Inelastic Behavior - Continued

DOE-STD-1020-2002

Some guidance on estimating F5% is given in Section C.4.4.3. However, such analyses are

often expensive and controversial. Therefore, a set of standard F values is provided in Chapter

2 for common elements. These F values may be used in lieu of performing nonlinear analyses,

so long as the following cautions are observed.

A significant difference between the R factors from the IBC and the F factors to be

used for PC-3 and PC-4 SSCs is that R applies to the entire lateral force resisting system and F

applies to individual elements of the lateral force resisting system. For elements for which F is

not specified in Chapter 2, it is permissible to use the F value which applies to the overall

structural system. For example, to evaluate diaphragm elements, footings, pile foundations, etc.:

(1) F of 3.0 may be used for a steel special moment resisting frame (SMRF) or (2) in the case

of a steel concentric braced frame, F of 1.75 may be used.

In the evaluation of existing facilities, it is necessary to evaluate an appropriate value of

F to be used. The F values in Chapter 2 assume good seismic detailing practice along with

reasonably uniform inelastic behavior. Otherwise, lower values should be used. Good detailing

practice corresponds to that specified in the current International Building Code (IBC, Ref. C-

28) or ACI 349 (Reference C-48). It is highly unlikely that existing facilities will satisfy the

seismic detailing requirements of the current IBC if they were designed and constructed many

years ago. If structures have less ductility than the IBC provisions require, those structures must

be able to withstand larger lateral forces than specified by this criteria to compensate for

non-conforming structural details. As a result, F values must be reduced from the values given

in Chapter 2. One acceptable option is that existing structural elements are adequate if they can

resist seismic demand forces in an elastic manner (i.e., F of unity). To arrive at reduced F

factors (i.e., between the full value and unity) requires judgment and care by the engineer

performing the evaluation. It is suggested that ATC-14, "Evaluating the Seismic Resistance of

Existing Buildings" (Ref. C-46) and ATC-22, "A Handbook for Seismic Evaluation of Existing

Buildings" (Ref. C-47) be reviewed for guidance on this subject.

The use of F values of 1.5 and greater for concrete walls is conditional on wall

cracking occurring as described in Table C-2, with stable wall behavior constituting acceptable

wall performance. If a lesser amount of wall cracking is required, then F should be 1.0.

The values for ductile failure modes (i.e., greater than 1.0) assume that steel reinforcing

bars, structural steels, metal tank shells, and anchorage will remain ductile during the

component's entire service life. It is assumed that the metal will retain at least a 6% uniaxial

elongation strain capability including the effects of welding. If this metal can become embrittled

at some time during the service life, F should be 1.0.

In some cases, reinforcement details in older facilities do not satisfy the development

length requirement of current codes (Refs. C-48 and C-49). In these instances, the potential

exists for a ductile failure mode associated with yielding of the reinforcement to become a less

ductile mode associated with bond failure. Data exists (Ref. C-50), however, indicating that

bond failure modes retain a reasonable amount of ductility provided that the reinforcement is

suitably confined within the region of the potential bond failure. The confinement may be

C-32