Civil Engineering Reference
In-Depth Information
12
Ribbed slabs
12.1 General
When the maximum bending stress on the bottom fi bre of a solid slab deck is below
the allowable limit, it is possible to remove material without increasing the depth of
the deck, leaving a shallow ribbed slab as described in
11.3.7
. However, if more depth
is available, the deck may be made more economical by making the ribs deeper and
narrower.
As explained in Chapter 8, it is important for economy to limit the number of
webs in a deck. In fact, for most bridge decks up to a width of 20 m and more, two
ribs are found to be adequate. Such bridges are usually called 'twin rib bridges' and
this chapter will concentrate on this deck arrangement. Figure 12.1 shows one of the
Doornhoek bridges in South Africa, a typical twin rib deck designed by the author at
Arup with the architectural assistance of Humphrey Wood of Renton Howard Wood
Levine for the design of the columns.
Where the depth is not constrained, the width of the ribs is defi ned by the ULS
bending or shear at the supports. This width is maintained constant throughout the
span. Economy is achieved by the simplicity of the construction, despite a relatively
high consumption of concrete and prestressing steel. The prestressing tendons have
simple profi les; the reinforcing cage is uncomplicated and may be prefabricated; casting
and compacting the concrete is quick; a complete span may be cast in one continuous
pour. Most twin rib bridges do not need diaphragms except at abutments where they
support the roadway expansion joint. However, pier diaphragms are necessary in some
particular situations, as explained in
12.3
below. Where there are no pier diaphragms,
the central tunnel formwork can be launched forwards from span to span without
dismantling.
These decks are well received by contractors in both developing and developed
economies. In the former, they are suitable for an unskilled work force; in the latter,
they allow high productivity from expensive labour. With a semi-mechanised falsework
of the type shown in Figure 12.2, used for the railway viaduct forming Contract 304
of the Tsuen Wan Extension of the Hong Kong Mass Transit Railway, and designed by
the author at Arup, construction proceeded at the rate of a span in two weeks. With
a mechanised self-launching rig and prefabrication of the reinforcing cage, developed
from that used for the Viaduc d'Incarville, Figure 12.3, two spans per week is possible
(
15.2.5
).
