13-09-2012, 05:15 PM
THE INCREMENTAL LAUNCHING METHOD IN PRESTRESSED CONCRETE BRIDGE CONSTRUCTION
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INTRODUCTION
The incremental launching method is one of the highly mechanised
erection methods used in bridge construction. The method consists
of manufacturing the superstructure of a bridge by sections in a
prefabrication area behind one of the abutments; each new unit is
concreted directly against the preceding one and after it has
hardened the resultant structure is moved forward by the length of
one unit (fig. 1). This principle has already been used for many
years in the construction of steel bridges. This is hardly surprising,
in view of the equal strength of steel in tension and compression
since, provided the design is suitable, the alternating stresses which
occur when the bridge is slid forwards can be accepted without
difficulty. This is not so with concrete, which can withstand only
small tensile stresses without damage. Special measures are
therefore necessary to enable concrete bridges to be slid forward by
steps; the skilful use of prestressing is the most important of these
measures.
Preconditions for use of the method
The incremental launching method can be used for straight bridges,
or where the superstructure has a spatial curve of constant radius
throughout the length. This means that it is even possible to construct
bridges which are curved both horizontally and vertically, provided
that the radii are constant.
The superstructure should consist of a beam of constant section, for
which the slenderness ratio, that is the span-to-depth ratio, is not
more than 17 when completed. Normally, the ratio lies between 12
and 15, the first value applying to larger, the second to smaller spans.
It is of advantage, with regard to design and detailing, if all the spans
except the end ones are equal or almost equal in length; the length of
the end spans should not exceed 75 0% of that of the standard
Loading cases
In addition to those loading cases which must in gene ral be
considered, the erection conditions are of espe cial importance to
the structural design when the in cremental launching method is to
be used; these con ditions influence both the superstructure and
also the piers and abutments.
During launching the superstructure is subjected to continually
alternating bending moments (fig. 4). Each cross-section moves
from regions of positive moment: into regions of negative moments
and vice-versa, so that tensile stresses occur alternately at the
botton and top parts of the section. The use of central pre stressing
reduces the tensile stresses to the permissiblE value.
Prestressing
In contrast to all other construction methods, a central prestress is
required during the construction stage in the incremental launching
method. As already mentioned in this section, this is due to the
alternating bending moments. What however does central
prestressing really mean? Central means that prestressing cables
are so arranged that the resultant compressive stresses at all points
of the cross-section are equal and therefore it makes no difference
whether the tensile stresses produced during launching occur in the
upper or lower parts of the section.