Civil Engineering Reference
In-Depth Information
essence, is that a signifi cant section of the deck may be built off the critical path with
resources additional to the balanced cantilevering teams.
15.4.6 Bending moments in cantilever built bridges
The self-weight bending moments in a cantilever built bridge are entirely hogging at
the end of cantilever construction, and are balanced by the sagging moments of the
construction prestress cables. The weight of the mid-span stitch concrete is carried
by the statically determinate cantilevers, and thus produces only hogging moments.
When the stitch has hardened suffi ciently, the deck is no longer statically determinate;
it has become a continuous structure. As the statical defi nition of the structure has
been changed, creep will gradually modify the self weight and construction prestress
moments, as explained in
6.21.2
. As creep is a slow process the fi nal bending moment
diagram will only be attained after many years.
The removal of the stitch falsework is equivalent to applying an upward force equal
to its weight producing hogging moments at mid-span and sagging moments at the
supports. Similarly, if a prop has been used to provide stability, its removal equates to
applying a force equal and opposite to the prop reaction.
As bottom fi bre continuity prestressing cables are stressed, they will produce
sagging parasitic moments. Subsequently, deck fi nishes and live loads are applied to
the continuous structure, producing further hogging moments at supports and sagging
moments at mid-span.
15.4.7 Prestress layout
The cantilevering prestress cables, called Stage 1 cables, are usually installed in
each pair of segments as they are built. As the bending moments close to the pier
are changing rapidly, it is frequently necessary to anchor several tendons in each of
the early segments. Closer to mid-span it is possible to install them in every other
segment, with the un-prestressed segments being carried in reinforced concrete action.
The tendons may be anchored in the end face of the webs, in the end face of the
thicker parts of the top slab, or in blisters inside the box. Tendons anchored in the face
of the webs have the advantage that they may be defl ected downwards to help carry
the shear, Figure 15.19 (a). This allows the web to be made thinner, saving precious
weight. However, their size may be limited by the width of the web, and they have to
be stressed before the next segment may be cast. Tendons anchored in blisters may be
stressed at any stage in the construction sequence and are not limited in size, although
it is good practice to use only one size of tendon in a deck, except for large projects
and special circumstances.
Stage 1 prestress is sometimes provided by bars, which are coupled at the face of
each segment. Although simple to install, in the author's experience such a form of
prestress is considerably more expensive than tendons; not only do the bars work at
a lower stress than strand, leading to a greater tonnage of steel, but their cost per ton
of steel is higher.
During construction, the Stage 1 tendons need to be near the top fi bre to carry
the self-weight hogging moments. However, in service the moment diagram will have
dropped, and, in the central half of the span, Stage 1 prestress that is too high in the
section has to be counteracted by additional bottom fi bre, Stage 2 prestress, wasting
