Civil Engineering Reference
In-Depth Information
on the deck caused by constraining it in this way are simply calculated using a space
frame, and in the great majority of cases are small compared with the other design
actions. In such a calculation, it is important to appreciate accurately the transverse
fl exibility of the piers, including their foundations, as very small lateral defl ections of
the piers will greatly reduce the transverse forces calculated on the assumption of rigid
supports. In some cases, the lateral forces may be found to be larger than desirable
on the abutments, which are usually rigid and cannot defl ect sideways. Omitting the
bearing guides on the penultimate pier, leaving the bearings as free sliding, gives the
deck extra freedom and generally reduces these forces to manageable proportions. Of
course, it is then necessary to ensure that the guided bearings on the second pier and
on the abutment are designed to carry the increased lateral wind loads.
7.9.9 Temporary fi xity during construction
When building a long viaduct span-by-span, it is necessary to ensure that the deck is
safely held in place at each stage of the construction. The extreme lower bound friction
coeffi cient of sliding bearings is so low, less than 0.5 per cent, that an unsecured bridge
could be displaced by accidental loads or even by exceptional winds.
In general, the completed viaduct would be held in place by a series of pinned
bearings near the centre, with all the other bearings being free sliding or longitudinally
guided. As construction proceeds from one end, the fi rst spans will be carried only by
sliding bearings. Some of these bearings need to be designed with locking bolts that
temporarily stop them sliding. Alternatively, the deck may be held in place during
construction by temporary locking devices that are independent of the bearings. The
load capacity of these temporary fi xings will be determined by the differential bearing
friction that may be applied at a particular stage of construction. If the temporary fi xed
point is moved forwards frequently as the deck erection progresses, the forces in the
temporary fi xing may be kept low.
When construction reaches the fi rst bearing that is pinned in the fi nal confi guration,
if no precautions were taken, it could be grossly overloaded by all the bearing
friction forces being applied from one side only. Consequently, it may be necessary to
temporarily free this bearing so that it can slide in response to a temporary point of
fi xity situated in the completed portion of the deck. Once suffi cient spans have been
erected beyond the permanent fi xed bearing to bring the differential friction loads
within its design capacity, it may be permanently locked. A simple example of this
procedure is given in Figure 7.16.
It is very important to consider these aspects of temporary fi xity and temporary
release of bearings suffi ciently early in the design. Bearings require long lead times for
ordering, and if these considerations are approached only at the last minute, they can
lead to expensive modifi cations to bearings that have already been fabricated.
7.9.10 Use of shock absorbers
High capacity shock absorbers that allow a slow extension of the piston but lock up
under rapid movements, can be fi xed between a bridge deck and a fi xed point such
as a pier or an abutment, or between successive sections of a bridge deck in a long
viaduct. Thus they allow the bridge to expand and contract freely under the effects of
