Effective Peak Ground Motion

DOE-STD-1020-2002

C.3.3 Effective Peak Ground Motion

The peak ground accelerations reported in probabilistic seismic hazard assessments

typically correspond to acceleration that would be recorded during an earthquake by a motion

instrument. This instrumental acceleration may, in some cases, provide an excessively

conservative estimate of the damage potential of the earthquake. Instead the effective peak

acceleration based on repeatable acceleration levels with frequency content corresponding to that

of structures is a better measure of earthquake damage potential. It is acceptable, but often quite

conservative, to use the instrumental ground motion as direct input to the dynamic model of the

structure. It is also acceptable, and encouraged, for the seismic evaluation to include additional

studies to remove sources of excessive conservatism on an individual facility basis, following the

guidance described below.

The instrumental acceleration is a poor measure of the damage potential of ground

motion associated with earthquakes at short epicentral ranges (less than about 20 km). Many

structures located close to the epicentral region, which were subjected to high values of peak

instrumental acceleration, have sustained much less damage than would be expected considering

the acceleration level. In these cases, the differences in measured ground motion, design levels,

and observed behavior were so great that it could not be reconciled by considering typical safety

factors associated with seismic design. The problem with instrumental acceleration is that a

limited number of high frequency spikes of high acceleration are not significant to structural

response. Instead, it can be more appropriate to utilize a lower acceleration value that has more

repeatable peaks and is within the frequency range of structures. Such a value, called effective

peak acceleration, has been evaluated by many investigators who believe it to be a good measure

of earthquake ground motion amplitude related to performance of structures. Reference C-24

contains a suggested approach for defining the effective peak acceleration. However, this

approach would require the development of representative ground motion time histories

appropriate for the earthquake magnitudes and epicentral distances that are expected to dominate

the seismic hazard at the site. Generally, special studies would be required for any site to take

advantage of the resultant reduction. The reductions that are likely to be justifiable from such

studies would most probably be significant for sites with peak instrumental accelerations in

excess of about 0.4g. The benefits would be expected to increase with increasing peak

instrumental accelerations. These higher ground accelerations most probably are associated with

short duration ground motion from earthquakes with short epicentral ranges. If such

characteristics can be demonstrated for a particular site, then reductions would be warranted

from an instrumental acceleration to an effective acceleration.

C. 4

Evaluation of Seismic Demand (Response)

The earthquake design and evaluation criteria in DOE-STD-1020 generally follow the

International Building Code, IBC 2000 provisions (Ref. C-28) for Performance Category 2 and

lower SSCs and the DOD Tri-service manual for essential buildings (Ref. C-5) for Performance

Category 3 and 4 SSCs as indicated in Figure C-6. For Performance Category 2 and lower SSCs,

these seismic design and evaluation criteria employ the IBC 2000 seismic provisions with the

C-18