Civil Engineering Reference
In-Depth Information
safety, the overall factor may be lower than when no component can be allowed
to fail. With passive lifting devices, it is desirable to create redundancy by using
multiple bars per lifting point.
Where the satisfactory anchorage of lifting devices cast into the concrete depends
on careful supervision during casting, in some circumstances it may be necessary to
load test each anchorage before suspending the segment from it. Despite all design
precautions, passive lifting systems are more subject to human error and material
failure than active ones, and are consequently less safe. Where they involve cast-in
products, they are also more expensive. However there are many instances where they
are the most suitable for the project.
The lifting beams can be designed with a hierarchy of sophistication. If the deck
to be erected is on a vertical gradient or crossfall, the lifting beam may be equipped
to incline the segment in both directions, using either a series of alternative lifting
positions or hydraulics. Similarly, the segment may be rotated about the axis of the
suspension system by hydraulic jacks, or this may be achieved by pulling on cables
from the erected deck.
14.3.14 Storing the segments
Depending on the size of the site and the philosophy of the contractor, the storage
may be highly organised and mechanised, or segments may be stored more informally.
In the most organised sites, the segments are handled by purpose-built transporters,
Figure 14.15, and are stored in regular rows, rather like a container yard, Figure 14.16.
Segments may be stacked two or three high, Figure 14.17, as long as no signifi cant
bending moments are imposed on the webs and slabs. In such a storage yard, it will
usually be necessary to build foundations for the rows of segments.
Figure 14.15
STAR: segment handler (Photo: Benaim)
