Civil Engineering Reference
In-Depth Information
The length of the side spans of cantilever built bridges are usually little more than
half the main span. This suits the construction technology as explained in
15.4.5.
When the side span is this short, it must be checked that the abutment bearings are
not decompressed by factored live loads in the main span. A counter-weight consisting
of mass concrete in the last metres of the box may be used to increase the bearing
reaction. Alternatively, the deck may be held down by ties which permit the length
changes of the deck, but this constitutes an additional maintenance and durability
liability. If the deck continues beyond the balanced cantilever spans, the reaction of the
adjacent span may be used to hold down the end of the cantilevered span. However,
in bridges with very long main spans it may be cost effective to limit the overall length
of the deck by curtailing the side spans, which will then need to be anchored down
with very substantial forces. This is clearly easier to achieve in Norwegian rock than
in British clay!
As described in
7.11
and
7.12
, decks should be built into their piers wherever
possible. The principal penalty for building the deck into the piers is the bending
moment that is applied to the foundations when the bridge is in service. For most
internal piers, where the adjacent spans are equal, this moment is almost entirely due
to live loads, the dead loads being virtually in balance. However, short end spans
lead to signifi cant unbalanced dead as well as live load moments being applied to
the foundations, principally of the fi rst and last pier, Figure 15.14. For the dead load
bending moments of the end span and of the fi rst internal span to be equal over the
pier, the end span length should be approximately 80 per cent of the internal span.
This causes problems in building the unbalanced portion of the end span, and the
sagging moment in the end span will be greater than in an internal span. A compromise
needs to be found.
With spans above about 150 m, it becomes worthwhile to use different qualities
of concrete for different areas of the deck. For instance, the support section may
use very high strength concrete, minimising the thickness of webs and bottom slab,
although very thin deep webs may start to meet problems of elastic stability. Further
out in the span, the concrete will be less highly stressed and the use of more expensive
lightweight concrete may be cost effective. Several large bridges, including the Stolma
Bridge, were designed in this way.
The Resal effect is described in
9.4.7
. For spans over about 150 m, where the
support depth is typically 8-10 m, it is worthwhile considering a trussed start to the
span, Figure 9.23.
Figure 15.14
Typical dead load moments in deck fi xed to piers
