Civil Engineering Reference
In-Depth Information
Deck
Column
Arch
Abutment
Span
(a)
Arch
Hanger
Deck
F
IGURE
1.6
Arches as
bridge deck supports
(b)
ARCHES
The use of trusses to support bridge decks becomes impracticable for longer than
moderate spans. In this situation arches are often used. Figure 1.6(a) shows an arch
in which the bridge deck is carried by columns supported, in turn, by the arch. Alter-
natively the bridge deck may be suspended from the arch by hangers, as shown in
Fig. 1.6(b). Arches carry most of their loads by developing compressive stresses within
the arch itself and therefore in the past were frequently constructed using materials
of high compressive strength and low tensile strength such as masonry. In addition
to bridges, arches are used to support roofs. They may be constructed in a variety of
geometries; they may be semicircular, parabolic or even linear where the members
comprising the arch are straight. The vertical loads on an arch would cause the ends
of the arch to
spread
, in other words the arch would flatten, if it were not for the
abutments which support its ends in both horizontal and vertical directions. We shall
see in Chapter 6 that the effect of this horizontal support is to reduce the bending
moment in the arch so that for the same loading and span the cross section of the arch
would be much smaller than that of a horizontal beam.
CABLES
For exceptionally long-span bridges, and sometimes for short spans, cables are used
to support the bridge deck. Generally, the cables pass over saddles on the tops of
