Prestressed/post-tensioned concrete bridges - Computational Analysis and Design of Bridge Structures - page 140

model, moments and shears are direct results from analysis, and there is
no need to integrate stresses to get beam moments for strength limit state
capacity check. No matter which code is adopted for design, stress limits
for concrete and steel are always given.
On the other hand, the 3D modeling technique has become more sophis-
ticated and more popular nowadays to understand the behavior of a
bridge during different construction stages. Instead of modeling tendons
as applied loading, they are modeled as resisting elements as described in
Section 5.2.2. In routine bridge analyses, prestressed beams are usually
modeled as beams while tendons are modeled as a series of truss elements
with embedded pretensioning forces. For a complete 3D bridge model, in
which deck are simulated by shell or solid elements with rigid connection
to beam elements, tendons can be modeled by spatial truss elements sharing
appropriate nodes with shell, solid or beam elements. An illustration of 2D
modeling is described in Section 5.3, and a more detailed demonstration of
3D modeling is covered in Sections 5.4 through 5.7.
5.3 2d illustrated exaMPle of a PrototyPe
Prestressed/Post-tensioned concrete
Bridge in the united states
Based on AASHTO specifications (2013), a design case for a concrete alter-
nate with a continuous prestressed and then post-tensioned precast I-beam
bridge is analyzed as a single beam staged from simple to continuous beams.
The total length of the bridge is 198.86 m (652′-5″), with five continuous
spans of 39.5 m (129′-7″) each (Figure 5.14a). The clear roadway width
is 13.41 m (44′), and out-to-out distance is 17.98 m (59′) with 3-3.66 m
(12′) lanes. Five 1880-mm (74″) deep precast bulb-T girders are used in the
design with 3.81-m (12′-6″) girder spacing (Figure 5.14b). A 200-mm (8″)
deck slab is used in the composite construction with another 13-mm (1/2″)
wearing surface.
Precast girder is formed by the semi-light weight concrete with initial
concrete strength ( f ci ′) of 31 MPa (4500 psi) and final concrete strength
( f c ′) of 48.3 MPa (7000 psi). Concrete strength of the cast-in-place con-
crete is 34.5 MPa (5000 psi). All the prestressing tendons are 1862-MPa
(270-ksi) stress-relieved seven-wire strands with modulus of elasticity of
1.9 × 10 5 MPa (28 × 106 psi). The prestressing steel strand's diameter is
13 mm (1/2″), and the post-tensioning steel strand's diameter is 15 mm
(0.6″). Figure 5.15 shows the profile of the post-tensioning conduits (pre-
stressing strands are not shown) and three cross sections at the end spans.
Cross sections A-A and C-C (Figure 5.15b) show the thickened webs at the
ends of the precast beam. The construction sequence is listed as follows: