Page 26 - ICJ Jan 2023

TECHNICAL PAPER COLLECTOR’S EDITION

cast and stressed, the statical conditions of each PRESTRESSING
stage is changed, with the result that a succession The BBR system of post-tensioning is used
of moments is built up in existing spans as further throughout. Transverse cables in the deck slab and
spans are completed. The net result, Fig 5(b), cantilevers are tendons made of thirty-eight 7-mm
is that the dead load moments obtained from 1
8
the summation of separate stages are diameter high strength steel wires (f = 235,000
2
different from those which might be obtained if the lb/in ) placed radially on the bridge at about 44 in
bridge could be constructed as a single unit (M ). (111.8 cm) centres.
G
The general effect is to increase span moments
and to diminish support moments. This elastic The main girder reinforcing is located in the webs
redistribution of moments can be useful in frame and consists of fourteen 48-wire cables which are
design, although it is generally difficult to predict continuous throughout the six spans. Cables are
in advance the distribution of secondary moments coupled at the existing anchorages as a new stage
1
in the structure. Lee has investigated the change is built. The post-tensioning system used is well
in girder moments which result from varying adapted to such splicing. In addition, four 38-wire
cantilever lengths in stage construction.
cables extend over a portion of the span and are
anchored through the deck. All cables are grouted,
Fig 6(a).

The Canadian Standards Association highway
bridge code, S6-1966, was used in the design. At
release, maximum wire tensions cannot exceed 0.7
1
1
f , nor 0.8 f during the post-tensioning operation.
8
8
After all losses have occurred the wire stress shall
not exceed 0.6 f . Because of the large friction
1
8
losses due to the change in cable direction over
the supports, it was necessary to stress and release
the tendon alternately to produce a reasonably
uniform tension near the stressing end. Fig 6(b)
shows the general prestressing force distribution
in stage 1.

Fig 4 Cross-section of the bridge
As with the girder moments, the construction by
stages has an effect on secondary moments. This
is illustrated in Fig 5(c) where smaller positive
secondary moments are produced in stage
construction than those which might be
obtained if the beam could be built and stressed
as a unit (M ).
R

The Indian Concrete Journal | November 2018 27