Inelastic Behavior - Continued

DOE-STD-1020-2002

demand and the resulting total demand is compared to the capacity determined from

member forces at ultimate/limit-state stress level to determine structural adequacy.

The spectral reduction factors and inelastic energy absorption factors are evaluated

based upon the permissible inelastic behavior level, which depends on the materials and type of

construction. For ductile steel moment frames, relatively large reduction factors or inelastic

energy absorption factors are used. For less ductile shear walls or braced frames, lower

reduction values or inelastic energy absorption factors are employed. For more hazardous

facilities, lower reduction factors or inelastic energy absorption factors may be used to add

conservatism to the design or evaluation process, such that increased probability of surviving any

given earthquake motion may be achieved.

The inelastic energy absorption factor approach is employed for design or evaluation of

Performance Category 3 and higher SSCs by these criteria. This approach is recommended in

the DOD manual for seismic design of essential buildings (Ref. C-5). Inelastic energy

absorption factors are called F in these criteria. Inelastic energy absorption factors have the

advantage over spectral reduction factors in that different values may be specified for individual

elements of the facility instead of a single spectral reduction factor for the entire lateral force

resisting system. As a result, critical elements such as columns or connections can be easily

designed for larger forces by specifying a smaller inelastic energy absorption factor than for

other elements.

Base shear reduction coefficients that account for energy absorption due to inelastic

behavior and other factors are called R by the IBC provisions. R is more like a spectral

reduction factor in that it is applied to the entire lateral force resisting system. There are special

IBC provisions which require critical elements such as columns or connections to be designed

for larger loads than those corresponding to the base shear equation using R. The IBC

provisions are followed for Performance Category 2 and lower SSCs by these criteria.

Reduction coefficients, R, to be used for evaluation of PC-1 and PC-2 are as per IBC

2000. Recommended inelastic energy absorption factors, F, for PC-3 and PC-4 SSCs are

presented in Chapter 2 for various structural systems. The F factors presented in Chapter 2

were established to approximately meet the performance goals for structural behavior of the

SSCs as defined in Chapter 2 and as discussed in Section C.2. These factors are based both on

values given in Reference C-5 and on values calculated from code reduction coefficients in a

manner based on the performance goals. Reference C-20 describes the detailed method for

establishing the values of F.

The response modification factor coefficients, R, by the IBC approach and inelastic

energy absorption factors, F, by the DOD approach differ in the procedures that define

permissible inelastic response under extreme earthquake loading. By the IBC approach, only the

element forces due to earthquake loads are reduced by the reduction coefficient, R, in evaluating

demand; while by the DOD approach, element forces due to both earthquake and dead and live

loads are reduced by the inelastic energy absorption factor, in evaluating demand. The effect of

this difference is that the DOD approach may be less conservative for beam or brace members

C-30