Prestressed/post-tensioned concrete bridges - Computational Analysis and Design of Bridge Structures - page 129

Prestressed/post-tensioned
concrete bridges
5.1 Prestressing Basics
There are two prestressing methods available for prestressed concrete
girders—pretensioning and post-tensioning. The main objective of pre-
stressed concrete girders is to increase the load-carrying capacity for
both strength and serviceability of concrete girders. Both prestressing
methods and their modeling techniques will be discussed in the follow-
ing sections.
For the pretensioning method, the process of producing prestressed
concrete girders is similar to that of reinforced concrete. However, unlike
reinforced concrete, special steel strands are used and pretensioned prior
to placing the concrete. Prestressed concrete bridge girders are typically
designed to resist high tensile stresses in the bottom flange of the girders at
midspan. This is achieved by placing the pretensioning steel strands in the
lower portion of the girders (McDonald 2005).
One consequence in attaining this desired strength at midspan is that
tensile stresses at the ends of the member in the top flange exceed design
code limits. Figure 5.1a provides a brief overview of the loading stages
for prestressed concrete girders. Figure 5.1b demonstrates the linear stress
distribution at the various stages of the prestressed concrete girder fabrica-
tion to the final installed condition. Stage 3, location 1 (at transfer length),
which is the primary concern, illustrates the tensile stress that develops in
the top flange of the girder.
For precast prestressed concrete girders, two techniques have been avail-
able for handling the tensile stresses that develop at the release of the pre-
stressing force. These two techniques are based on the position and pattern
of the prestressing strands. The two strand patterns consist of (1) all straight
strands with debonding at the ends of the member or (2) straight strands
with a certain number of the strands deflected upward at the ends of the
girder. Figure 5.2 illustrates the strand profiles for these two detensioning
techniques. Figure 5.3 illustrates the harping technique and displays the
hold-down devices used prior to the placement of formwork.
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