Civil Engineering Reference
In-Depth Information
a ribbed or strutted slab and side cantilever (
9.2
and
9.3
) or to increase the number
of boxes in the cross section. The choice will depend on the methods of construction
adopted, and on the preferences of designer and builder. The author's preference is,
wherever possible, to build single-cell boxes, accepting the resultant complications to
the construction procedure, or to increase the number of boxes.
When three webs are used, it should be noted that the shear force is not evenly
distributed, with the centre web taking more than one-third. The prestress should be
distributed between the webs in proportion to the shear they carry.
13.6 Number of boxes in the deck cross section
13.6.1 General
Once it has been decided that it is appropriate to adopt a box section for the bridge
deck, the designer must decide how many boxes should be used across the width of
the deck. This decision governs not only the material content of the deck, but also
the arrangement and cost of the substructure, and the type and scale of the temporary
works that need to be mobilised. It is intimately linked to the other basic decision that
must be made, whether to cast in-situ or to precast.
There are many factors that infl uence these linked decisions. Some of the most
important are the following:
•
the scale of the project and subdivision of the scale; many short bridges; several
longer bridges; one very long bridge (Chapter 16);
complication of the project; change of crossfall; variable width of the deck;
bifurcations of the bridge deck; presence of a family of bridge decks; presence of
slip roads etc;
criteria for sub-structure and foundations (Chapter 7).
•
•
13.6.2 Constant-width decks
Consider a 15 m wide deck carried by a single-cell box. This would typically have
3.25 m long side cantilevers, and an overall box width of 8.5 m. If the span were
50 m and the depth of the box 2.8 m, the thickness of the webs at the piers would be
625 mm (
9.5.2
), while at mid-span the webs could be reduced to 350 mm or 400 mm,
Figure 13.10 (a). The top slab would be 250 mm thick with 450 mm haunches. If
two boxes were to be used, the side cantilevers would typically be 1.25 m, the overall
width of each box 4.25 m, and the slab between boxes 4 m, Figure 13.10 (b). For
the same span and depth, the webs would be 350 mm thick at the piers, and this
thickness would be maintained at mid-span. The top slab would be 200 mm thick
with 350 mm haunches. This is not a very economical scheme as there is too much
web material at mid-span, and the 200 mm top slab will be under-used. Furthermore,
the side cantilever length is somewhat short for the appearance of the deck and the
3.55 m internal width of the box is too small for economical striking and handling of
the shutter for precast segmental construction. One bonus is that the webs may be of
constant thickness, simplifying construction.
