an alternative proposal for the design of balanced cantilever bridges ...

2 Proceedings IBSBI 2011
20% of the length of the second span are casted together. The construction of
the next bridge segment follows after the application of the prestressing force,
while keeping the immediate prestress losses within normal levels. The final
loading of the bridge due to the self-weight of the superstructure is varying with
time due to the influence of the creep effect [3] [4].
A new bridge construction method is investigated in this paper. The method has
similarities with the balanced cantilever method. The connection of the
cantilevers is achieved by the use of tendon couplers. The tendons are straight
and the scaffolding, which is used for the deck casting, is removed after the
application of the prestressing force. The applicability of the proposed
construction method has been attempted to a cast-in-situ benchmark bridge
actually built along a major motorway that runs across Northern Greece.
2 THE PROPOSED CONSTRUCTION METHOD
2.1 Structural assumptions
The proposed structural method, which can be utilised for the construction of
cast-in-situ bridges, is based on the following structural assumptions: (a) The
deck cross section has a variable height along the longitudinal direction of the
bridge with a symmetrical bottom flange, which is modulated by a polygonal
shape inscribed in a parabolic arch, as shown in Figure 1. The cross section of
the deck can be either a box girder or a voided slab. (b) The prestressing
tendons are straight and continuous in all the deck spans and they are installed
in the top flange of the deck. The appropriate concrete cover [5] [6] is provided
to protect the tendons against corrosion. Within the bottom flange of the deck
only ordinary strength steel is utilised. (c) The construction of the end spans can
follow two different design alternatives: (c1) The first alternative introduces the
construction of the end spans by maintaining the geometry of the intermediate
spans for reasons of aesthetics. In that case, the deck is chosen to be seated on a
wall-like abutment web, as shown in Figure 1 and 2. (c2) The second design
alternative introduces the construction of the end spans with lengths smaller
than the ones of the intermediate ones. Half of the length of the end span has a
deck cross section with variable height. This corresponds to the part of the deck
which extends from the end pier towards the abutment. The other part of the
span is seated through bearings to the abutment, as shown on the right abutment
of Figure 1. It extends from the abutment towards the pier and has a constant
cross section height. The need for the smaller length of the end spans was found
to be dictated by the relatively small height of the deck cross section that is 0,80
m and by the use of ordinary reinforcements in the bottom fibre of the deck.
It is noted that the use of prestressing within the bottom flange of the deck was
not deemed to be a rational design selection, as the tendons would induce a
large vertical load downwards, due to the variation of the height of the deck
cross section. This constraint loading, namely the one induced by possible