Prestressed/post-tensioned concrete bridges - Computational Analysis and Design of Bridge Structures - page 148

• Section 1 —In the middle of the spans
• Section 2 —With thickened webs and a thickened bottom slab, used
where negative vertical moments are expected, in the zones next to
and over the piers
• Section 3 —In the diaphragm areas
The vertical position of these elements is at the neutral axis of each cross
section, which, together with the horizontal measures, defines the geom-
etry of the beam.
To model the post-tensioning tendons, truss elements are created. Only
one truss element represents all eight of the individual tendons that are
distributed over the cross section, as seen in Figure 5.17. The geometry
of the tendon is approximately at the middle of the actual positions. It is
important that they end at the same vertical location as the beam elements,
so that they can be connected with vertical rigid elements. Therefore, the
line that represented the geometry of the tendon was cut with vertical
lines placed at every two, three, or even four elements of the beam. Again,
it was more important to have the connection at the suitable positions,
rather than have intervals with the same number of elements between
them. Suitable positions are (1) at the anchorage of the tendons, (2) at the
middle span, (3) over the pier, and (4) where section changes occur. The
tendon is divided between these points if the remaining length is longer
than four elements or if a straight line between these points would fail to
keep the geometry of the tendon.
Figure 5.21 shows the beam and tendon elements connected with rigid
elements. The two horizontal lines are not used for this model but are used
to assist in visualizing the upper and lower edges of the cross section. The
shade scale changes of beam elements as shown in Figure 5.21 indicate the
change of cross section. Different cross sections with different areas of steel
are used for truss elements to simulate the changes of total strands in the
longitudinal direction.
The boundary conditions stay the same as those for Model 1. In
Figure 5.22 the right part of Model 3 is shown. Notice that the support
Figure 5.21 Detail of Verzasca 2 Bridge model 3.