Civil Engineering Reference
In-Depth Information
Figure 15.19
Typical layout of prestress tendons for cast-in-situ balanced cantilever decks
materials as explained below. The designer should lower the Stage 1 tendons from
about the quarter point of the span as much as the construction moments allow, and
use reinforced concrete action or partial prestressing to resist these moments to the
limit allowed by the code of practice.
Generally, the top fi bre cables required at the support for cantilever erection are not
enough to carry the fi nishes and live load moments and additional top fi bre cables are
required. These may be straight capping cables stressed between top fi bre blisters, or
the lengthened Stage 2 tendons described below.
Stage 2 cables provide the sagging moment capacity in the span, Figure 15.19 (b),
and they also create substantial sagging parasitic moments, which for preliminary
sizing may be assumed to be equal to the mid-span live loads moments. Initially there
are no other signifi cant bending moments at mid-span, and the prestress is likely to
cause tension stresses on the top fi bre. These stresses will be small, and can be carried
