site. This should be aA
anticipated at the beginning of the
design process (Figure 470-31) .
m
4. Maintaining safe working conditions: n
only by a
accidents can be avoided .
adopting safe working methods ri
which are carefully supervised .
e
MAINTENANCE OF BRIDGES e
a.
All bridges dete