Civil Engineering Reference
In-Depth Information
12.10 The development of ribbed slabs
12.10.1 General
Twin rib type decks are typically used for bridges with spans up to 35 m and deck
widths up to 15 m. Such decks have ribs that may be up to 2 m deep and 1-1.4 m wide,
side cantilevers of 3 m and slab spans of up to 7 m. However, they can be stretched
to cover greater spans and widths. The 15 m wide Viaduc d'Incarville had spans of
45 m, with a rib depth of 2.5 m. The Super Twin Rib decks described below can take
the width above 20 m under UK loading, wider under lighter live loading. In some
cases, the extra material implicit in the simplicity of the constant section solid web
may not always be justifi ed, and twin rib bridges can be designed with profi led webs
for particular situations.
12.10.2 Super twin rib decks
The principal prestressed concrete options for Structure 18 of the Harlequin
Interchange, a 375 m long, 20.3 m wide bridge deck with spans of 39 m designed
for tender to carry the M4 motorway, were either a precast segmental twin box deck,
or a cast-in-situ deck. As the total number of precast segments would have been only
about 220, which is considered below the economical threshold, Benaim proposed, as
an alternative to the offi cial scheme, the Super Twin Rib deck shown in Figure 12.17.
Each rib is carried by a single bearing on each pier, minimising the torque on the ribs.
Although the piers are slightly staggered, giving a skew deck, each rib is effectively
right. The 2.4 m deep ribs have been lightened by 1.8 m diameter voids which were
to consist of concrete drainage pipe sections. Unfortunately the alternative was not
accepted.
12.10.3 Profi led ribs
Under some circumstances it may be economical to design twin rib bridges with
profi led webs, similar to those of precast Tee beam decks. This is usually only justifi ed
if the benefi ts of simplicity have already been lost by the details of the project, or
Figure 12.17
Super Twin Rib project
