Civil Engineering Reference
In-Depth Information
Post-tensioning involves fi rst casting the concrete deck, and then installing cables,
which are stressed by proprietary jacks reacting against the concrete. The cables are
then locked to the concrete by anchors. This topic is principally concerned with the
design of post-tensioned structures, although mention is made of the option of pre-
tensioning when appropriate.
4.4 Conclusion
Designing a prestressed concrete structure requires a higher degree of skill from
the designer than if he was designing a conventional, 'passive' structure. He has to
understand more fully the range of actions on his structure, the effects of creep and
shrinkage of concrete, the difference between internal and external forces and between
loads and imposed deformations. Whereas an unskilled designer of a reinforced
concrete structure can hide his lack of ability by over-reinforcing it, over-prestressing
a structure may reduce its safety. For instance, increasing the prestressing in the tie
of the above example 'for safety' would in fact increase the compressive stress in the
unloaded tie, and increase the risk of failure rather than providing any additional
margin of safety.
The example of a tension member was chosen for its simplicity and clarity. However,
the benefi ts of prestressing are even greater in beams and slabs, as in addition to those
summarised in
4.2.6
, the self-weight defl ections of the member may be cancelled,
and, in continuous beams, manipulation of the prestress secondary moments allows
a degree of optimisation of the ratio of support and span moments. Prestressing also
frequently allows a reduction in the depth and in the cross-section area of beams and a
reduction in the thickness of slabs as compared with reinforced concrete.
In Chapter 5, the general principles of prestressing are illustrated by following step
by step the preliminary design of a statically determinate beam. In Chapter 6, these
principles are extended to the design of continuous beams.
