Civil Engineering Reference
In-Depth Information
Articulation of bridges and the
design of substructure
7.1 General
Concrete bridges expand and contract with temperature changes, they shorten under
the effects of concrete shrinkage and creep and they defl ect under the effect of
applied loads, prestress and temperature gradients. The deck must be held in place
when subjected to wind, seismic activity and to forces applied by the traffi c carried.
If the deck is long, it may be necessary to divide it into several expansion lengths by
intermediate joints. The articulation of a bridge may be defi ned as all the measures
taken to hold it fi rmly in position while allowing it to change in length and width and
to rotate at supports.
The articulation interacts closely with the type of bridge deck chosen, with the
method of construction of the deck and with the design of the foundations. It cannot
be considered in isolation; it is part of the overall concept of the bridge and should be
present in the fi rst 'back of the envelope' schemes.
7.2 Design parameters
7.2.1 Temperature change
Concrete bridge decks have considerable thermal inertia, and so do not follow daily
extremes of temperature. The bridge deck temperatures which should be considered
are defi ned by the local codes of practice. For instance, in the London area of the
UK it is accepted that the temperature of a concrete bridge may vary from -7°C in
winter to +36°C in summer, while in an equatorial climate such as that relevant to
Kuala Lumpur, for instance, there are no signifi cant seasons, and designers use bridge
temperatures of 30 ± 10°C.
One cannot of course expect that the bridge will be built at the mean temperature;
consequently it is necessary to consider carefully the actual range for which the arti-
culation should be designed. This may not be the same for all parts of the structure.
For instance, consider a bridge built in the London area consisting of several
continuous spans, with the deck built into the piers, and erected by balanced cantilever.
While the balanced cantilever is under construction, it is free to expand and contract
with changing temperature without any bending moments being created. Once it is
connected to the adjacent balanced cantilever it becomes a portal and expansion and
contraction of the spans due to temperature change will cause bending moments in
