earthquake (OBE) and safe shutdown earthquake (SSE) seismic levels. These two seismic
levels form part of the requirements of this standard for the design of LNG containers,
system components required to isolate the container and maintain it in a safe shutdown
condition, and any structures or systems the failure of which could affect the integrity of the
aforementioned.
When computing lateral forces and shears associated with these two seismic levels, an
importance factor, I, of 1.0 can be used. That is because, to the extent that the importance
factors in ASCE 7, Minimum Design Loads for Buildings and Other Structures, and the ICC
International Building Code (IBC) are identified as “occupancy importance factors” that
apply to a broad range of building and structure types, they do not apply to this standard,
which deals with a rather narrow, welldefined range of structures (primarily LNG containers
and their impounding systems). The designer using this standard should rely on the fact that
the “importance” of these structures and their “occupancy category” have already been
factored in, as reflected in the definitions of OBE and SSE and the provisions and
performance criteria of 7.2.2.
B.2 Operating Basis Earthquake (OBE). The OBE is a probable earthquake to which a facility can be subjected during its design life.
All elements of the facility are designed to withstand this event in accordance with
conventional engineering procedures and criteria, and, therefore, the facility is expected to
remain in operation. The OBE is defined as ground motion having a 10 percent probability of exceedance within a
50year period (mean return interval of 475 years). For design, this motion is typically
represented by design response spectra covering the appropriate ranges of natural period and
damping ratio.
B.3 Safe Shutdown Earthquake (SSE). B.3.1 The SSE is the “maximum considered earthquake (MCE) ground motion,” per the
definition in ASCE 7, Minimum Design Loads for Buildings and Other Structures. For most
locations, except possibly those near active faults, the MCE is ground motion that has a 2
percent probability of exceedance in a 50year period. The corresponding design response
spectrum is 50 percent greater than the “design earthquake” spectrum as defined in ASCE 7.
(The ASCE 7 “design earthquake” represents the ground motion for which buildings are
designed so as to safeguard life, although they are allowed to sustain some tolerable
damage). In this standard, the LNG facility is designed to contain the LNG and prevent
catastrophic failure of critical facilities under an SSE event. Plastic behavior and significant
finite movements and deformations are permissible. The facility is not required to remain
operational following the SSE event. Following such an event, the facility is expected to be
inspected and repaired as necessary. The provisions of this standard are considered
equivalent to those applied to an ASCE 7 Seismic Group III structure.
B.3.2 The objective of the selection and use of the SSE is to provide a minimum level of
public safety in the event of a very low probability seismic event. It is recognized that the
required probability level to achieve acceptable public safety varies from project to project,