Civil Engineering Reference
In-Depth Information
The diaphragms must of course be designed to carry wheel loads that are applied
to them directly, or that are carried to them by longitudinal spanning of the slab. The
top slab must also be designed for longitudinal hogging moments where it frames into
the diaphragm.
Pier diaphragms hinder the launching of mechanised falsework, which is important
in the construction of long viaducts. Where diaphragms are necessary, it is sometimes
possible, but not very convenient to leave them out in a fi rst phase of construction,
casting them once the falsework rig has moved on. This was the solution adopted for
the Viaduc d'Incarville shown in Figure 12.10.
12.3.3 Abutment diaphragms
Abutment diaphragms are required for the reasons described in
9.2.6.
The presence
of the diaphragm then attracts other loads, as described above. It is important to keep
the end diaphragm as fl exible as possible compatible with its tasks as this will minimise
the torque in the ribs.
12.4 Proportioning of twin rib decks
12.4.1 Height and width of ribs
Generally, twin rib decks are up to 2 m deep, with 2.5 m being exceptional. Span/depth
ratios generally are a minimum of 1/20, with 1/18 being more typical. Consequently
suitable spans range from 20 m where this type of deck takes over from solid or voided
slabs, to about 45 m. Figure 12.9 shows diagrammatically how the proportions of a
twin rib deck of 35 m span evolve towards a voided slab as the depth decreases.
As the design intent for ribbed slab bridges is to keep the dimensions of the rib
constant over the full span, economy requires that the width be minimised. The ribs
are often slightly tapered towards the soffi t. A taper of about 1/12 allows the tunnel
shutter between the ribs to be struck without being collapsed or folded. The outside
face does not need to be tapered for that reason, as the shutter is not trapped. However,
it is often tapered to the same degree to improve the appearance of the bridge. Parallel
sided ribs may be designed, as long as the tunnel shutter may be collapsed or struck in
stages, Figure 12.10.
The width of the ribs may be defi ned by one of fi ve criteria:
•
•
•
•
•
SLS compressive bending stresses in the deck;
ULS bending adjacent to the pier;s
ULS shear and torsion;
width necessary to accommodate bridge bearings;
width necessary to support the top slab.
For very shallow twin rib decks, the width of the bottom fl ange may be controlled
by the need to provide an adequate bottom fi bre modulus to reduce the variation of
stress under live loads. However, this is unusual, the width generally being controlled
by one of the other criteria listed.
Generally, the critical criterion is Ultimate Limit State bending at the piers. There is
always a trade-off between section depth and rib width. As the section is made shallower,
