B.1 Introduction. The purpose of Annex B is to provide information on the selection and use of operating basis copyright NFPA 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.