Civil Engineering Reference
In-Depth Information
Figure 6.5
Typical cable zone
prestress profi le, the more positive (sagging) becomes the parasitic moment. This is a
vital tool in the hands of the engineer, but is neglected by many bridge designers.
6.9 Details of the sample bridge deck
The sample bridge has spans of 36 m, 40 m, 40 m, 36 m; Figure 6.6
(a). Note that the
side span is 90 per cent of the main span, which is the appropriate proportion for a
prestressed deck, as the sagging parasitic moment at the critical design section of the
side span is only 40 per cent of the value in a typical internal span. For the design of
continuous prestressed concrete bridge decks, it is normal practice to check stresses at
least at every tenth point of each span. The numbering of the design sections we will
adopt is shown in Figure 6.6 (b).
The deck consists of a 12 m wide box section bridge deck as shown in Figure 6.7. The
depth is 2.2 m, giving an economical span/depth ratio of 18, and providing suffi cient
headroom for work inside the box. The web thickness in the spans is 350 mm, which
is close to the practical minimum for a cast-in-situ deck with internal prestressing
tendons. At the piers, the webs thicken to 600 mm to provide suffi cient width to carry
the shear force.
The top slab is 200 mm thick which is a practical minimum for a slab carrying
traffi c (
9.1
). Also, for the 5.3 m clear span it provides a span/depth ratio of 26.5, which
is within the guidelines given in Chapter 9. The bottom slab is 200 mm thick, which is
also close to the practical minimum for cast-in-situ construction.
Figure 6.6
Span arrangement of sample bridge
