Prestressed/post-tensioned concrete bridges - Computational Analysis and Design of Bridge Structures - page 147

Table 5.1 Spans of Verzasca 2
Span

Length (m) Elements

1 33.57 30

2 36.26 30

3 39.69 30

4 36.51 30

5 29.40 25

6 25.24 25

section. Truss elements are placed as supports. This model is depicted in
Figure 5.18. The magnitudes of moment of inertia of the beam elements are
represented in gray. While all beam elements have the same properties in
this example, the gray elements form a straight line.
The spans are described in Table 5.1. A uniform weight of 219.3 kN/m
(15 kip/ft) is applied on each beam element. This represents the structural
weight for a reinforced concrete cross section of 8.6 m 2 (92.6 ft 2 ), with a
density of 25.5 kN/m 3 (162 lb/ft 3 ) assumed.

5.4.2.2 Model 2: Continuous girder with skew supports
Skew supports have an influence mainly on the torsional moment of the
superstructure. The skew supports would be taken into account only if they
generate a change in the distribution of the vertical moments. To analyze the
influences on the torsional moment, skew supports are added in Model 2,
which is presented in Figure 5.19.
To model the skew supports, further elements have been created. They
are aligned in the direction of the supports and have other section prop-
erties as the already-existent beam elements. To prevent deformations of
these elements, their moment of inertia has been set 10 times higher than
the moment of inertia of the beam elements. Notice that the beam elements
need torsional stiffness to obtain the actual distribution of the internal
forces.

5.4.2.3 Model 3: One girder built in a single stage
Model 3 is further developed from Model 1. There are two key differences.
First, the cross section changes across the beam, with wider webs and bot-
tom flanges in the zones of the piers. Second, the post-tensioning tendons
are also included in the model.
The girder, which is modeled into 3D beam elements, is divided into
three different cross sections: