Prestressed/post-tensioned concrete bridges - Computational Analysis and Design of Bridge Structures - page 166

• The conventional layout of longitudinal tendons is shown in
Figure 5.44, including cantilever tendons in the top slabs, cantilever
bent-down tendons in the webs, and continuity tendons in the bottom
slabs and webs. The layout and number of tendons are mainly deter-
mined based on the envelope of the bending moment of the girder
under all kinds of loads.
• From the end of 1980s, the elimination of the bent-down tendons in the
webs, and instead the addition of vertical prestressing rods in the webs,
was proposed, as shown in Figure 5.45. This straight layout method
was well received by the construction industry because there were few
ducts in the webs, which was more convenient to the construction of
the box girder. However, a large number of vertical prestressing rods
may have led to rising costs. Also, after more than 10 years, exces-
sive deflections at midspan and inclined cracks in the webs appeared
in many long-span concrete cantilever bridges with this design method.
As a result, designers began to throw doubt on the elimination of the
webs' bent-down tendons (as demonstrated in Figure 5.46).
• In the early 2000s, designers brought their attention back to the webs'
bent-down tendons, and the common layout of longitudinal tendons
is shown in Figure 5.46. The phenomena of the excessive deflections
at midspan and the inclined cracks in the webs are seldom seen in
the long-span concrete cantilever box girder bridges constructed either
more than 20 years ago or more recently in the 2000s. It was therefore
concluded that the elimination of the webs' bent-down tendons is one
Figure 5.44 Conventional layout of longitudinal tendons.
Figure 5.45 Straight layout of longitudinal tendons.
Figure 5.46 Current layout of longitudinal tendons with the webs' bent-down tendons.