Inelastic Behavior - Continued

DOE-STD-1020-2002

provided by a cover of at least 2.5 bar diameters or by ties (stirrups) spaced no further than 5 bar

diameters apart. If this confinement is provided, a strength of the reinforcement equal to the

yield strength of the steel times the ratio of actual to required development length may be used in

the capacity evaluations. In these cases, the factor F should be limited to 1.0. If the

confinement is not provided, the reinforcement should be omitted in the capacity evaluations.

For low-ductility failure modes such as axial compression or shear in concrete walls or

columns and wall-to-diaphragm, wall-to-column, or column-to-base connections, the F values

are 1.0. In most cases, such stringent limits can be relaxed somewhat, as described below,

because most components also have a ductile failure mode which when reached is likely to limit

the demand in the low-ductility failure modes. Unless the component has a seismic capacity in

the ductile failure mode significantly in excess of its required capacity, inelastic distortions in

this ductile failure mode will likely limit the scaled inelastic seismic demand DSI in the

low-ductility failure modes. Since greater conservatism exists in code capacities CC for

low-ductility failure modes than for ductile failure modes, the failure will be controlled by the

ductile failure mode so long as the low-ductility failure mode code capacity is at least equal to

the ductile failure mode capacity. Thus, for low-ductility failure modes, the factored seismic

demand DSI can be limited to the lesser of the following:

DSI given by dividing elastic demand by F, or

DSI = CC - DNS computed for the ductile failure mode, where CC is the ductile failure

mode capacity.

Therefore, for example, connections do not have to be designed to have code capacities

CC greater than the code capacities CC of the members being connected, or the total factored

demand DTI, whichever is less. Similarly, the code shear capacity of a wall does not have to

exceed the total shear load which can be supported by the wall at the code flexural capacity of

the wall. Finally, the horizontal seismic-induced axial force in a moment frame column can be

limited to the axial force which can be transmitted to the column when a full plastic hinge

mechanism develops in the frame where the plastic hinge capacities are defined by the code

ultimate flexural capacities.

Several other factors may be noted about the inelastic energy absorption factors, F:

Chapter 2 values assume that good seismic design detailing practice (Reference

C-21) has been employed such that ductile behavior is maximized. If this is not the

case (e.g., an existing facility constructed a number of years ago), lower inelastic

energy absorption factors should be used instead of those presented in Chapter 2.

Chapter 2 F values assume that inelastic behavior will occur in a reasonably

uniformly manner throughout the lateral load-carrying system. If inelastic behavior

during seismic response is concentrated in local regions of the lateral load carrying

system, lower inelastic energy absorption factors should be used than those presented

herein.

C-33