Tiến Sĩ Numerical analysis of externally prestressed concrete beams

1.1 GENERAL

One of the latest developments in prestressed concrete technology has been the use of
external cables, which may be defined as a method of prestressing where major portions of
the cables are placed outside the concrete section of a structural member. The prestressing
force is transferred to the beam section through end anchorages, deviators or saddles. This
type of prestressing could be applied not only to new structures, but also to those being
strengthened. Substantial economic and construction time saving have been indicated for this
innovative type of construction.

External prestressing system was used in the bridge construction in the early days of
prestressing. However, due to a generally inadequate technology, external prestressing has
received a bad image and was almost abandoned in the 1950’s. This is because the corrosion
problem for the external cables was serious, and the internal prestressing system with the
bonded cable was emphasized. With the development of partial prestressing techniques and
protective system for the external cables, it is possible to have structures with external cables,
whose performance is as good as the structures with bonded cables. In recent years, external
prestressing revives in the construction of new structures and has a great development in the
bridge construction.

The deterioration of existing bridges due to increased traffic loading, progressive structural
aging, and reinforcement corrosion from severe weathering condition has become a major
problem around the world. The number of heavy trucks and the traffic volume on these
bridges has both risen to a level exceeding the value used at the time of their design, as a
result of which many of these bridges are suffered fatigue damage and are therefore in urgent need of strengthening and repair. A method for strengthening and rehabilitation of such
structures has become increasingly important.

External prestressing is considered one of the most powerful techniques used for
strengthening or rehabilitation of existing structures and has grow recently to occupy a
significant share of the construction market. The adoption of external cables has been
proposed as a very effective method for repairing and strengthening damaged structures.
Although external prestressing is a primary method for rehabilitation and strengthening of
existing structures, it is being increasingly considered for the construction of new structures,
particularly bridges. Since the external prestressing system is simpler to construct and easier
to inspect and maintain as compared with the internal prestressing system, the beams
prestressed with external cables have attracted the engineer’s attention in recent years, and it
has been proposed in the design and construction of new bridges. A large number of bridges
with monolithic or precast segmental block have been already built in the United States,
European countries and Japan by using the external prestressing technique. Recently, a new
type of structures using the external cables or combination with either bonded cables or
unbonded cables has been increasingly developed around the world such as externally
prestressed concrete bridges consisting of concrete flanges and folded steel web or extra-
dosed bridges with a short tower.

In this chapter, the definition of post-tensioned prestressed concrete beams and
classification of beams prestressed with external cables is initially presented. The application
of external prestressing is discussed together with its advantages and disadvantages. The
historical development of external prestressing is also discussed, following by literature
reviews of the previous studies. A general overview of problem arisen from the application of
external prestressing is highlighted. The differences between internally unbonded cables and
external cables at all loading stage are also briefly presented and discussed. Finally, the
objectives and scope of the present study as well as the organization of the course of study are