Civil Engineering Reference
In-Depth Information
The successful outcome of this method of construction depends on using a casting
cell of high quality. Top class design, materials and fabrication are worth paying for,
and the cost difference with a cut price product will be recovered many times over
for a long bridge deck. The author has had experience of a site in the UK with several
casting cells whose construction was too light, and strengthening beams had to be
attached at each cast to limit the defl ections of the side shutters under the pressure
of the concrete and to resist the shaking caused by the external vibrators, and then
detached to allow the shutters to be struck. The cells were eventually abandoned and
a better product purchased. On the site of a very long viaduct in South East Asia,
the local contractor needed a large number of cells. He had some made by Ninive
Casseforme [1] in Italy, and then had a local fi rm fabricate others. When more cells
were needed, the contractor opted for the better quality Italian product despite the
greater cost and the delay of shipping.
14.3.4 The calculation of the casting geometry
In order to develop the geometric instructions for precasting the segments, their relative
position in the bridge deck must be calculated with precision. The so-called 'second
order' effects must be included in this calculation. Any combination of longitudinal
gradient, crossfall, horizontal or vertical curvature creates secondary curvatures or
twists that must be included in the casting geometry. For instance, if a horizontally
curved deck is given a crossfall by rotating it about a longitudinal axis at the pier it will
progressively develop a vertical gradient, Figure 14.6, which must be corrected in the
casting cell. Similarly if a deck curved in the horizontal plane is tilted onto a vertical
gradient, the carriageway will progressively develop a crossfall unless a corrective twist
is built in. It needs a clear head to understand these geometric interactions and to
transform the mathematical defi nition of the alignment into a set of casting instructions.
Most of the failures of erected alignment when using this technique of bridge deck
construction can be traced to mathematical errors in defi ning the casting geometry.
Software is available to ease the task of the designer, but these programs should not
be used unless someone within the team understands the mathematics of the transfer
of the deck alignment to the casting yard; when mistakes or accidents occur during
the casting of a segment, it is essential to understand the basic mathematics in order to
develop corrective action.
Prestressed concrete bridge decks need to be pre-cambered to cancel out the deck
defl ections under long-term dead loads and prestress. Once the geometry of the deck
has been converted into a set of casting instructions, the pre-cambers must be added.
Errors independent of the casting geometry may be introduced into the alignment
during the construction of the segments, some of which are unpredictable. Consequently,
the calculation of the casting geometry should be as rigorous as possible to avoid
adding avoidable errors to those that are unplanned. When the deck is erected, even
relatively small errors in alignment can be troublesome and require costly remedial
works. For instance, the mid-span cast-in-situ stitch for a balanced cantilever deck is
usually between 150 mm and 250 mm wide. Consequently, even a positional error
of 25 mm between the ends of the two cantilevers will give diffi culties in linking the
prestressing ducts and in aligning the parapet.
Every effect that may infl uence the erected alignment should be taken into account.
For instance, on the STAR railway viaduct in Kuala Lumpur that was erected in
