Civil Engineering Reference
In-Depth Information
A very economical form of abutment is achieved by separating the functions of
soil retention and support of the deck. The soil is retained by a reinforced earth wall,
while the deck is carried on piles that are allowed to rock within the fi ll. The piles
may be concrete or steel, and may be encased in pipes to give them the freedom to
rock. It needs to be demonstrated that steel piles would be adequately protected from
corrosion; they would appear to be more at risk than similar piles driven into the
embankment.
Integral bridges have been designed in the USA with lengths up to 352 m according
to [5]. However, current UK guidelines in BA42/96 suggest a very conservative limit
of 60 m. The same standard also suggests limiting the skew to 30°.
Building in piers and abutments and eliminating bearings and expansion joints
should be a major aim of the bridge designer.
7.14 Continuity versus statical determinacy
7.14.1 General
The designer of small to medium span bridges often has the choice between decks
that are continuous and those that are statically determinate. The choice is not
straightforward, and depends on the consideration of many factors, which may be
grouped under the headings:
•
•
•
•
economy of materials;
safety;
method of construction;
maintenance and use.
7.14.2 Economy of materials
Theoretically, the weight of reinforcement required for a reinforced concrete beam
is proportional to the area beneath the bending moment diagram. Figure 7.21 (a)
shows a simply supported beam of span
l
subjected to a uniformly distributed load.
The maximum bending moment is 0.125
wl
2
, where
w
is the unit load. The area
beneath the diagram is 0.0833
wl
3
. Figure 7.21 (b) shows a built-in beam of identical
span hinged at mid-span, subject to the same load. Although the maximum bending
moment is also 0.125
wl
2
, the area beneath the diagram is 0.0417
wl
3
, half that of
the simply supported beam. Figure 7.21 (c) shows a built-in beam without the mid-
span hinge, where the bending moment at the supports is 0.0833
wl
2
and at mid-
span is 0.0417
wl
2
, and the area beneath the diagram is 0.0278
wl
3
, one-third that
of the statically determinate span. This simple demonstration shows that hogging
moments are cheaper to reinforce than sagging moments, because the bars can
be shorter, and that a continuous beam should be much more economical than a
statically determinate beam.
However, for bridge decks the theoretical advantage of the continuous beam is
eroded in several ways. Live loading may be applied to individual spans of a continuous
beam, giving rise to an envelope of moments that has a much larger requirement
in reinforcement, Figure 7.21 (d). Furthermore, a continuous bridge deck must be
designed for additional bending moments due to differential settlement of foundations,
