Civil Engineering Reference
In-Depth Information
rigid bodies or beams should locate in all hanger locations. For truss stiffened
girder, it is preferable to use the 3D model. Each truss member can be mod-
eled as a beam or truss element so the girder's properties can be accurately
modeled. For cases where the deck is comprised of floor beams and string-
ers, stringers too can be included as beam elements. The superimposed dead
loads will be applied only on main beams, main trusses, and/or stringers.
12.4.4 pylons
The modeling of pylons is similar to cable-stayed bridges. 3D beam ele-
ments are usually used to model pylons. The longitudinal bending stiffness
of pylons is an important factor to influence the analysis of saddle offsets.
When 2D model is used for preliminary analyses, cross-sectional properties
should accurately reflect the pylon stiffness.
12.4.5 Saddles
The modeling of saddles and their connections to pylons and cables are
critical in the entire bridge model, especially for construction control analy-
ses in which large displacement iterations will be involved. The moving
between saddle and pylon usually is locked during the erection of the stiff-
ened girder and is unlocked when horizontal adjustment is needed between
erections. After the deck is superimposed, as in most suspension bridges,
the connection between the saddle and the top of the pylon will be changed
to rigid so the pylon will work to resist unbalanced cable forces due to live
loads. This change should be incorporated into modeling according to the
type of analyses.
Figure 12.16a shows a general model of the saddle and its connection to
cables and the pylon. A temporary horizontal rigid truss element is needed to
simulate the locking between saddle and pylon during erection and adjust-
ment. By applying an initial displacement of Δ, as shown in Figure 12.9,
on the temporary right truss element, a load case of jacking saddle can be
simulated. Changing its stiffness to significantly small or simply removing
it, free moving between the saddle and pylon can be simulated during live
load analyses. However, when conducting certain types of analysis, the
saddle and its connections can be simplified as shown in Figure 12.16b.
For example, when only extreme live load responses are of concern in sche-
matic analysis, a single rigid truss or rigid body can be used. If the saddle is
designed not to rigidly connect to pylon when bridge is in services, a simple
truss from the top of the pylon to the intersection of cables can be used.
Figure 12.16c shows a general model of splay saddle and cable anchorage.
When the splay saddle is built into anchor box or a rotational splay saddle
is used as shown in Figure 12.8, the splay saddle shown in Figure 12.16
is either the arc center of the saddle surface or the saddle hinge. When a
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