Dynamic/earthquake analysis - Computational Analysis and Design of Bridge Structures - page 528

Equations 17.16 and 17.17 can be used to replace the forcing function
shown on the right-hand side of Equation 17.1. Special technique and spe-
cialized program have to be adopted for the analysis. For details, please
refer to Scanlan (1978a, 1978b) and Cai et al. (1999) for FEM formulation.
For wind analysis, many used ESA in their studies. To demonstrate blast
analysis, two examples are illustrated in Chapter 15 as part of the redun-
dancy analysis.
Long-span bridge design should follow special guidance for aerodynamic
issues. Wind tunnel testing may be unavoidable for the design of long-span
bridges. The aerodynamic stability issue is not covered in this chapter while
wind load can be considered as a static wind load pressure. Its application is
discussed in Chapter 11 for cable-stayed bridges and illustrated in Section
11.5 for the Sutong Bridge, China.
17.3 moDeling of BriDge for Dynamic analysis
As introduced in the last section, bridge dynamic analyses can be catego-
rized into five different types: (1) dynamic interaction between vehicle and
bridge, (2) pedestrian bridge dynamics, (3) bridge earthquake analysis, (4)
blast analysis, and (5) long-span bridge wind analysis. The first two types
of bridge dynamic analysis, except few special cases, can be modeled with
superstructure only where the substructure and foundation have little con-
tribution on the dynamic behavior. Modeling for the other three types of
analysis will include the whole system, super- and substructures, where the
earthquake analysis even includes the foundation. The first, second, and
fifth types can be handled by linear dynamic analysis, whereas the third
and fourth types may involve nonlinear dynamic analysis.
Due to its uniqueness in analysis and popularity in usage, only the model-
ing technique of bridge earthquake analysis is discussed in detail here. The
earthquake-resistant system (ERS) for bridges may be modeled with the
entire super- and substructures (the global model) or an individual bent or
column (the local model). Individual bridge components (the local model)
shall have displacement capacities greater than the displacement demand
from the global model to satisfy the performance requirement.
17.3.1 linear elastic dynamic analysis
Linear elastic dynamic analysis (EDA) will be a minimum requirement for
the global response analysis. The global analytical model should include
the stiffness and mass distributions of the bridge. Commonly a three-
dimensional (3D) model is used where it, as shown in Figure 17.11 (NHI
1996), can be a spine model, a grillage model, and a 3D FEM model where
the spine and grillage models are the popular kinds. Because elastic analysis