Civil Engineering Reference
In-Depth Information
Figure 15.23
Location of temporary bar anchors
Finally, temporary stressing may be incorporated in ducts within the top slab of the
box. Although apparently a neat option, this method requires pockets to be left to allow
the bars to be de-stressed and dismantled, which is a laborious task. This temporary
prestress is sometimes designed to be incorporated into the fi nal prestressing scheme.
Although superfi cially attractive, the bars tend to account for only a small percentage
of the total prestress required, at a very high unit price. In general, it is better to design
to recover and re-use the temporary prestressing bars.
15.5.4 Prestress layout
The prestress layout for precast decks is broadly similar to that described for cast-in-situ
decks in
15.4.7
and only matters specifi c to precast construction will be discussed.
Precasting to a daily routine requires greater discipline than cast-in-situ construction,
and repetition of prestress details should be sought. Thinner webs may be adopted,
which can limit the size of face anchored tendons. Alternatively, the web may be
increased in thickness to accommodate the anchor chosen, or thickened locally. As
the concrete is mature at the time of stressing, a tendon consisting of 12 No 13 mm
strand requires a 320 mm thick web, 19 No 13 mm strand or 12 No 15 mm strand
require a 450 mm thick web, while 19 No 15 mm strand requires a 500 mm web. For
an economically designed bridge, the web in the central half of the span rarely needs
to be thicker than 400 mm, and 300 mm is usually adequate.
For example, in the East Moors Viaduct the webs were 500 mm thick close to the
pier, thinning to 300 mm in the second segment. The Stage 1 prestress units were
19 No 13 mm strand, face anchored towards the bottom of the web. This size of tendon
was chosen as it allowed one or two pairs of tendons to be anchored in each segment
except the last which was attached by temporary stressing alone. A local thickening
of the 300 mm web to 450 mm was therefore required. This thickening was faired
into the bottom corner blisters used to anchor the Stage 2 tendons, Figure 15.21 and
Figure 9.28. Intelligent design of the temporary stressing allows the Stage 1 tendons
to be lowered from about the quarter point of the span onwards, to optimise their in-
service performance, as described in
15.4.7
.
