20622Design Approaches

The displacement-based design approach is used to ensure that the structural system and its individual components have enough displacement capacities to withstand the deformation imposed by the design earthquake. Using displacements rather than forces as a measurement of earthquake damages allows a structure to fulfill the required functions. The displacements of the global system and the local ductile system shall satisfy the following requirement:

where AD is the displacement demand determined by the global analysis, the stand-alone analysis, or the larger of the two if both types of analyses are necessary and AC is the displacement capacity when any plastic hinge capacity reaches its ultimate capacity.

In a displacement-based analysis, proportioning of the structure is made first based on strength and stiffness requirements. The appropriate seismic analysis is performed and the resulting displacements are compared to the displacement capacity, which is dependent on the formation and rotational capacity of plastic hinges and can be evaluated by an inelastic static pushover analysis. This procedure has been used widely in seismic bridge design in California since 1994. Figure 20.16 shows the design flowchart for the ordinary standard bridges.

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