Civil Engineering Reference
In-Depth Information
Figure 15.37
Blackwater Viaduct, Ireland (Photo: Strabag SE)
15.8.5 Launching in reinforced concrete
A further development is to launch the bridge entirely in reinforced concrete. For
launching decks with a span/depth of the order of 8-10, which is typical when
intermediate props are employed, this concept does not give rise to excessive quantities
of passive reinforcement and makes major savings in prestress, eliminating the 20 per
cent defi cit noted in
15.8.1
. The permanent cables are installed when the launch is
complete, as described in
15.8.16
.
If the fi nal prestress is designed to a code of practice that accepts partial prestressing,
much of the launching reinforcement is utilised for the fi nal design. This gives rise to
a very economical form of construction that is ductile during the critical launching
phase. If the fi nal prestress is external, it is feasible to install tendons that are hundreds
of metres long, eliminating, or much reducing, the cost of intermediate anchorages.
This technique was pioneered by Benaim on the 300 m long Broadmeadow Viaduct
in Dublin, Ireland, and re-used by Benaim on the design and build project for the 450 m
long Blackwater Crossing Viaduct in Cork, Ireland, Figure 15.37, built by Strabag. The
spans which ranged from 51 m to 58 m were halved by temporary supports for the
launching phase. Once the 2.9 m deep deck had been launched, external prestressing
cables were threaded from end to end without intermediate anchors, and the deck
designed as partially prestressed.
15.8.6 Design of the webs during launching
The launch bearings should if possible be located beneath the point of conjunction of
the axes of the web and of the bottom slab. However with webs which are typically
400 mm thick and launch bearings that are typically 500 mm to 800 mm wide, that is
not always possible, Figure 15.38 (a). The bending moments caused by this eccentricity
