Suspension bridges - Computational Analysis and Design of Bridge Structures - page 389

For extreme long-span suspension bridge proposals, such as the Gibraltar
Strait Bridge or cable-stayed-and-suspended hybrid bridge as shown in
Figure 12.14, the girder is stiffened by stay cables anchored from pylons or
hangers from rigid components in areas close to the pylon.
12.4 principLe and modeLing
of SuSpenSion BridgeS
Similar to modeling a cable-stayed bridge, the modeling of a suspension
bridge needs to identify the analysis tool. The following specific issues for
suspension bridge analyses make general-purposed FEA packages not a
suitable tool for many types of analysis in general.
1. Analyses to determine the initial state from the design state
2. Geometric nonlinear construction and control analyses
3. Simulation of saddle adjustment
4. Tracking the changes of cable ends in saddles
5. Live load analyses with the consideration of initial stresses and large
displacements
Whether or not to use a 3D model depends on the purpose of the analyses.
A 3D model is always preferable for all types of analyses, not only because
the lateral distribution can be included but also because the modeling of
pylons and stiffened girders can be simplified in 3D modeling. For example,
the discrete truss members can accurately and easily reflect the properties of
the stiffening girder than beam elements. Having a modern graphical tool
aided, modeling a suspension bridge in 3D is no longer a challenge as it was
many years before. Also the computer capacity and performance nowadays
guarantees that a full-scale nonlinear analysis simulating multiple-stage
construction in 3D is doable.
12.4.1 main cables
How to model the main cables is an important question in modeling a sus-
pension bridge, and it should be answered first. Due to its special character-
istics such as flexibility, large displacements, and catenary behaviors, some
analysis tools may have a catenary element type included. When a catenary
cable element type is adopted for modeling cables, some special consider-
ations should be taken as properties to describe that such a cable element
may vary among different tools. For example, one may use stressed state
to describe the cable geometry and the other may use its zero-stress state.
In general practice, the simple truss element type can be used to model the
main cables.