Dual systems

A dual system is one in which, according to the results of the analysis, between 35 and 65% of the seismic base shear is (or rather should be) resisted by frames of primary seismic beams and columns, and the rest of the seismic base shear resisted by primary seismic walls. Dual systems combine the satisfactory stiffness, force resistance and cost-effectiveness of walls with the ductility and large deformation capacity of frames, which can act as a second line of defence in case (some of) the more brittle walls of the system fail. Moreover, dual systems use to advantage the beams and columns that carry (most of the) gravity loads for the lateral force resistance, as well as the capacity of columns to resist lateral forces in both horizontal directions. Their inelastic behaviour, though, is much more uncertain than that of pure frame or wall systems. Examples of uncertainties include:

(1) the capacity of floor diaphragms to transfer forces from walls to frames or vice versa, as these subsystems share the storey shear differently at different storeys

(2) the sharing of lateral forces between walls and frames depending on the rotation at the base of walls and columns due to compliance of the foundation (in systems with vertical elements of about the same size, such rotations do not appreciably affect the distribution of storey shears forces among the vertical elements).

The sensitivity of the response to such uncertainties should be reduced through proper conceptual design and/or addressed through sensitivity analyses.

If more than 50% of the base shear is resisted by primary seismic walls, the dual system is classified as wall-equivalent; otherwise it is defined as frame-equivalent. As noted on p. 90, the distinction between wall- and frame-equivalent dual systems has important practical consequences, as it determines whether the columns of the dual system should be capacity designed against plastic hinging above their base and whether design should account for the presence and the effects of masonry infills.

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